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 The Software-RAID HOWTO
 Jakob OEstergaard and Emilio Bueso
 v. 1.1 3rd of June 2004
 This HOWTO describes how to use Software RAID under Linux. It
 addresses a specific version of the Software RAID layer, namely the
 0.90 RAID layer made by Ingo Molnar and others. This is the RAID layer
 that is the standard in Linux-2.4, and it is the version that is also
 used by Linux-2.2 kernels shipped from some vendors. The 0.90 RAID
 support is available as patches to Linux-2.0 and Linux-2.2, and is by
 many considered far more stable that the older RAID support already in
 those kernels.
 Table of Contents

 1. Introduction
    1.1 Disclaimer
    1.2 What is RAID?
    1.3 Terms
    1.4 The RAID levels
    1.5 Requirements
 2. Why RAID?
    2.1 Device and filesystem support
    2.2 Performance
    2.3 Swapping on RAID
    2.4 Why mdadm?
 3. Devices
    3.1 Spare disks
    3.2 Faulty disks
 4. Hardware issues
    4.1 IDE Configuration
    4.2 Hot Swap
       4.2.1 Hot-swapping IDE drives
       4.2.2 Hot-swapping SCSI drives
       4.2.3 Hot-swapping with SCA
 5. RAID setup
    5.1 General setup
    5.2 Downloading and installing the RAID tools
    5.3 Downloading and installing mdadm
    5.4 Linear mode
    5.5 RAID-0
    5.6 RAID-1
    5.7 RAID-4
    5.8 RAID-5
    5.9 The Persistent Superblock
    5.10 Chunk sizes
       5.10.1 RAID-0
       5.10.2 RAID-0 with ext2
       5.10.3 RAID-1
       5.10.4 RAID-4
       5.10.5 RAID-5
    5.11 Options for mke2fs
 6. Detecting, querying and testing
    6.1 Detecting a drive failure
    6.2 Querying the arrays status
    6.3 Simulating a drive failure
       6.3.1 Force-fail by hardware
       6.3.2 Force-fail by software
    6.4 Simulating data corruption
    6.5 Monitoring RAID arrays
 7. Tweaking, tuning and troubleshooting
    7.1 raid-level and raidtab
    7.2 Autodetection
    7.3 Booting on RAID
    7.4 Root filesystem on RAID
       7.4.1 Method 1
       7.4.2 Method 2
    7.5 Making the system boot on RAID
       7.5.1 Booting with RAID as module
       7.5.2 Modular RAID on Debian GNU/Linux after move to RAID
    7.6 Converting a non-RAID RedHat System to run on Software RAID
       7.6.1 Introduction
       7.6.2 Scope
       7.6.3 Pre-conversion example system
       7.6.4 Step-1 - boot rescue cd/floppy
       7.6.5 Step-2 - create a /etc/raidtab file
       7.6.6 Step-3 - create the md devices
       7.6.7 Step-4 - unmount filesystems
       7.6.8 Step-5 - start raid devices
       7.6.9 Step-6 - remount filesystems
       7.6.10 Step-7 - change root
       7.6.11 Step-8 - edit config files
       7.6.12 Step-9 - run LILO
       7.6.13 Step-10 - change partition types
       7.6.14 Step-11 - resize filesystem
       7.6.15 Step-12 - checklist
       7.6.16 Step-13 - reboot
    7.7 Sharing spare disks between different arrays
    7.8 Pitfalls
 8. Reconstruction
    8.1 Recovery from a multiple disk failure
 9. Performance
    9.1 RAID-0
    9.2 RAID-0 with TCQ
    9.3 RAID-5
    9.4 RAID-10
    9.5 Fresh benchmarking tools
 10. Related tools
    10.1 RAID resizing and conversion
    10.2 Backup
 11. Partitioning RAID / LVM on RAID
    11.1 Partitioning RAID devices
    11.2 LVM on RAID
 12. Credits
 13. Changelog
    13.1 Version 1.1

 1.  Introduction
 This HOWTO describes the "new-style" RAID present in the 2.4 and 2.6
 kernel series only. It does not describe the "old-style" RAID
 functionality present in 2.0 and 2.2 kernels.
 The home site for this HOWTO is
 RAID.HOWTO/, where updated versions appear first.  The howto was
 originally written by Jakob OEstergaard based on a large number of
 emails between the author and Ingo Molnar (
 -- one of the RAID developers --, the linux-raid mailing list (linux- and various other people. Emilio Bueso
 ( co-wrote the 1.0 version.
 If you want to use the new-style RAID with 2.0 or 2.2 kernels, you
 should get a patch for your kernel, from The standard 2.2 kernels does not have
 direct support for the new-style RAID described in this HOWTO.
 Therefore these patches are needed.  The old-style RAID support in
 standard 2.0 and 2.2 kernels is buggy and lacks several important
 features present in the new-style RAID software.
 Some of the information in this HOWTO may seem trivial, if you know
 RAID all ready. Just skip those parts.

 1.1.  Disclaimer
 The mandatory disclaimer:
 All information herein is presented "as-is", with no warranties
 expressed nor implied.  If you lose all your data, your job, get hit
 by a truck, whatever, it's not my fault, nor the developers'.  Be
 aware, that you use the RAID software and this information at your own
 risk!  There is no guarantee whatsoever, that any of the software, or
 this information, is in any way correct, nor suited for any use
 whatsoever. Back up all your data before experimenting with this.
 Better safe than sorry.

 1.2.  What is RAID?
 In 1987, the University of California Berkeley, published an article
 entitled A Case for Redundant Arrays of Inexpensive Disks (RAID).
 This article described various types of disk arrays, referred to by
 the acronym RAID. The basic idea of RAID was to combine multiple
 small, independent disk drives into an array of disk drives which
 yields performance exceeding that of a Single Large Expensive Drive
 (SLED). Additionally, this array of drives appears to the computer as
 a single logical storage unit or drive.
 The Mean Time Between Failure (MTBF) of the array will be equal to the
 MTBF of an individual drive, divided by the number of drives in the
 array.  Because of this, the MTBF of an array of drives would be too
 low for many application requirements. However, disk arrays can be
 made fault-tolerant by redundantly storing information in various
 Five types of array architectures, RAID-1 through RAID-5, were defined
 by the Berkeley paper, each providing disk fault-tolerance and each
 offering different trade-offs in features and performance. In addition
 to these five redundant array architectures, it has become popular to
 refer to a non-redundant array of disk drives as a RAID-0 array.
 Today some of the original RAID levels (namely level 2 and 3) are only
 used in very specialized systems (and in fact not even supported by
 the Linux Software RAID drivers). Another level, "linear" has emerged,
 and especially RAID level 0 is often combined with RAID level 1.

 1.3.  Terms
 In this HOWTO the word "RAID" means "Linux Software RAID". This HOWTO
 does not treat any aspects of Hardware RAID. Furthermore, it does not
 treat any aspects of Software RAID in other operating system kernels.
 When describing RAID setups, it is useful to refer to the number of
 disks and their sizes. At all times the letter N is used to denote the
 number of active disks in the array (not counting spare-disks). The
 letter S is the size of the smallest drive in the array, unless
 otherwise mentioned. The letter P is used as the performance of one
 disk in the array, in MB/s. When used, we assume that the disks are
 equally fast, which may not always be true in real-world scenarios.
 Note that the words "device" and "disk" are supposed to mean about the
 same thing.  Usually the devices that are used to build a RAID device
 are partitions on disks, not necessarily entire disks.  But combining
 several partitions on one disk usually does not make sense, so the
 words devices and disks just mean "partitions on different disks".

 1.4.  The RAID levels
 Here's a short description of what is supported in the Linux RAID
 drivers. Some of this information is absolutely basic RAID info, but
 I've added a few notices about what's special in the Linux
 implementation of the levels.  You can safely skip this section if you
 know RAID already.
 The current RAID drivers in Linux supports the following levels:
 o  Linear mode
    o  Two or more disks are combined into one physical device. The
       disks are "appended" to each other, so writing linearly to the
       RAID device will fill up disk 0 first, then disk 1 and so on.
       The disks does not have to be of the same size. In fact, size
       doesn't matter at all here :)
    o  There is no redundancy in this level. If one disk crashes you
       will most probably lose all your data.  You can however be lucky
       to recover some data, since the filesystem will just be missing
       one large consecutive chunk of data.
    o  The read and write performance will not increase for single
       reads/writes. But if several users use the device, you may be
       lucky that one user effectively is using the first disk, and the
       other user is accessing files which happen to reside on the
       second disk. If that happens, you will see a performance gain.
 o  RAID-0
    o  Also called "stripe" mode. The devices should (but need not)
       have the same size. Operations on the array will be split on the
       devices; for example, a large write could be split up as 4 kB to
       disk 0, 4 kB to disk 1, 4 kB to disk 2, then 4 kB to disk 0
       again, and so on. If one device is much larger than the other
       devices, that extra space is still utilized in the RAID device,
       but you will be accessing this larger disk alone, during writes
       in the high end of your RAID device. This of course hurts
    o  Like linear, there is no redundancy in this level either. Unlike
       linear mode, you will not be able to rescue any data if a drive
       fails. If you remove a drive from a RAID-0 set, the RAID device
       will not just miss one consecutive block of data, it will be
       filled with small holes all over the device. e2fsck or other
       filesystem recovery tools will probably not be able to recover
       much from such a device.
    o  The read and write performance will increase, because reads and
       writes are done in parallel on the devices. This is usually the
       main reason for running RAID-0. If the busses to the disks are
       fast enough, you can get very close to N*P MB/sec.
 o  RAID-1
    o  This is the first mode which actually has redundancy. RAID-1 can
       be used on two or more disks with zero or more spare-disks. This
       mode maintains an exact mirror of the information on one disk on
       the other disk(s). Of Course, the disks must be of equal size.
       If one disk is larger than another, your RAID device will be the
       size of the smallest disk.
    o  If up to N-1 disks are removed (or crashes), all data are still
       intact. If there are spare disks available, and if the system
       (eg. SCSI drivers or IDE chipset etc.) survived the crash,
       reconstruction of the mirror will immediately begin on one of
       the spare disks, after detection of the drive fault.
    o  Write performance is often worse than on a single device,
       because identical copies of the data written must be sent to
       every disk in the array. With large RAID-1 arrays this can be a
       real problem, as you may saturate the PCI bus with these extra
       copies. This is in fact one of the very few places where
       Hardware RAID solutions can have an edge over Software solutions
       - if you use a hardware RAID card, the extra write copies of the
       data will not have to go over the PCI bus, since it is the RAID
       controller that will generate the extra copy. Read performance
       is good, especially if you have multiple readers or seek-
       intensive workloads. The RAID code employs a rather good read-
       balancing algorithm, that will simply let the disk whose heads
       are closest to the wanted disk position perform the read
       operation. Since seek operations are relatively expensive on
       modern disks (a seek time of 6 ms equals a read of 123 kB at 20
       MB/sec), picking the disk that will have the shortest seek time
       does actually give a noticeable performance improvement.
 o  RAID-4
    o  This RAID level is not used very often. It can be used on three
       or more disks. Instead of completely mirroring the information,
       it keeps parity information on one drive, and writes data to the
       other disks in a RAID-0 like way.  Because one disk is reserved
       for parity information, the size of the array will be (N-1)*S,
       where S is the size of the smallest drive in the array. As in
       RAID-1, the disks should either be of equal size, or you will
       just have to accept that the S in the (N-1)*S formula above will
       be the size of the smallest drive in the array.
    o  If one drive fails, the parity information can be used to
       reconstruct all data.  If two drives fail, all data is lost.
    o  The reason this level is not more frequently used, is because
       the parity information is kept on one drive. This information
       must be updated every time one of the other disks are written
       to. Thus, the parity disk will become a bottleneck, if it is not
       a lot faster than the other disks.  However, if you just happen
       to have a lot of slow disks and a very fast one, this RAID level
       can be very useful.
 o  RAID-5
    o  This is perhaps the most useful RAID mode when one wishes to
       combine a larger number of physical disks, and still maintain
       some redundancy. RAID-5 can be used on three or more disks, with
       zero or more spare-disks. The resulting RAID-5 device size will
       be (N-1)*S, just like RAID-4. The big difference between RAID-5
       and -4 is, that the parity information is distributed evenly
       among the participating drives, avoiding the bottleneck problem
       in RAID-4.
    o  If one of the disks fail, all data are still intact, thanks to
       the parity information. If spare disks are available,
       reconstruction will begin immediately after the device failure.
       If two disks fail simultaneously, all data are lost. RAID-5 can
       survive one disk failure, but not two or more.
    o  Both read and write performance usually increase, but can be
       hard to predict how much. Reads are similar to RAID-0 reads,
       writes can be either rather expensive (requiring read-in prior
       to write, in order to be able to calculate the correct parity
       information), or similar to RAID-1 writes. The write efficiency
       depends heavily on the amount of memory in the machine, and the
       usage pattern of the array. Heavily scattered writes are bound
       to be more expensive.

 1.5.  Requirements
 This HOWTO assumes you are using Linux 2.4 or later. However, it is
 possible to use Software RAID in late 2.2.x or 2.0.x Linux kernels
 with a matching RAID patch and the 0.90 version of the raidtools. Both
 the patches and the tools can be found at The RAID patch, the raidtools
 package, and the kernel should all match as close as possible. At
 times it can be necessary to use older kernels if raid patches are not
 available for the latest kernel.
 If you use and recent GNU/Linux distribution based on the 2.4 kernel
 or later, your system most likely already has a matching version of
 the raidtools for your kernel.

 2.  Why RAID?
 There can be many good reasons for using RAID. A few are; the ability
 to combine several physical disks into one larger "virtual" device,
 performance improvements, and redundancy.
 It is, however, very important to understand that RAID is not a
 substitute for good backups. Some RAID levels will make your systems
 immune to data loss from single-disk failures, but RAID will not allow
 you to recover from an accidental "rm -rf /". RAID will also not help
 you preserve your data if the server holding the RAID itself is lost
 in one way or the other (theft, flooding, earthquake, Martian invasion
 RAID will generally allow you to keep systems up and running, in case
 of common hardware problems (single disk failure). It is not in itself
 a complete data safety solution. This is very important to realize.

 2.1.  Device and filesystem support
 Linux RAID can work on most block devices. It doesn't matter whether
 you use IDE or SCSI devices, or a mixture. Some people have also used
 the Network Block Device (NBD) with more or less success.
 Since a Linux Software RAID device is itself a block device, the above
 implies that you can actually create a RAID of other RAID devices.
 This in turn makes it possible to support RAID-10 (RAID-0 of multiple
 RAID-1 devices), simply by using the RAID-0 and RAID-1 functionality
 together. Other more exotic configurations, such a RAID-5 over RAID-5
 "matrix" configurations are equally supported.
 The RAID layer has absolutely nothing to do with the filesystem layer.
 You can put any filesystem on a RAID device, just like any other block

 2.2.  Performance
 Often RAID is employed as a solution to performance problems. While
 RAID can indeed often be the solution you are looking for, it is not a
 silver bullet. There can be many reasons for performance problems, and
 RAID is only the solution to a few of them.
 See Chapter one for a mention of the performance characteristics of
 each level.

 2.3.  Swapping on RAID
 There's no reason to use RAID for swap performance reasons. The kernel
 itself can stripe swapping on several devices, if you just give them
 the same priority in the /etc/fstab file.
 A nice /etc/fstab looks like:
 /dev/sda2       swap           swap    defaults,pri=1   0 0
 /dev/sdb2       swap           swap    defaults,pri=1   0 0
 /dev/sdc2       swap           swap    defaults,pri=1   0 0
 /dev/sdd2       swap           swap    defaults,pri=1   0 0
 /dev/sde2       swap           swap    defaults,pri=1   0 0
 /dev/sdf2       swap           swap    defaults,pri=1   0 0
 /dev/sdg2       swap           swap    defaults,pri=1   0 0

 This setup lets the machine swap in parallel on seven SCSI devices. No
 need for RAID, since this has been a kernel feature for a long time.
 Another reason to use RAID for swap is high availability.  If you set
 up a system to boot on eg. a RAID-1 device, the system should be able
 to survive a disk crash. But if the system has been swapping on the
 now faulty device, you will for sure be going down.  Swapping on a
 RAID-1 device would solve this problem.
 There has been a lot of discussion about whether swap was stable on
 RAID devices. This is a continuing debate, because it depends highly
 on other aspects of the kernel as well.  As of this writing, it seems
 that swapping on RAID should be perfectly stable, you should however
 stress-test the system yourself until you are satisfied with the
 You can set up RAID in a swap file on a filesystem on your RAID
 device, or you can set up a RAID device as a swap partition, as you
 see fit.  As usual, the RAID device is just a block device.

 2.4.  Why mdadm?
 The classic raidtools are the standard software RAID management tool
 for Linux, so using mdadm is not a must.
 However, if you find raidtools cumbersome or limited, mdadm (multiple
 devices admin) is an extremely useful tool for running RAID systems.
 It can be used as a replacement for the raidtools, or as a supplement.
 The mdadm tool, written by Neil Brown, a software engineer at the
 University of New South Wales and a kernel developer, is now at
 version 1.4.0 and has proved to be quite stable. There is much
 positive response on the Linux-raid mailing list and mdadm is likely
 to become widespread in the future.
 The main differences between mdadm and raidtools are:

 o  mdadm can diagnose, monitor and gather detailed information about
    your arrays
 o  mdadm is a single centralized program and not a collection of
    disperse programs, so there's a common syntax for every RAID
    management command
 o  mdadm can perform almost all of its functions without having a
    configuration file and does not use one by default
 o  Also, if a configuration file is needed, mdadm will help with
    management of it's contents

 3.  Devices
 Software RAID devices are so-called "block" devices, like ordinary
 disks or disk partitions. A RAID device is "built" from a number of
 other block devices - for example, a RAID-1 could be built from two
 ordinary disks, or from two disk partitions (on separate disks -
 please see the description of RAID-1 for details on this).
 There are no other special requirements to the devices from which you
 build your RAID devices - this gives you a lot of freedom in designing
 your RAID solution. For example, you can build a RAID from a mix of
 IDE and SCSI devices, and you can even build a RAID from other RAID
 devices (this is useful for RAID-0+1, where you simply construct two
 RAID-1 devices from ordinary disks, and finally construct a RAID-0
 device from those two RAID-1 devices).
 Therefore, in the following text, we will use the word "device" as
 meaning "disk", "partition", or even "RAID device". A "device" in the
 following text simply refers to a "Linux block device". It could be
 anything from a SCSI disk to a network block device. We will commonly
 refer to these "devices" simply as "disks", because that is what they
 will be in the common case.
 However, there are several roles that devices can play in your arrays.
 A device could be a "spare disk", it could have failed and thus be a
 "faulty disk", or it could be a normally working and fully functional
 device actively used by the array.
 In the following we describe two special types of devices; namely the
 "spare disks" and the "faulty disks".

 3.1.  Spare disks
 Spare disks are disks that do not take part in the RAID set until one
 of the active disks fail.  When a device failure is detected, that
 device is marked as "bad" and reconstruction is immediately started on
 the first spare-disk available.
 Thus, spare disks add a nice extra safety to especially RAID-5 systems
 that perhaps are hard to get to (physically). One can allow the system
 to run for some time, with a faulty device, since all redundancy is
 preserved by means of the spare disk.
 You cannot be sure that your system will keep running after a disk
 crash though. The RAID layer should handle device failures just fine,
 but SCSI drivers could be broken on error handling, or the IDE chipset
 could lock up, or a lot of other things could happen.
 Also, once reconstruction to a hot-spare begins, the RAID layer will
 start reading from all the other disks to re-create the redundant
 information. If multiple disks have built up bad blocks over time, the
 reconstruction itself can actually trigger a failure on one of the
 "good" disks. This will lead to a complete RAID failure. If you do
 frequent backups of the entire filesystem on the RAID array, then it
 is highly unlikely that you would ever get in this situation - this is
 another very good reason for taking frequent backups. Remember, RAID
 is not a substitute for backups.

 3.2.  Faulty disks
 When the RAID layer handles device failures just fine, crashed disks
 are marked as faulty, and reconstruction is immediately started on the
 first spare-disk available.
 Faulty disks still appear and behave as members of the array. The RAID
 layer just treats crashed devices as inactive parts of the filesystem.

 4.  Hardware issues
 This section will mention some of the hardware concerns involved when
 running software RAID.
 If you are going after high performance, you should make sure that the
 bus(ses) to the drives are fast enough. You should not have 14 UW-SCSI
 drives on one UW bus, if each drive can give 20 MB/s and the bus can
 only sustain 160 MB/s.  Also, you should only have one device per IDE
 bus. Running disks as master/slave is horrible for performance. IDE is
 really bad at accessing more that one drive per bus.  Of Course, all
 newer motherboards have two IDE busses, so you can set up two disks in
 RAID without buying more controllers. Extra IDE controllers are rather
 cheap these days, so setting up 6-8 disk systems with IDE is easy and

 4.1.  IDE Configuration
 It is indeed possible to run RAID over IDE disks. And excellent
 performance can be achieved too. In fact, today's price on IDE drives
 and controllers does make IDE something to be considered, when setting
 up new RAID systems.
 o  Physical stability: IDE drives has traditionally been of lower
    mechanical quality than SCSI drives. Even today, the warranty on
    IDE drives is typically one year, whereas it is often three to five
    years on SCSI drives.  Although it is not fair to say, that IDE
    drives are per definition poorly made, one should be aware that IDE
    drives of some brand may fail more often that similar SCSI drives.
    However, other brands use the exact same mechanical setup for both
    SCSI and IDE drives. It all boils down to: All disks fail, sooner
    or later, and one should be prepared for that.
 o  Data integrity: Earlier, IDE had no way of assuring that the data
    sent onto the IDE bus would be the same as the data actually
    written to the disk. This was due to total lack of parity,
    checksums, etc.  With the Ultra-DMA standard, IDE drives now do a
    checksum on the data they receive, and thus it becomes highly
    unlikely that data get corrupted. The PCI bus however, does not
    have parity or checksum, and that bus is used for both IDE and SCSI
 o  Performance: I am not going to write thoroughly about IDE
    performance here. The really short story is:
    o  IDE drives are fast, although they are not (as of this writing)
       found in 10.000 or 15.000 rpm versions as their SCSI
    o  IDE has more CPU overhead than SCSI (but who cares?)
    o  Only use one IDE drive per IDE bus, slave disks spoil
 o  Fault survival: The IDE driver usually survives a failing IDE
    device. The RAID layer will mark the disk as failed, and if you are
    running RAID levels 1 or above, the machine should work just fine
    until you can take it down for maintenance.
 It is very important, that you only use one IDE disk per IDE bus. Not
 only would two disks ruin the performance, but the failure of a disk
 often guarantees the failure of the bus, and therefore the failure of
 all disks on that bus.  In a fault-tolerant RAID setup (RAID levels
 1,4,5), the failure of one disk can be handled, but the failure of two
 disks (the two disks on the bus that fails due to the failure of the
 one disk) will render the array unusable. Also, when the master drive
 on a bus fails, the slave or the IDE controller may get awfully
 confused. One bus, one drive, that's the rule.
 There are cheap PCI IDE controllers out there. You often get two or
 four busses for around $80. Considering the much lower price of IDE
 disks versus SCSI disks, an IDE disk array can often be a really nice
 solution if one can live with the relatively low number (around 8
 probably) of disks one can attach to a typical system.
 IDE has major cabling problems when it comes to large arrays. Even if
 you had enough PCI slots, it's unlikely that you could fit much more
 than 8 disks in a system and still get it running without data
 corruption caused by too long IDE cables.
 Furthermore, some of the newer IDE drives come with a restriction that
 they are only to be used a given number of hours per day. These drives
 are meant for desktop usage, and it can lead to severe problems if
 these are used in a 24/7 server RAID environment.

 4.2.  Hot Swap
 Although hot swapping of drives is supported to some extent, it is
 still not something one can do easily.

 4.2.1.  Hot-swapping IDE drives
 Don't ! IDE doesn't handle hot swapping at all.  Sure, it may work for
 you, if your IDE driver is compiled as a module (only possible in the
 2.2 series of the kernel), and you re-load it after you've replaced
 the drive.  But you may just as well end up with a fried IDE
 controller, and you'll be looking at a lot more down-time than just
 the time it would have taken to replace the drive on a downed system.
 The main problem, except for the electrical issues that can destroy
 your hardware, is that the IDE bus must be re-scanned after disks are
 swapped. While newer Linux kernels do support re-scan of an IDE bus
 (with the help of the hdparm utility), re-detecting partitions is
 still something that is lacking.  If the new disk is 100% identical to
 the old one (wrt. geometry etc.), it may work, but really, you are
 walking the bleeding edge here.

 4.2.2.  Hot-swapping SCSI drives
 Normal SCSI hardware is not hot-swappable either. It may however work.
 If your SCSI driver supports re-scanning the bus, and removing and
 appending devices, you may be able to hot-swap devices. However, on a
 normal SCSI bus you probably shouldn't unplug devices while your
 system is still powered up. But then again, it may just work (and you
 may end up with fried hardware).
 The SCSI layer should survive if a disk dies, but not all SCSI drivers
 handle this yet. If your SCSI driver dies when a disk goes down, your
 system will go with it, and hot-plug isn't really interesting then.

 4.2.3.  Hot-swapping with SCA
 With SCA, it is possible to hot-plug devices. Unfortunately, this is
 not as simple as it should be, but it is both possible and safe.
 Replace the RAID device, disk device, and host/channel/id/lun numbers
 with the appropriate values in the example below:

 o  Dump the partition table from the drive, if it is still readable:
    sfdisk -d /dev/sdb > partitions.sdb

 o  Remove the drive to replace from the array:
    raidhotremove /dev/md0 /dev/sdb1

 o  Look up the Host, Channel, ID and Lun of the drive to replace, by
    looking in

 o  Remove the drive from the bus:
    echo "scsi remove-single-device 0 0 2 0" > /proc/scsi/scsi

 o  Verify that the drive has been correctly removed, by looking in

 o  Unplug the drive from your SCA bay, and insert a new drive
 o  Add the new drive to the bus:
    echo "scsi add-single-device 0 0 2 0" > /proc/scsi/scsi

 (this should spin up the drive as well)
 o  Re-partition the drive using the previously dumped partition table:

    sfdisk /dev/sdb < partitions.sdb

 o  Add the drive to your array:
    raidhotadd /dev/md0 /dev/sdb2

 The arguments to the "scsi remove-single-device" commands are: Host,
 Channel, Id and Lun. These numbers are found in the "/proc/scsi/scsi"
 The above steps have been tried and tested on a system with IBM SCA
 disks and an Adaptec SCSI controller. If you encounter problems or
 find easier ways to do this, please discuss this on the linux-raid
 mailing list.

 5.  RAID setup

 5.1.  General setup
 This is what you need for any of the RAID levels:
 o  A kernel. Preferably a kernel from the 2.4 series. Alternatively a
    2.0 or 2.2 kernel with the RAID patches applied.
 o  The RAID tools.
 o  Patience, Pizza, and your favorite caffeinated beverage.
 All of this is included as standard in most GNU/Linux distributions
 If your system has RAID support, you should have a file called
 /proc/mdstat.  Remember it, that file is your friend. If you do not
 have that file, maybe your kernel does not have RAID support. See what
 the contains, by doing a cat /proc/mdstat. It should tell you that you
 have the right RAID personality (eg. RAID mode) registered, and that
 no RAID devices are currently active.
 Create the partitions you want to include in your RAID set.

 5.2.  Downloading and installing the RAID tools
 The RAID tools are included in almost every major Linux distribution.
 IMPORTANT: If using Debian Woody (3.0) or later, you can install the
 package by running
 apt-get install raidtools2

 This raidtools2 is a modern version of the old raidtools package,
 which doesn't support the persistent-superblock and parity-algorithm

 5.3.  Downloading and installing mdadm
 You can download the most recent mdadm tarball at  Issue a nice make
 install to compile and then install mdadm and its documentation,
 manual pages and example files.
 tar xvf ./mdadm-1.4.0.tgz
 cd mdadm-1.4.0.tgz
 make install

 If using an RPM-based distribution, you can download and install the
 package file found at
 rpm -ihv mdadm-1.4.0-1.i386.rpm

 If using Debian Woody (3.0) or later, you can install the package by
 apt-get install mdadm

 Gentoo has this package available in the portage tree. There you can
 emerge mdadm

 Other distributions may also have this package available. Now, let's
 go mode-specific.

 5.4.  Linear mode
 Ok, so you have two or more partitions which are not necessarily the
 same size (but of course can be), which you want to append to each
 Set up the /etc/raidtab file to describe your setup. I set up a
 raidtab for two disks in linear mode, and the file looked like this:

 raiddev /dev/md0
         raid-level      linear
         nr-raid-disks   2
         chunk-size      32
         persistent-superblock 1
         device          /dev/sdb6
         raid-disk       0
         device          /dev/sdc5
         raid-disk       1

 Spare-disks are not supported here.  If a disk dies, the array dies
 with it. There's no information to put on a spare disk.
 You're probably wondering why we specify a chunk-size here when linear
 mode just appends the disks into one large array with no parallelism.
 Well, you're completely right, it's odd. Just put in some chunk size
 and don't worry about this any more.

 Ok, let's create the array. Run the command
   mkraid /dev/md0

 This will initialize your array, write the persistent superblocks, and
 start the array.
 If you are using mdadm, a single command like
    mdadm --create --verbose /dev/md0 --level=linear --raid-devices=2 /dev/sdb6 /dev/sdc5

 should create the array. The parameters talk for themselves.  The out-
 put might look like this
   mdadm: chunk size defaults to 64K
   mdadm: array /dev/md0 started.

 Have a look in /proc/mdstat. You should see that the array is running.
 Now, you can create a filesystem, just like you would on any other
 device, mount it, include it in your /etc/fstab and so on.

 5.5.  RAID-0
 You have two or more devices, of approximately the same size, and you
 want to combine their storage capacity and also combine their
 performance by accessing them in parallel.
 Set up the /etc/raidtab file to describe your configuration. An
 example raidtab looks like:
 raiddev /dev/md0
         raid-level      0
         nr-raid-disks   2
         persistent-superblock 1
         chunk-size     4
         device          /dev/sdb6
         raid-disk       0
         device          /dev/sdc5
         raid-disk       1

 Like in Linear mode, spare disks are not supported here either. RAID-0
 has no redundancy, so when a disk dies, the array goes with it.
 Again, you just run
   mkraid /dev/md0

 to initialize the array. This should initialize the superblocks and
 start the raid device.  Have a look in /proc/mdstat to see what's
 going on. You should see that your device is now running.
 /dev/md0 is now ready to be formatted, mounted, used and abused.

 5.6.  RAID-1
 You have two devices of approximately same size, and you want the two
 to be mirrors of each other. Eventually you have more devices, which
 you want to keep as stand-by spare-disks, that will automatically
 become a part of the mirror if one of the active devices break.
 Set up the /etc/raidtab file like this:
 raiddev /dev/md0
         raid-level      1
         nr-raid-disks   2
         nr-spare-disks  0
         persistent-superblock 1
         device          /dev/sdb6
         raid-disk       0
         device          /dev/sdc5
         raid-disk       1

 If you have spare disks, you can add them to the end of the device
 specification like
         device          /dev/sdd5
         spare-disk      0

 Remember to set the nr-spare-disks entry correspondingly.
 Ok, now we're all set to start initializing the RAID. The mirror must
 be constructed, eg. the contents (however unimportant now, since the
 device is still not formatted) of the two devices must be
 Issue the
   mkraid /dev/md0

 command to begin the mirror initialization.
 Check out the /proc/mdstat file. It should tell you that the /dev/md0
 device has been started, that the mirror is being reconstructed, and
 an ETA of the completion of the reconstruction.
 Reconstruction is done using idle I/O bandwidth. So, your system
 should still be fairly responsive, although your disk LEDs should be
 glowing nicely.
 The reconstruction process is transparent, so you can actually use the
 device even though the mirror is currently under reconstruction.
 Try formatting the device, while the reconstruction is running. It
 will work.  Also you can mount it and use it while reconstruction is
 running. Of Course, if the wrong disk breaks while the reconstruction
 is running, you're out of luck.

 5.7.  RAID-4
 Note! I haven't tested this setup myself. The setup below is my best
 guess, not something I have actually had up running. If you use
 RAID-4, please write to the author and share your experiences.

 You have three or more devices of roughly the same size, one device is
 significantly faster than the other devices, and you want to combine
 them all into one larger device, still maintaining some redundancy
 information.  Eventually you have a number of devices you wish to use
 as spare-disks.
 Set up the /etc/raidtab file like this:
 raiddev /dev/md0
         raid-level      4
         nr-raid-disks   4
         nr-spare-disks  0
         persistent-superblock 1
         chunk-size      32
         device          /dev/sdb1
         raid-disk       0
         device          /dev/sdc1
         raid-disk       1
         device          /dev/sdd1
         raid-disk       2
         device          /dev/sde1
         raid-disk       3

 If we had any spare disks, they would be inserted in a similar way,
 following the raid-disk specifications;
         device         /dev/sdf1
         spare-disk     0

 as usual.
 Your array can be initialized with the
    mkraid /dev/md0

 command as usual.
 You should see the section on special options for mke2fs before
 formatting the device.

 5.8.  RAID-5
 You have three or more devices of roughly the same size, you want to
 combine them into a larger device, but still to maintain a degree of
 redundancy for data safety. Eventually you have a number of devices to
 use as spare-disks, that will not take part in the array before
 another device fails.
 If you use N devices where the smallest has size S, the size of the
 entire array will be (N-1)*S. This "missing" space is used for parity
 (redundancy) information.  Thus, if any disk fails, all data stay
 intact. But if two disks fail, all data is lost.
 Set up the /etc/raidtab file like this:

 raiddev /dev/md0
         raid-level      5
         nr-raid-disks   7
         nr-spare-disks  0
         persistent-superblock 1
         parity-algorithm        left-symmetric
         chunk-size      32
         device          /dev/sda3
         raid-disk       0
         device          /dev/sdb1
         raid-disk       1
         device          /dev/sdc1
         raid-disk       2
         device          /dev/sdd1
         raid-disk       3
         device          /dev/sde1
         raid-disk       4
         device          /dev/sdf1
         raid-disk       5
         device          /dev/sdg1
         raid-disk       6

 If we had any spare disks, they would be inserted in a similar way,
 following the raid-disk specifications;
         device         /dev/sdh1
         spare-disk     0

 And so on.
 A chunk size of 32 kB is a good default for many general purpose
 filesystems of this size. The array on which the above raidtab is
 used, is a 7 times 6 GB = 36 GB (remember the (n-1)*s = (7-1)*6 = 36)
 device. It holds an ext2 filesystem with a 4 kB block size.  You could
 go higher with both array chunk-size and filesystem block-size if your
 filesystem is either much larger, or just holds very large files.
 Ok, enough talking. You set up the /etc/raidtab, so let's see if it
 works. Run the
   mkraid /dev/md0

 command, and see what happens.  Hopefully your disks start working
 like mad, as they begin the reconstruction of your array. Have a look
 in /proc/mdstat to see what's going on.
 If the device was successfully created, the reconstruction process has
 now begun.  Your array is not consistent until this reconstruction
 phase has completed. However, the array is fully functional (except
 for the handling of device failures of course), and you can format it
 and use it even while it is reconstructing.
 See the section on special options for mke2fs before formatting the
 Ok, now when you have your RAID device running, you can always stop it
 or re-start it using the
   raidstop /dev/md0

   raidstart /dev/md0

 With mdadm you can stop the device using
   mdadm -S /dev/md0

 and re-start it with
   mdadm -R /dev/md0

 Instead of putting these into init-files and rebooting a zillion times
 to make that work, read on, and get autodetection running.

 5.9.  The Persistent Superblock
 Back in "The Good Old Days" (TM), the raidtools would read your
 /etc/raidtab file, and then initialize the array.  However, this would
 require that the filesystem on which /etc/raidtab resided was mounted.
 This is unfortunate if you want to boot on a RAID.
 Also, the old approach led to complications when mounting filesystems
 on RAID devices. They could not be put in the /etc/fstab file as
 usual, but would have to be mounted from the init-scripts.
 The persistent superblocks solve these problems. When an array is
 initialized with the persistent-superblock option in the /etc/raidtab
 file, a special superblock is written in the beginning of all disks
 participating in the array. This allows the kernel to read the
 configuration of RAID devices directly from the disks involved,
 instead of reading from some configuration file that may not be
 available at all times.
 You should however still maintain a consistent /etc/raidtab file,
 since you may need this file for later reconstruction of the array.
 The persistent superblock is mandatory if you want auto-detection of
 your RAID devices upon system boot. This is described in the
 Autodetection section.

 5.10.  Chunk sizes
 The chunk-size deserves an explanation.  You can never write
 completely parallel to a set of disks. If you had two disks and wanted
 to write a byte, you would have to write four bits on each disk,
 actually, every second bit would go to disk 0 and the others to disk
 1. Hardware just doesn't support that.  Instead, we choose some chunk-
 size, which we define as the smallest "atomic" mass of data that can
 be written to the devices.  A write of 16 kB with a chunk size of 4
 kB, will cause the first and the third 4 kB chunks to be written to
 the first disk, and the second and fourth chunks to be written to the
 second disk, in the RAID-0 case with two disks.  Thus, for large
 writes, you may see lower overhead by having fairly large chunks,
 whereas arrays that are primarily holding small files may benefit more
 from a smaller chunk size.
 Chunk sizes must be specified for all RAID levels, including linear
 mode. However, the chunk-size does not make any difference for linear
 For optimal performance, you should experiment with the value, as well
 as with the block-size of the filesystem you put on the array.
 The argument to the chunk-size option in /etc/raidtab specifies the
 chunk-size in kilobytes. So "4" means "4 kB".

 5.10.1.  RAID-0
 Data is written "almost" in parallel to the disks in the array.
 Actually, chunk-size bytes are written to each disk, serially.
 If you specify a 4 kB chunk size, and write 16 kB to an array of three
 disks, the RAID system will write 4 kB to disks 0, 1 and 2, in
 parallel, then the remaining 4 kB to disk 0.
 A 32 kB chunk-size is a reasonable starting point for most arrays. But
 the optimal value depends very much on the number of drives involved,
 the content of the file system you put on it, and many other factors.
 Experiment with it, to get the best performance.

 5.10.2.  RAID-0 with ext2
 The following tip was contributed by
 There is more disk activity at the beginning of ext2fs block groups.
 On a single disk, that does not matter, but it can hurt RAID0, if all
 block groups happen to begin on the same disk.  Example:
 With 4k stripe size and 4k block size, each block occupies one stripe.
 With two disks, the stripe-#disk-product is 2*4k=8k.  The default
 block group size is 32768 blocks, so all block groups start on disk 0,
 which can easily become a hot spot, thus reducing overall performance.
 Unfortunately, the block group size can only be set in steps of 8
 blocks (32k when using 4k blocks), so you can not avoid the problem by
 adjusting the block group size with the -g option of mkfs(8).
 If you add a disk, the stripe-#disk-product is 12, so the first block
 group starts on disk 0, the second block group starts on disk 2 and
 the third on disk 1.  The load caused by disk activity at the block
 group beginnings spreads over all disks.
 In case you can not add a disk, try a stripe size of 32k.  The
 stripe-#disk-product is 64k.  Since you can change the block group
 size in steps of 8 blocks (32k), using a block group size of 32760
 solves the problem.
 Additionally, the block group boundaries should fall on stripe
 boundaries.  That is no problem in the examples above, but it could
 easily happen with larger stripe sizes.

 5.10.3.  RAID-1
 For writes, the chunk-size doesn't affect the array, since all data
 must be written to all disks no matter what.  For reads however, the
 chunk-size specifies how much data to read serially from the
 participating disks.  Since all active disks in the array contain the
 same information, the RAID layer has complete freedom in choosing from
 which disk information is read - this is used by the RAID code to
 improve average seek times by picking the disk best suited for any
 given read operation.
 5.10.4.  RAID-4
 When a write is done on a RAID-4 array, the parity information must be
 updated on the parity disk as well.
 The chunk-size affects read performance in the same way as in RAID-0,
 since reads from RAID-4 are done in the same way.

 5.10.5.  RAID-5
 On RAID-5, the chunk size has the same meaning for reads as for
 RAID-0. Writing on RAID-5 is a little more complicated: When a chunk
 is written on a RAID-5 array, the corresponding parity chunk must be
 updated as well. Updating a parity chunk requires either
 o  The original chunk, the new chunk, and the old parity block
 o  Or, all chunks (except for the parity chunk) in the stripe
    The RAID code will pick the easiest way to update each parity chunk
    as the write progresses. Naturally, if your server has lots of
    memory and/or if the writes are nice and linear, updating the
    parity chunks will only impose the overhead of one extra write
    going over the bus (just like RAID-1). The parity calculation
    itself is extremely efficient, so while it does of course load the
    main CPU of the system, this impact is negligible. If the writes
    are small and scattered all over the array, the RAID layer will
    almost always need to read in all the untouched chunks from each
    stripe that is written to, in order to calculate the parity chunk.
    This will impose extra bus-overhead and latency due to extra reads.
 A reasonable chunk-size for RAID-5 is 128 kB, but as always, you may
 want to experiment with this.
 Also see the section on special options for mke2fs.  This affects
 RAID-5 performance.

 5.11.  Options for mke2fs
 There is a special option available when formatting RAID-4 or -5
 devices with mke2fs. The -R stride=nn option will allow mke2fs to
 better place different ext2 specific data-structures in an intelligent
 way on the RAID device.
 If the chunk-size is 32 kB, it means, that 32 kB of consecutive data
 will reside on one disk. If we want to build an ext2 filesystem with 4
 kB block-size, we realize that there will be eight filesystem blocks
 in one array chunk. We can pass this information on the mke2fs
 utility, when creating the filesystem:
   mke2fs -b 4096 -R stride=8 /dev/md0

 RAID-{4,5} performance is severely influenced by this option. I am
 unsure how the stride option will affect other RAID levels. If anyone
 has information on this, please send it in my direction.
 The ext2fs blocksize severely influences the performance of the
 filesystem. You should always use 4kB block size on any filesystem
 larger than a few hundred megabytes, unless you store a very large
 number of very small files on it.
 6.  Detecting, querying and testing
 This section is about life with a software RAID system, that's
 communicating with the arrays and tinkertoying them.
 Note that when it comes to md devices manipulation, you should always
 remember that you are working with entire filesystems. So, although
 there could be some redundancy to keep your files alive, you must
 proceed with caution.

 6.1.  Detecting a drive failure
 No mistery here. It's enough with a quick look to the standard log and
 stat files to notice a drive failure.
 It's always a must for /var/log/messages to fill screens with tons of
 error messages, no matter what happened. But, when it's about a disk
 crash, huge lots of kernel errors are reported.  Some nasty examples,
 for the masochists,
    kernel: scsi0 channel 0 : resetting for second half of retries.
    kernel: SCSI bus is being reset for host 0 channel 0.
    kernel: scsi0: Sending Bus Device Reset CCB #2666 to Target 0
    kernel: scsi0: Bus Device Reset CCB #2666 to Target 0 Completed
    kernel: scsi : aborting command due to timeout : pid 2649, scsi0, channel 0, id 0, lun 0 Write (6) 18 33 11 24 00
    kernel: scsi0: Aborting CCB #2669 to Target 0
    kernel: SCSI host 0 channel 0 reset (pid 2644) timed out - trying harder
    kernel: SCSI bus is being reset for host 0 channel 0.
    kernel: scsi0: CCB #2669 to Target 0 Aborted
    kernel: scsi0: Resetting BusLogic BT-958 due to Target 0
    kernel: scsi0: *** BusLogic BT-958 Initialized Successfully ***

 Most often, disk failures look like these,
    kernel: sidisk I/O error: dev 08:01, sector 1590410
    kernel: SCSI disk error : host 0 channel 0 id 0 lun 0 return code = 28000002

 or these
    kernel: hde: read_intr: error=0x10 { SectorIdNotFound }, CHS=31563/14/35, sector=0
    kernel: hde: read_intr: status=0x59 { DriveReady SeekComplete DataRequest Error }

 And, as expected, the classic /proc/mdstat look will also reveal prob-
    Personalities : [linear] [raid0] [raid1] [translucent]
    read_ahead not set
    md7 : active raid1 sdc9[0] sdd5[8] 32000 blocks [2/1] [U_]

 Later on this section we will learn how to monitor RAID with mdadm so
 we can receive alert reports about disk failures. Now it's time to
 learn more about /proc/mdstat interpretation.

 6.2.  Querying the arrays status
 You can always take a look at /proc/mdstat. It won't hurt. Let's learn
 how to read the file. For example,
               Personalities : [raid1]
               read_ahead 1024 sectors
               md5 : active raid1 sdb5[1] sda5[0]
                     4200896 blocks [2/2] [UU]
               md6 : active raid1 sdb6[1] sda6[0]
                     2104384 blocks [2/2] [UU]
               md7 : active raid1 sdb7[1] sda7[0]
                     2104384 blocks [2/2] [UU]
               md2 : active raid1 sdc7[1] sdd8[2] sde5[0]
                     1052160 blocks [2/2] [UU]
               unused devices: none

 To identify the spare devices, first look for the [#/#] value on a
 line.  The first number is the number of a complete raid device as
 defined.  Lets say it is "n".  The raid role numbers [#] following
 each device indicate its role, or function, within the raid set. Any
 device with "n" or higher are spare disks. 0,1,..,n-1 are for the
 working array.
 Also, if you have a failure, the failed device will be marked with (F)
 after the [#]. The spare that replaces this device will be the device
 with the lowest role number n or higher that is not marked (F). Once
 the resync operation is complete, the device's role numbers are
 The order in which the devices appear in the /proc/mdstat output means
 Finally, remember that you can always use raidtools or mdadm to check
 the arrays out.
         mdadm --detail /dev/mdx
         lsraid -a /dev/mdx

 These commands will show spare and failed disks loud and clear.

 6.3.  Simulating a drive failure
 If you plan to use RAID to get fault-tolerance, you may also want to
 test your setup, to see if it really works.  Now, how does one
 simulate a disk failure?
 The short story is, that you can't, except perhaps for putting a fire
 axe thru the drive you want to "simulate" the fault on.  You can never
 know what will happen if a drive dies. It may electrically take the
 bus it is attached to with it, rendering all drives on that bus
 inaccessible. I have never heard of that happening though, but it is
 entirely possible. The drive may also just report a read/write fault
 to the SCSI/IDE layer, which in turn makes the RAID layer handle this
 situation gracefully.  This is fortunately the way things often go.
 Remember, that you must be running RAID-{1,4,5} for your array to be
 able to survive a disk failure.  Linear- or RAID-0 will fail
 completely when a device is missing.

 6.3.1.  Force-fail by hardware
 If you want to simulate a drive failure, you can just plug out the
 drive.  You should do this with the power off. If you are interested
 in testing whether your data can survive with a disk less than the
 usual number, there is no point in being a hot-plug cowboy here. Take
 the system down, unplug the disk, and boot it up again.
 Look in the syslog, and look at /proc/mdstat to see how the RAID is
 doing. Did it work?
 Faulty disks should appear marked with an (F) if you look at
 /proc/mdstat.  Also, users of mdadm should see the device state as
 When you've re-connected the disk again (with the power off, of
 course, remember), you can add the "new" device to the RAID again,
 with the raidhotadd command.

 6.3.2.  Force-fail by software
 Newer versions of raidtools come with a raidsetfaulty command.  By
 using raidsetfaulty you can just simulate a drive failure without
 unplugging things off.
 Just running the command
     raidsetfaulty /dev/md1 /dev/sdc2

 should be enough to fail the disk /dev/sdc2 of the array /dev/md1.  If
 you are using mdadm, just type
     mdadm --manage --set-faulty /dev/md1 /dev/sdc2

 Now things move up and fun appears. First, you should see something
 like the first line of this on your system's log. Something like the
 second line will appear if you have spare disks configured.
     kernel: raid1: Disk failure on sdc2, disabling device.
     kernel: md1: resyncing spare disk sdb7 to replace failed disk

 Checking /proc/mdstat out will show the degraded array. If there was a
 spare disk available, reconstruction should have started.
 Another fresh utility in newest raidtools is lsraid. Try with
     lsraid -a /dev/md1

 users of mdadm can run the command
     mdadm --detail /dev/md1

 and enjoy the view.
 Now you've seen how it goes when a device fails. Let's fix things up.
 First, we will remove the failed disk from the array. Run the command

     raidhotremove /dev/md1 /dev/sdc2

 users of mdadm can run the command
     mdadm /dev/md1 -r /dev/sdc2

 Note that raidhotremove cannot pull a disk out of a running array.
 For obvious reasons, only crashed disks are to be hotremoved from an
 array (running raidstop and unmounting the device won't help).
 Now we have a /dev/md1 which has just lost a device. This could be a
 degraded RAID or perhaps a system in the middle of a reconstruction
 process. We wait until recovery ends before setting things back to
 So the trip ends when we send /dev/sdc2 back home.
     raidhotadd /dev/md1 /dev/sdc2

 As usual, you can use mdadm instead of raidtools. This should be the
     mdadm /dev/md1 -a /dev/sdc2

 As the prodigal son returns to the array, we'll see it becoming an
 active member of /dev/md1 if necessary. If not, it will be marked as
 an spare disk.  That's management made easy.

 6.4.  Simulating data corruption
 RAID (be it hardware- or software-), assumes that if a write to a disk
 doesn't return an error, then the write was successful. Therefore, if
 your disk corrupts data without returning an error, your data will
 become corrupted.  This is of course very unlikely to happen, but it
 is possible, and it would result in a corrupt filesystem.
 RAID cannot and is not supposed to guard against data corruption on
 the media. Therefore, it doesn't make any sense either, to purposely
 corrupt data (using dd for example) on a disk to see how the RAID
 system will handle that.  It is most likely (unless you corrupt the
 RAID superblock) that the RAID layer will never find out about the
 corruption, but your filesystem on the RAID device will be corrupted.
 This is the way things are supposed to work.  RAID is not a guarantee
 for data integrity, it just allows you to keep your data if a disk
 dies (that is, with RAID levels above or equal one, of course).

 6.5.  Monitoring RAID arrays
 You can run mdadm as a daemon by using the follow-monitor mode.  If
 needed, that will make mdadm send email alerts to the system
 administrator when arrays encounter errors or fail. Also, follow mode
 can be used to trigger contingency commands if a disk fails, like
 giving a second chance to a failed disk by removing and reinserting
 it, so a non-fatal failure could be automatically solved.
 Let's see a basic example.  Running
    mdadm --monitor --mail=root@localhost --delay=1800 /dev/md2

 should release a mdadm daemon to monitor /dev/md2.  The delay parame-
 ter means that polling will be done in intervals of 1800 seconds.
 Finally, critical events and fatal errors should be e-mailed to the
 system manager. That's RAID monitoring made easy.
 Finally, the --program or --alert parameters specify the program to be
 run whenever an event is detected.
 Note that the mdadm daemon will never exit once it decides that there
 are arrays to monitor, so it should normally be run in the background.
 Remember that your are running a daemon, not a shell command.
 Using mdadm to monitor a RAID array is simple and effective. However,
 there are fundamental problems with that kind of monitoring - what
 happens, for example, if the mdadm daemon stops?  In order to overcome
 this problem, one should look towards "real" monitoring solutions.
 There is a number of free software, open source, and commercial
 solutions available which can be used for Software RAID monitoring on
 Linux. A search on FreshMeat should return a good number of matches.

 7.  Tweaking, tuning and troubleshooting

 7.1.  raid-level  and raidtab
 Some GNU/Linux distributions, like RedHat 8.0 and possibly others,
 have a bug in their init-scripts, so that they will fail to start up
 RAID arrays on boot, if your /etc/raidtab has spaces or tabs before
 the raid-level keywords.
 The simple workaround for this problem is to make sure that the raid-
 level keyword appears in the very beginning of the lines, without any
 leading spaces of any kind.

 7.2.  Autodetection
 Autodetection allows the RAID devices to be automatically recognized
 by the kernel at boot-time, right after the ordinary partition
 detection is done.
 This requires several things:
 1. You need autodetection support in the kernel. Check this
 2. You must have created the RAID devices using persistent-superblock
 3. The partition-types of the devices used in the RAID must be set to
    0xFD  (use fdisk and set the type to "fd")
 NOTE: Be sure that your RAID is NOT RUNNING before changing the
 partition types.  Use raidstop /dev/md0 to stop the device.
 If you set up 1, 2 and 3 from above, autodetection should be set up.
 Try rebooting.  When the system comes up, cat'ing /proc/mdstat should
 tell you that your RAID is running.
 During boot, you could see messages similar to these:

  Oct 22 00:51:59 malthe kernel: SCSI device sdg: hdwr sector= 512
   bytes. Sectors= 12657717 [6180 MB] [6.2 GB]
  Oct 22 00:51:59 malthe kernel: Partition check:
  Oct 22 00:51:59 malthe kernel:  sda: sda1 sda2 sda3 sda4
  Oct 22 00:51:59 malthe kernel:  sdb: sdb1 sdb2
  Oct 22 00:51:59 malthe kernel:  sdc: sdc1 sdc2
  Oct 22 00:51:59 malthe kernel:  sdd: sdd1 sdd2
  Oct 22 00:51:59 malthe kernel:  sde: sde1 sde2
  Oct 22 00:51:59 malthe kernel:  sdf: sdf1 sdf2
  Oct 22 00:51:59 malthe kernel:  sdg: sdg1 sdg2
  Oct 22 00:51:59 malthe kernel: autodetecting RAID arrays
  Oct 22 00:51:59 malthe kernel: (read) sdb1's sb offset: 6199872
  Oct 22 00:51:59 malthe kernel: bind<sdb1,1>
  Oct 22 00:51:59 malthe kernel: (read) sdc1's sb offset: 6199872
  Oct 22 00:51:59 malthe kernel: bind<sdc1,2>
  Oct 22 00:51:59 malthe kernel: (read) sdd1's sb offset: 6199872
  Oct 22 00:51:59 malthe kernel: bind<sdd1,3>
  Oct 22 00:51:59 malthe kernel: (read) sde1's sb offset: 6199872
  Oct 22 00:51:59 malthe kernel: bind<sde1,4>
  Oct 22 00:51:59 malthe kernel: (read) sdf1's sb offset: 6205376
  Oct 22 00:51:59 malthe kernel: bind<sdf1,5>
  Oct 22 00:51:59 malthe kernel: (read) sdg1's sb offset: 6205376
  Oct 22 00:51:59 malthe kernel: bind<sdg1,6>
  Oct 22 00:51:59 malthe kernel: autorunning md0
  Oct 22 00:51:59 malthe kernel: running: <sdg1><sdf1><sde1><sdd1><sdc1><sdb1>
  Oct 22 00:51:59 malthe kernel: now!
  Oct 22 00:51:59 malthe kernel: md: md0: raid array is not clean --
   starting background reconstruction

 This is output from the autodetection of a RAID-5 array that was not
 cleanly shut down (eg. the machine crashed).  Reconstruction is auto-
 matically initiated.  Mounting this device is perfectly safe, since
 reconstruction is transparent and all data are consistent (it's only
 the parity information that is inconsistent - but that isn't needed
 until a device fails).
 Autostarted devices are also automatically stopped at shutdown.  Don't
 worry about init scripts.  Just use the /dev/md devices as any other
 /dev/sd or /dev/hd devices.
 Yes, it really is that easy.
 You may want to look in your init-scripts for any raidstart/raidstop
 commands. These are often found in the standard RedHat init scripts.
 They are used for old-style RAID, and has no use in new-style RAID
 with autodetection. Just remove the lines, and everything will be just

 7.3.  Booting on RAID
 There are several ways to set up a system that mounts it's root
 filesystem on a RAID device. Some distributions allow for RAID setup
 in the installation process, and this is by far the easiest way to get
 a nicely set up RAID system.
 Newer LILO distributions can handle RAID-1 devices, and thus the
 kernel can be loaded at boot-time from a RAID device. LILO will
 correctly write boot-records on all disks in the array, to allow
 booting even if the primary disk fails.
 If you are using grub instead of LILO, then just start grub and
 configure it to use the second (or third, or fourth...) disk in the
 RAID-1 array you want to boot off as its root device and run setup.
 And that's all.
 For example, on an array consisting of /dev/hda1 and /dev/hdc1 where
 both partitions should be bootable you should just do this:

 grub>device (hd0) /dev/hdc
 grub>root (hd0,0)
 grub>setup (hd0)

 Some users have experienced problems with this, reporting that
 although booting with one drive connected worked, booting with both
 two drives failed.  Nevertheless, running the described procedure with
 both disks fixed the problem, allowing the system to boot from either
 single drive or from the RAID-1
 Another way of ensuring that your system can always boot is, to create
 a boot floppy when all the setup is done. If the disk on which the
 /boot filesystem resides dies, you can always boot from the floppy. On
 RedHat and RedHat derived systems, this can be accomplished with the
 mkbootdisk command.

 7.4.  Root filesystem on RAID
 In order to have a system booting on RAID, the root filesystem (/)
 must be mounted on a RAID device. Two methods for achieving this is
 supplied bellow. The methods below assume that you install on a normal
 partition, and then - when the installation is complete - move the
 contents of your non-RAID root filesystem onto a new RAID device.
 Please not that this is no longer needed in general, as most newer
 GNU/Linux distributions support installation on RAID devices (and
 creation of the RAID devices during the installation process).
 However, you may still want to use the methods below, if you are
 migrating an existing system to RAID.

 7.4.1.  Method 1
 This method assumes you have a spare disk you can install the system
 on, which is not part of the RAID you will be configuring.

 o  First, install a normal system on your extra disk.
 o  Get the kernel you plan on running, get the raid-patches and the
    tools, and make your system boot with this new RAID-aware kernel.
    Make sure that RAID-support is in the kernel, and is not loaded as
 o  Ok, now you should configure and create the RAID you plan to use
    for the root filesystem. This is standard procedure, as described
    elsewhere in this document.
 o  Just to make sure everything's fine, try rebooting the system to
    see if the new RAID comes up on boot. It should.
 o  Put a filesystem on the new array (using mke2fs), and mount it
    under /mnt/newroot
 o  Now, copy the contents of your current root-filesystem (the spare
    disk) to the new root-filesystem (the array). There are lots of
    ways to do this, one of them is
     cd /
     find . -xdev | cpio -pm /mnt/newroot

 another way to copy everything from / to /mnt/newroot could be
   cp -ax / /mnt/newroot

 o  You should modify the /mnt/newroot/etc/fstab file to use the
    correct device (the /dev/md? root device) for the root filesystem.
 o  Now, unmount the current /boot filesystem, and mount the boot
    device on /mnt/newroot/boot instead. This is required for LILO to
    run successfully in the next step.
 o  Update /mnt/newroot/etc/lilo.conf to point to the right devices.
    The boot device must still be a regular disk (non-RAID device), but
    the root device should point to your new RAID. When done, run
     lilo -r /mnt/newroot
    complete with no errors.
 o  Reboot the system, and watch everything come up as expected  :)
 If you're doing this with IDE disks, be sure to tell your BIOS that
 all disks are "auto-detect" types, so that the BIOS will allow your
 machine to boot even when a disk is missing.

 7.4.2.  Method 2
 This method requires that your kernel and raidtools understand the
 failed-disk directive in the /etc/raidtab file - if you are working on
 a really old system this may not be the case, and you will need to
 upgrade your tools and/or kernel first.
 You can only use this method on RAID levels 1 and above, as the method
 uses an array in "degraded mode" which in turn is only possible if the
 RAID level has redundancy. The idea is to install a system on a disk
 which is purposely marked as failed in the RAID, then copy the system
 to the RAID which will be running in degraded mode, and finally making
 the RAID use the no-longer needed "install-disk", zapping the old
 installation but making the RAID run in non-degraded mode.

 o  First, install a normal system on one disk (that will later become
    part of your RAID). It is important that this disk (or partition)
    is not the smallest one. If it is, it will not be possible to add
    it to the RAID later on!
 o  Then, get the kernel, the patches, the tools etc. etc. You know the
    drill. Make your system boot with a new kernel that has the RAID
    support you need, compiled into the kernel.
 o  Now, set up the RAID with your current root-device as the failed-
    disk in the /etc/raidtab file. Don't put the failed-disk as the
    first disk in the raidtab, that will give you problems with
    starting the RAID. Create the RAID, and put a filesystem on it.  If
    using mdadm, you can create a degraded array just by running
    something like mdadm -C /dev/md0 --level raid1 --raid-disks 2
    missing /dev/hdc1, note the missing parameter.
 o  Try rebooting and see if the RAID comes up as it should
 o  Copy the system files, and reconfigure the system to use the RAID
    as root-device, as described in the previous section.
 o  When your system successfully boots from the RAID, you can modify
    the /etc/raidtab file to include the previously failed-disk as a
    normal raid-disk. Now, raidhotadd the disk to your RAID.
 o  You should now have a system that can boot from a non-degraded

 7.5.  Making the system boot on RAID
 For the kernel to be able to mount the root filesystem, all support
 for the device on which the root filesystem resides, must be present
 in the kernel. Therefore, in order to mount the root filesystem on a
 RAID device, the kernel must have RAID support.
 The normal way of ensuring that the kernel can see the RAID device is
 to simply compile a kernel with all necessary RAID support compiled
 in. Make sure that you compile the RAID support into the kernel, and
 not as loadable modules. The kernel cannot load a module (from the
 root filesystem) before the root filesystem is mounted.
 However, since RedHat-6.0 ships with a kernel that has new-style RAID
 support as modules, I here describe how one can use the standard
 RedHat-6.0 kernel and still have the system boot on RAID.

 7.5.1.  Booting with RAID as module
 You will have to instruct LILO to use a RAM-disk in order to achieve
 this. Use the mkinitrd command to create a ramdisk containing all
 kernel modules needed to mount the root partition. This can be done
  mkinitrd --with=<module> <ramdisk name> <kernel>

 For example:
  mkinitrd --preload raid5 --with=raid5 raid-ramdisk 2.2.5-22

 This will ensure that the specified RAID module is present at boot-
 time, for the kernel to use when mounting the root device.

 7.5.2.  Modular RAID on Debian GNU/Linux after move to RAID
 Debian users may encounter problems using an initrd to mount their
 root filesystem from RAID, if they have migrated a standard non-RAID
 Debian install to root on RAID.
 If your system fails to mount the root filesystem on boot (you will
 see this in a "kernel panic" message), then the problem may be that
 the initrd filesystem does not have the necessary support to mount the
 root filesystem from RAID.
 Debian seems to produce its initrd.img files on the assumption that
 the root filesystem to be mounted is the current one.  This will
 usually result in a kernel panic if the root filesystem is moved to
 the raid device and you attempt to boot from that device using the
 same initrd image.  The solution is to use the mkinitrd command but
 specifying the proposed new root filesystem.  For example, the
 following commands should create and set up the new initrd on a Debian
 % mkinitrd -r /dev/md0 -o /boot/initrd.img-2.4.22raid
 % mv /initrd.img /initrd.img-nonraid
 % ln -s /boot/initrd.img-raid /initrd.img"

 7.6.  Converting a non-RAID RedHat System to run on Software RAID
 This section was written and contributed by Mark Price, IBM. The text
 has undergone minor changes since his original work.
 Notice: the following information is provided "AS IS" with no
 representation or warranty of any kind either express or implied. You
 may use it freely at your own risk, and no one else will be liable for
 any damages arising out of such usage.

 7.6.1.  Introduction
 The technote details how to convert a linux system with non RAID
 devices to run with a Software RAID configuration.

 7.6.2.  Scope
 This scenario was tested with Redhat 7.1, but should be applicable to
 any release which supports Software RAID (md) devices.

 7.6.3.  Pre-conversion example system
 The test system contains two SCSI disks, sda and sdb both of of which
 are the same physical size. As part of the test setup, I configured
 both disks to have the same partition layout, using fdisk to ensure
 the number of blocks for each partition was identical.
 /dev/sda1   /           2048MB      /dev/sdb1               2048MB
 /dev/sda2   /boot       80MB        /dev/sdb2               80MB
 /dev/sda3   /var/       100MB       /dev/sdb3               100MB
 /dev/sda4   SWAP        1024MB      /dev/sdb4   SWAP        1024MB

 In our basic example, we are going to set up a simple RAID-1 Mirror,
 which requires only two physical disks.

 7.6.4.  Step-1 - boot rescue cd/floppy
 The redhat installation CD provides a rescue mode which boots into
 linux from the CD and mounts any filesystems it can find on your
 At the lilo prompt type
     lilo: linux rescue

 With the setup described above, the installer may ask you which disk
 your root filesystem in on, either sda or sdb. Select sda.
 The installer will mount your filesytems in the following way.
 /dev/sda1   /           /mnt/sysimage
 /dev/sda2   /boot       /mnt/sysimage/boot
 /dev/sda3   /var        /mnt/sysimage/var
 /dev/sda6   /home       /mnt/sysimage/home

 Note: - Please bear in mind other distributions may mount your
 filesystems on different mount points, or may require you to mount
 them by hand.

 7.6.5.  Step-2 - create a /etc/raidtab  file
 Create the file /mnt/sysimage/etc/raidtab (or wherever your real /etc
 file system has been mounted.
 For our test system, the raidtab file would like like this.
 raiddev /dev/md0
     raid-level              1
     nr-raid-disks           2
     nr-spare-disks          0
     chunk-size              4
     persistent-superblock   1
     device                  /dev/sda1
     raid-disk               0
     device                  /dev/sdb1
     raid-disk               1
 raiddev /dev/md1
     raid-level              1
     nr-raid-disks           2
     nr-spare-disks          0
     chunk-size              4
     persistent-superblock   1
     device                  /dev/sda2
     raid-disk               0
     device                  /dev/sdb2
     raid-disk               1
 raiddev /dev/md2
     raid-level              1
     nr-raid-disks           2
     nr-spare-disks          0
     chunk-size              4
     persistent-superblock   1
     device                  /dev/sda3
     raid-disk               0
     device                  /dev/sdb3
     raid-disk               1

 Note: - It is important that the devices are in the correct order. ie.
 that /dev/sda1 is raid-disk 0 and not raid-disk 1. This instructs the
 md driver to sync from /dev/sda1, if it were the other way around it
 would sync from /dev/sdb1 which would destroy your filesystem.

 Now copy the raidtab file from your real root filesystem to the
 current root filesystem.
 (rescue)# cp /mnt/sysimage/etc/raidtab /etc/raidtab

 7.6.6.  Step-3 - create the md devices
 There are two ways to do this, copy the device files from
 /mnt/sysimage/dev or use mknod to create them. The md device, is a
 (b)lock device with major number 9.
 (rescue)# mknod /dev/md0 b 9 0
 (rescue)# mknod /dev/md1 b 9 1
 (rescue)# mknod /dev/md2 b 9 2

 7.6.7.  Step-4 - unmount filesystems
 In order to start the raid devices, and sync the drives, it is
 necessary to unmount all the temporary filesystems.
 (rescue)# umount /mnt/sysimage/var
 (rescue)# umount /mnt/sysimage/boot
 (rescue)# umount /mnt/sysimage/proc
 (rescue)# umount /mnt/sysimage

 Please note, you may not be able to umount /mnt/sysimage. This problem
 can be caused by the rescue system - if you choose to manually mount
 your filesystems instead of letting the rescue system do this automat-
 ically, this problem should go away.

 7.6.8.  Step-5 - start raid devices
 Because there are filesystems on /dev/sda1, /dev/sda2 and /dev/sda3 it
 is necessary to force the start of the raid device.
 (rescue)# mkraid --really-force /dev/md2

 You can check the completion progress by cat'ing the /proc/mdstat
 file. It shows you status of the raid device and percentage left to
 Continue with /boot and /
 (rescue)# mkraid --really-force /dev/md1
 (rescue)# mkraid --really-force /dev/md0

 The md driver syncs one device at a time.

 7.6.9.  Step-6 - remount filesystems
 Mount the newly synced filesystems back into the /mnt/sysimage mount
 (rescue)# mount /dev/md0 /mnt/sysimage
 (rescue)# mount /dev/md1 /mnt/sysimage/boot
 (rescue)# mount /dev/md2 /mnt/sysimage/var

 7.6.10.  Step-7 - change root
 You now need to change your current root directory to your real root
 file system.
 (rescue)# chroot /mnt/sysimage

 7.6.11.  Step-8 - edit config files
 You need to configure lilo and /etc/fstab appropriately to boot from
 and mount the md devices.
 Note: - The boot device MUST be a non-raided device. The root device
 is your new md0 device.  eg.

 Alter /etc/fstab
 /dev/md0               /                       ext3    defaults        1 1
 /dev/md1               /boot                   ext3    defaults        1 2
 /dev/md2               /var                    ext3    defaults        1 2
 /dev/sda4              swap                    swap    defaults        0 0

 7.6.12.  Step-9 - run LILO
 With the /etc/lilo.conf edited to reflect the new root=/dev/md0 and
 with /dev/md1 mounted as /boot, we can now run /sbin/lilo -v on the
 chrooted filesystem.

 7.6.13.  Step-10 - change partition types
 The partition types of the all the partitions on ALL Drives which are
 used by the md driver must be changed to type 0xFD.
 Use fdisk to change the partition type, using option 't'.

 (rescue)# fdisk /dev/sda
 (rescue)# fdisk /dev/sdb

 Use the 'w' option after changing all the required partitions to save
 the partion table to disk.

 7.6.14.  Step-11 - resize filesystem
 When we created the raid device, the physical partion became slightly
 smaller because a second superblock is stored at the end of the
 partition. If you reboot the system now, the reboot will fail with an
 error indicating the superblock is corrupt.
 Resize them prior to the reboot, ensure that the all md based
 filesystems are unmounted except root, and remount root read-only.
 (rescue)# mount / -o remount,ro

 You will be required to fsck each of the md devices. This is the
 reason for remounting root read-only. The -f flag is required to force
 fsck to check a clean filesystem.
 (rescue)# e2fsck -f /dev/md0

 This will generate the same error about inconsistent sizes and
 possibly corrupted superblock.Say N to 'Abort?'.
 (rescue)# resize2fs /dev/md0

 Repeat for all /dev/md devices.

 7.6.15.  Step-12 - checklist
 The next step is to reboot the system, prior to doing this run through
 the checklist below and ensure all tasks have been completed.
 o  All devices have finished syncing. Check /proc/mdstat
 o  /etc/fstab has been edited to reflect the changes to the device
 o  /etc/lilo.conf has beeb edited to reflect root device change.
 o  /sbin/lilo has been run to update the boot loader.
 o  The kernel has both SCSI and RAID(MD) drivers built into the
 o  The partition types of all partitions on disks that are part of an
    md device have been changed to 0xfd.
 o  The filesystems have been fsck'd and resize2fs'd.

 7.6.16.  Step-13 - reboot
 You can now safely reboot the system, when the system comes up it will
 auto discover the md devices (based on the partition types).
 Your root filesystem will now be mirrored.

 7.7.  Sharing spare disks between different arrays
 When running mdadm in the follow/monitor mode you can make different
 arrays share spare disks. That will surely make you save storage space
 without losing the comfort of fallback disks.
 In the world of software RAID, this is a brand new never-seen-before
 feature: for securing things to the point of spare disk areas, you
 just have to provide one single idle disk for a bunch of arrays.
 With mdadm is running as a daemon, you have an agent polling arrays at
 regular intervals. Then, as a disk fails on an array without a spare
 disk, mdadm removes an available spare disk from another array and
 inserts it into the array with the failed disk. The reconstruction
 process begins now in the degraded array as usual.
 To declare shared spare disks, just use the spare-group parameter when
 invoking mdadm as a daemon.

 7.8.  Pitfalls
 Never NEVER never re-partition disks that are part of a running RAID.
 If you must alter the partition table on a disk which is a part of a
 RAID, stop the array first, then repartition.
 It is easy to put too many disks on a bus. A normal Fast-Wide SCSI bus
 can sustain 10 MB/s which is less than many disks can do alone today.
 Putting six such disks on the bus will of course not give you the
 expected performance boost. It is becoming equally easy to saturate
 the PCI bus - remember, a normal 32-bit 33 MHz PCI bus has a
 theoretical maximum bandwidth of around 133 MB/sec, considering
 command overhead etc. you will see a somewhat lower real-world
 transfer rate. Some disks today has a throughput in excess of 30
 MB/sec, so just four of those disks will actually max out your PCI
 bus! When designing high-performance RAID systems, be sure to take the
 whole I/O path into consideration - there are boards with more PCI
 busses, with 64-bit and 66 MHz busses, and with PCI-X.
 More SCSI controllers will only give you extra performance, if the
 SCSI busses are nearly maxed out by the disks on them.  You will not
 see a performance improvement from using two 2940s with two old SCSI
 disks, instead of just running the two disks on one controller.
 If you forget the persistent-superblock option, your array may not
 start up willingly after it has been stopped.  Just re-create the
 array with the option set correctly in the raidtab. Please note that
 this will destroy the information on the array!
 If a RAID-5 fails to reconstruct after a disk was removed and re-
 inserted, this may be because of the ordering of the devices in the
 raidtab. Try moving the first "device ..." and "raid-disk ..." pair to
 the bottom of the array description in the raidtab file.

 8.  Reconstruction
 If you have read the rest of this HOWTO, you should already have a
 pretty good idea about what reconstruction of a degraded RAID
 involves. Let us summarize:
 o  Power down the system
 o  Replace the failed disk
 o  Power up the system once again.
 o  Use raidhotadd /dev/mdX /dev/sdX to re-insert the disk in the array
 o  Have coffee while you watch the automatic reconstruction running
    And that's it.
 Well, it usually is, unless you're unlucky and your RAID has been
 rendered unusable because more disks than the ones redundant failed.
 This can actually happen if a number of disks reside on the same bus,
 and one disk takes the bus with it as it crashes. The other disks,
 however fine, will be unreachable to the RAID layer, because the bus
 is down, and they will be marked as faulty.  On a RAID-5 where you can
 spare one disk only, loosing two or more disks can be fatal.
 The following section is the explanation that Martin Bene gave to me,
 and describes a possible recovery from the scary scenario outlined
 above. It involves using the failed-disk directive in your
 /etc/raidtab (so for people running patched 2.2 kernels, this will
 only work on kernels 2.2.10 and later).

 8.1.  Recovery from a multiple disk failure
 The scenario is:
 o  A controller dies and takes two disks offline at the same time,
 o  All disks on one scsi bus can no longer be reached if a disk dies,
 o  A cable comes loose...
    In short: quite often you get a temporary failure of several disks
    at once; afterwards the RAID superblocks are out of sync and you
    can no longer init your RAID array.
 If using mdadm, you could first try to run:
   mdadm --assemble --force

 If not, there's one thing left: rewrite the RAID superblocks by mkraid
 To get this to work, you'll need to have an up to date /etc/raidtab -
 if it doesn't EXACTLY match devices and ordering of the original disks
 this will not work as expected, but will most likely completely
 obliterate whatever data you used to have on your disks.
 Look at the sylog produced by trying to start the array, you'll see
 the event count for each superblock; usually it's best to leave out
 the disk with the lowest event count, i.e the oldest one.
 If you mkraid without failed-disk, the recovery thread will kick in
 immediately and start rebuilding the parity blocks - not necessarily
 what you want at that moment.
 With failed-disk you can specify exactly which disks you want to be
 active and perhaps try different combinations for best results. BTW,
 only mount the filesystem read-only while trying this out... This has
 been successfully used by at least two guys I've been in contact with.

 9.  Performance
 This section contains a number of benchmarks from a real-world system
 using software RAID. There is some general information about
 benchmarking software too.
 Benchmark samples were done with the bonnie program, and at all times
 on files twice- or more the size of the physical RAM in the machine.
 The benchmarks here only measures input and output bandwidth on one
 large single file.  This is a nice thing to know, if it's maximum I/O
 throughput for large reads/writes one is interested in.  However, such
 numbers tell us little about what the performance would be if the
 array was used for a news spool, a web-server, etc. etc.  Always keep
 in mind, that benchmarks numbers are the result of running a
 "synthetic" program. Few real-world programs do what bonnie does, and
 although these I/O numbers are nice to look at, they are not ultimate
 real-world-appliance performance indicators. Not even close.
 For now, I only have results from my own machine. The setup is:
 o  Dual Pentium Pro 150 MHz
 o  256 MB RAM (60 MHz EDO)
 o  Three IBM UltraStar 9ES 4.5 GB, SCSI U2W
 o  Adaptec 2940U2W
 o  One IBM UltraStar 9ES 4.5 GB, SCSI UW
 o  Adaptec 2940 UW
 o  Kernel 2.2.7 with RAID patches
 The three U2W disks hang off the U2W controller, and the UW disk off
 the UW controller.
 It seems to be impossible to push much more than 30 MB/s thru the SCSI
 busses on this system, using RAID or not. My guess is, that because
 the system is fairly old, the memory bandwidth sucks, and thus limits
 what can be sent thru the SCSI controllers.

 9.1.  RAID-0
 Read is Sequential block input, and Write is Sequential block output.
 File size was 1GB in all tests. The tests where done in single-user
 mode. The SCSI driver was configured not to use tagged command

 >From this it seems that the RAID chunk-size doesn't make that much of
 a difference. However, the ext2fs block-size should be as large as
 possible, which is 4kB (eg. the page size) on IA-32.
       |            |              |             |              |
       |Chunk size  |  Block size  |  Read kB/s  |  Write kB/s  |
       |            |              |             |              |
       |4k          |  1k          |  19712      |  18035       |
       |4k          |  4k          |  34048      |  27061       |
       |8k          |  1k          |  19301      |  18091       |
       |8k          |  4k          |  33920      |  27118       |
       |16k         |  1k          |  19330      |  18179       |
       |16k         |  2k          |  28161      |  23682       |
       |16k         |  4k          |  33990      |  27229       |
       |32k         |  1k          |  19251      |  18194       |
       |32k         |  4k          |  34071      |  26976       |
 9.2.  RAID-0 with TCQ
 This time, the SCSI driver was configured to use tagged command
 queuing, with a queue depth of 8. Otherwise, everything's the same as
       |            |              |             |              |
       |Chunk size  |  Block size  |  Read kB/s  |  Write kB/s  |
       |            |              |             |              |
       |32k         |  4k          |  33617      |  27215       |

 No more tests where done. TCQ seemed to slightly increase write
 performance, but there really wasn't much of a difference at all.

 9.3.  RAID-5
 The array was configured to run in RAID-5 mode, and similar tests
 where done.
       |            |              |             |              |
       |Chunk size  |  Block size  |  Read kB/s  |  Write kB/s  |
       |            |              |             |              |
       |8k          |  1k          |  11090      |  6874        |
       |8k          |  4k          |  13474      |  12229       |
       |32k         |  1k          |  11442      |  8291        |
       |32k         |  2k          |  16089      |  10926       |
       |32k         |  4k          |  18724      |  12627       |

 Now, both the chunk-size and the block-size seems to actually make a

 9.4.  RAID-10
 RAID-10 is "mirrored stripes", or, a RAID-1 array of two RAID-0
 arrays. The chunk-size is the chunk sizes of both the RAID-1 array and
 the two RAID-0 arrays. I did not do test where those chunk-sizes
 differ, although that should be a perfectly valid setup.
       |            |              |             |              |
       |Chunk size  |  Block size  |  Read kB/s  |  Write kB/s  |
       |            |              |             |              |
       |32k         |  1k          |  13753      |  11580       |
       |32k         |  4k          |  23432      |  22249       |

 No more tests where done. The file size was 900MB, because the four
 partitions involved where 500 MB each, which doesn't give room for a
 1G file in this setup (RAID-1 on two 1000MB arrays).
 9.5.  Fresh benchmarking tools
 To check out speed and performance of your RAID systems, do NOT use
 hdparm.  It won't do real benchmarking of the arrays.
 Instead of hdparm, take a look at the tools described here: IOzone and
 IOzone is a small, versatile and modern tool to use. It benchmarks
 file I/O performance for read, write, re-read, re-write, read
 backwards, read strided, fread, fwrite, random read, pread, mmap,
 aio_read and aio_write operations.  Don't worry, it can run on any of
 the ext2, ext3, reiserfs, JFS, or XFS filesystems in OSDL STP.
 You can also use IOzone to show throughput performance as a function
 of number of processes and number of disks used in a filesystem,
 something interesting when it's about RAID striping.
 Although documentation for IOzone is available in Acrobat/PDF,
 PostScript, nroff, and MS Word formats, we are going to cover here a
 nice example of IOzone in action:
 iozone -s 4096

 This would run a test using a 4096KB file size.
 And this is an example of the output quality IOzone gives
         File size set to 4096 KB
         Output is in Kbytes/sec
         Time Resolution = 0.000001 seconds.
         Processor cache size set to 1024 Kbytes.
         Processor cache line size set to 32 bytes.
         File stride size set to 17 * record size.
                                                             random  random    bkwd  record  stride
               KB  reclen   write rewrite    read    reread    read   write    read rewrite    read   fwrite frewrite   fread  freread
             4096       4   99028  194722   285873   298063  265560  170737  398600  436346  380952    91651   127212  288309   292633

 Now you just need to know about the feature that makes IOzone useful
 for RAID benchmarking: the file operations involving RAID are the read
 strided.  The example above shows a 380.952Kb/sec. for the read
 strided, so you can go figure.

 Bonnie++ seems to be more targeted at benchmarking single drives that
 at RAID, but it can test more than 2Gb of storage on 32-bit machines,
 and tests for file creat, stat, unlink operations.

 10.  Related tools
 While not described in this HOWTO, some useful tools for Software-RAID
 systems have been developed.

 10.1.  RAID resizing and conversion
 It is not easy to add another disk to an existing array. A tool to
 allow for just this operation has been developed, and is available
 from The tool will allow for
 conversion between RAID levels, for example converting a two-disk
 RAID-1 array into a four-disk RAID-5 array.  It will also allow for
 chunk-size conversion, and simple disk adding.
 Please note that this tool is not really "production ready". It seems
 to have worked well so far, but it is a rather time-consuming process
 that, if it fails, will absolutely guarantee that your data will be
 irrecoverably scattered over your disks. You absolutely must keep good
 backups prior to experimenting with this tool.

 10.2.  Backup
 Remember, RAID is no substitute for good backups. No amount of
 redundancy in your RAID configuration is going to let you recover week
 or month old data, nor will a RAID survive fires, earthquakes, or
 other disasters.
 It is imperative that you protect your data, not just with RAID, but
 with regular good backups. One excellent system for such backups, is
 the Amanda backup system.

 11.  Partitioning RAID / LVM on RAID
 RAID devices cannot be partitioned, like ordinary disks can. This can
 be a real annoyance on systems where one wants to run, for example,
 two disks in a RAID-1, but divide the system onto multiple different
 filesystems. A horror example could look like:
 # df -h
 Filesystem            Size  Used Avail Use% Mounted on
 /dev/md2              3.8G  640M  3.0G  18% /
 /dev/md1               97M   11M   81M  12% /boot
 /dev/md5              3.8G  1.1G  2.5G  30% /usr
 /dev/md6              9.6G  8.5G  722M  93% /var/www
 /dev/md7              3.8G  951M  2.7G  26% /var/lib
 /dev/md8              3.8G   38M  3.6G   1% /var/spool
 /dev/md9              1.9G  231M  1.5G  13% /tmp
 /dev/md10             8.7G  329M  7.9G   4% /var/www/html

 11.1.  Partitioning RAID devices
 If a RAID device could be partitioned, the administrator could simply
 have created one single /dev/md0 device device, partitioned it as he
 usually would, and put the filesystems there. Instead, with today's
 Software RAID, he must create a RAID-1 device for every single
 filesystem, even though there are only two disks in the system.
 There have been various patches to the kernel which would allow
 partitioning of RAID devices, but none of them have (as of this
 writing) made it into the kernel. In short; it is not currently
 possible to partition a RAID device - but luckily there is another
 solution to this problem.

 11.2.  LVM on RAID
 The solution to the partitioning problem is LVM, Logical Volume
 Management. LVM has been in the stable Linux kernel series for a long
 time now - LVM2 in the 2.6 kernel series is a further improvement over
 the older LVM support from the 2.4 kernel series. While LVM has
 traditionally scared some people away because of its complexity, it
 really is something that an administrator could and should consider if
 he wishes to use more than a few filesystems on a server.
 We will not attempt to describe LVM setup in this HOWTO, as there
 already is a fine HOWTO for exactly this purpose. A small example of a
 RAID + LVM setup will be presented though. Consider the df output
 below, of such a system:
 # df -h
 Filesystem            Size  Used Avail Use% Mounted on
 /dev/md0              942M  419M  475M  47% /
 /dev/vg0/backup        40G  1.3M   39G   1% /backup
 /dev/vg0/amdata       496M  237M  233M  51% /var/lib/amanda
 /dev/vg0/mirror        62G   56G  2.9G  96% /mnt/mirror
 /dev/vg0/webroot       97M  6.5M   85M   8% /var/www
 /dev/vg0/local        2.0G  458M  1.4G  24% /usr/local
 /dev/vg0/netswap      3.0G  2.1G 1019M  67% /mnt/netswap

 "What's the difference" you might ask... Well, this system has only
 two RAID-1 devices - one for the root filesystem, and one that cannot
 be seen on the df output - this is because /dev/md1 is used as a
 "physical volume" for LVM. What this means is, that /dev/md1 acts as
 "backing store" for all "volumes" in the "volume group" named vg0.
 All this "volume" terminology is explained in the LVM HOWTO - if you
 do not completely understand the above, there is no need to worry -
 the details are not particularly important right now (you will need to
 read the LVM HOWTO anyway if you want to set up LVM). What matters is
 the benefits that this setup has over the many-md-devices setup:
 o  No need to reboot just to add a new filesystem (this would
    otherwise be required, as the kernel cannot re-read the partition
    table from the disk that holds the root filesystem, and re-
    partitioning would be required in order to create the new RAID
    device to hold the new filesystem)
 o  Resizing of filesystems: LVM supports hot-resizing of volumes (with
    RAID devices resizing is difficult and time consuming - but if you
    run LVM on top of RAID, all you need in order to resize a
    filesystem is to resize the volume, not the underlying RAID
    device). With a filesystem such as XFS, you can even resize the
    filesystem without un-mounting it first (!) Ext3 does not (as of
    this writing) support hot-resizing, you can, however, resize the
    filesystem without rebooting, you just need to un-mount it first.
 o  Adding new disks: Need more storage?  Easy! Simply insert two new
    disks in your system, create a RAID-1 on top of them, make your new
    /dev/md2 device a physical volume and add it to your volume group.
    That's it! You now have more free space in your volume group for
    either growing your existing logical volumes, or for adding new
 All in all - for servers with many filesystems, LVM (and LVM2) is
 definitely a fairly simple solution which should be considered for use
 on top of Software RAID. Read on in the LVM HOWTO if you want to learn
 more about LVM.

 12.  Credits
 The following people contributed to the creation of this

 o  Mark Price and IBM
 o  Steve Boley of Dell
 o  Damon Hoggett
 o  Ingo Molnar
 o  Jim Warren
 o  Louis Mandelstam
 o  Allan Noah
 o  Yasunori Taniike
 o  Martin Bene
 o  Bennett Todd
 o  Kevin Rolfes
 o  Darryl Barlow
 o  Brandon Knitter
 o  Hans van Zijst
 o  Matthew Mcglynn
 o  Jimmy Hedman
 o  Tony den Haan
 o  The Linux-RAID mailing list people
 o  The ones I forgot, sorry :)
 Please submit corrections, suggestions etc. to the author. It's the
 only way this HOWTO can improve.

 13.  Changelog
 13.1.  Version 1.1

 o  New sub-section: "Downloading and installing the RAID tools"
 o  Grub support at section "Booting on RAID"
 o  Mention LVM on top of RAID
 o  Other minor corrections.
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