[Beowulf] Software RAID?
landman at scalableinformatics.com
Wed Nov 21 19:52:03 PST 2007
Ekechi Nwokah wrote:
> Does anyone know of any software RAID solutions that come close to the
> performance of a commodity RAID card such as LSI/3ware/Areca for
> direct-attached drives?
For small numbers of drives, yes, the MD driver is superb with two
(well, really three) caveats.
First: No hot swap. You can do a kind-of-cold swap (have to take the
mount offline, and can execute a few MD raw-disassemble, and then turn
the device off, swap, force Linux to rescan the scsi bus, mark the drive
as a hot spare, and force reassembly ... then remount). This may or may
not work, depending upon the linux driver for the SATA port. Some get
very unhappy if the drive goes away after it found it.
Second (and third): Context switches (and interrupts) tend to quickly
swamp even fast systems with lots of processors. This is because the
SATA drivers on Linux, while good for basic SATA operations, may have a
few issues with multiple CSW needed for each transfer. You can drive a
fast system to become slow with a simple RAID0 across two drives. Run
bonnie++ on it (not IOzone, unless you want to measure memory cache).
Now imagine that system serving NFS requests. Additionally, the
interrupts driven by these hard IO operations also often drive the
system performance into the ground. We see 15-20k CSW and 20+k
interrupts under heavy load for a simple two drive RAID0 serving NFS
That is, it is not a bad idea, and it is possible to do it. But be
aware that you are going to need a fairly beefy machine (lots of RAM,
lots of cores) to handle the buffering and the interrupts. Can't help
much on the CSW's, you will just have to pay that price.
> With the availability multi-core chips and SSE instruction sets, it
> would seem to me that this is doable. Would be nice to not have to pay
> for those RAID cards if I don't have to. Just wondering if anything
> already exists.
The extra you pay for those RAID cards buys you hot swap, and if you
choose carefully, reasonable RAID engines. They aren't perfect, their
small random IO performance on large files leaves something to be
desired (as do all RAID controllers from what I can see, unless you want
to buy Bluearc or other units)
If you do choose to go the MD route, check out which SATA drivers are
well performing (low CSW/interrupts), and focus upon them. There are a
few out there.
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Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics LLC,
email: landman at scalableinformatics.com
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