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concerning 'optimal' strip size on RAID disks...

To: xfs-oss <xfs@xxxxxxxxxxx>
Subject: concerning 'optimal' strip size on RAID disks...
From: "Linda A. Walsh" <xfs@xxxxxxxxx>
Date: Fri, 17 Jul 2009 17:33:21 -0700
User-agent: Thunderbird 2.0.0.22 (Windows/20090605)

Concerning strip-size.  What are the considerations for that?  Any reason
not to chose the largest?  Seems that at sizes up to 1MB, the larger the
better.

For direct I/O (which is what the RAID controller would be doing to
it's disks, seems a larger write would be better). Would I be naïve to assume that if a RAID controller only needed to update 1 block, it wouldn't need to update the entire strip? Would there be a benefit in running a smaller strip size?
I know when I can control the hard disks, I can enable their write caches,
so having them do physical writes the keep their write buffers saturated
would optimize write performance, at least, but if it's a BIOS or hardware
controlled RAID, I don't know if I'd have the option to turn the disk's
write buffer on or off.  So that likely wouldn't matter much.

Ideally, I think, it be optimal if a RAID controller (hardware or software) really knew the the layout of the data on disk -- as in sectors/track.
Then it really might be able to interleave tracks among disk units (unless
all the tracks can be written contiguously w/no delay, then I'd guess there'd
be no benefit...oh well..

But how does one decide a strip size for RAID disks?

What criteria does one use?

Thanks!
-linda


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