Introduction
Contents |
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 http://unthought.net/Software- 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 (mingo@chiara.csoma.elte.hu) -- one of the RAID developers --, the linux-raid mailing list (linux- raid@vger.kernel.org) and various other people. Emilio Bueso (bueso@vives.org) 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 http://people.redhat.com/mingo/ 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.
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.
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 ways.
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.
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".
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
performance.
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.
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 http://people.redhat.com/mingo/. 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.
