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| This section will mention some of the hardware concerns involved when | | This section will mention some of the hardware concerns involved when |
Revision as of 08:14, 23 July 2008
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
channel. Running disks as master/slave is horrible for performance. IDE is
really bad at accessing more that one drive per channel. Most
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
affordable.
Also most newer motherboards have one or more controllers for SATA disks.
SATA disks do not connect more than one disk on one channel so the problems
mentined aboce with multiple IO for 2 disks on one channel is not
relevant for SATA disks.
See also the section on bottlenecks.
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.
- 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.
- 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 systems.
- Performance: I am not going to write thoroughly about IDE performance here. The really short story is:
- IDE drives are fast, although they are not (as of this writing) found in 10.000 or 15.000 rpm versions as their SCSI counterparts
- IDE has more CPU overhead than SCSI (but who cares?)
- Only use one IDE drive per IDE bus, slave disks spoil performance
- 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.
Hot Swap
Although hot swapping of drives is supported to some extent, it is
still not something one can do easily.
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.
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.
Hot-swapping with SATA
SATA supports hot swapping a drive but the linux kernel is not quite there yet.
see http://linux-ata.org/driver-status.html for more information on SATA hotpug status.
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:
- Dump the partition table from the drive, if it is still readable:
sfdisk -d /dev/sdb > partitions.sdb
- Mark faulty and remove the drive to replace from the array:
mdadm -f /dev/md0 /dev/sdb1
mdadm -r /dev/md0 /dev/sdb1
- Look up the Host, Channel, ID and Lun of the drive to replace, by looking in
/proc/scsi/scsi
- Remove the drive from the bus:
echo "scsi remove-single-device 0 0 2 0" > /proc/scsi/scsi
- Verify that the drive has been correctly removed, by looking in
/proc/scsi/scsi
- Unplug the drive from your SCA bay, and insert a new drive
- 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)
- Re-partition the drive using the previously dumped partition table:
sfdisk /dev/sdb < partitions.sdb
- Add the drive to your array:
mdadm -a /dev/md0 /dev/sdb1
The arguments to the "scsi remove-single-device" commands are: Host,
Channel, Id and Lun. These numbers are found in the "/proc/scsi/scsi"
file.
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.