On 19.12.2009 01:37, Matt Tehonica wrote:
I have a 4 disk RAID5 using a 2048K chunk size and using XFS filesystem. Typical file size is about 2GB-5GB. I usually get around 50MB/sec transfer speed when writting files to the array. Is this typcial or is it below normal? A friend has a 20 disk RAID6 using the same filesystem and chunk size and gets around 150MB/sec. Any input on this??
Software RAID performance should not be that slow, unless the drives are connected to controller on 32bit/33Mhz PCI slots of course. There is a few things to keep in mind though.
Controllers and bus topology of the motherboard matters a great deal on I/O performance, but even on recent (up to around 3 years back, I think) desktop motherboards you should be able to go very fast when using the right slots and busses. PCI-Express was the game changer here, but you should try to get most SATA ports on a slot connected to the north bridge if you want to go REALLY fast.
Filesystem alignment and stripe size awareness helps quite a bit, and I guess even more on a machine that is already bus starved (if thats your problem) as it helps reduce "invisible" I/O - operations spanning multiple stripes when they could have spanned one, for example, and a reduction in read-modify-write cycles in general.
A bigger stripe_cache on the array might help, especially if things aren't aligned/aware, if you got the memory (check /sys/block/<md dev>/md/stripe_cache_active and _size.)
On a old Intel core 2 duo, with a MD RAID6 set using 128k chunks over 8 1.5TB 7200rpm SATA drives I'm seeing about 600MB/s writes and 700-750MB/s reads with sequential I/O - which is very near the maximum for the resulting stripe size with those drives. Changing to RAID5 would probably net me another ~100MB/s as the stripe would span one more drive.
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