On 18/01/2011 15:59, Igor Podlesny wrote:
I had experience of using FreeBSD's vinum (another software RAID).
Its author, Greg Lehey, stated in vinum's manual: "... A good
guideline for stripe size is between 256 kB and 512 kB. Avoid powers
of 2, however: they tend to cause all superblocks to be placed on the
first subdisk. ..."
Meanwhile, with LSR we're given exactly 2^n choices, for e. g.,
neither 768 KiB, nor 387 KiB won't go: "mdadm: invalid chunk/rounding
value: 387".
So, why... ($Subj) and how complex would it be to abolish this
restriction? I think this could be a key to performance increase.
You're slightly confusing things. Stripe size and chunk size are
different things, but related. Stripe size = number of data discs *
chunk size. Linux software RAID does insist on a power of two chunk
size, which is almost certainly done for performance reasons, but
obviously it can't insist on a power of two stripe size, e.g. a 3-disc
RAID-0, 4-disc RAID-5 or 5-disc RAID-6 would all have 3 data discs so
the stripe size won't be a power of two.
Secondly, as far as I know all Linux filesystems are RAID-aware so know
how to distribute their superblocks and other metadata evenly across all
the discs, so they've already got any performance improvement that might
be had.
Cheers,
John.
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