Re: Hi! Why having LSR's chunk size 2^n limitation?

maybe we could do betters read/write algorithm for devices (hard disks
/ solid state disks) for example (round robin, closer head, minimal
read/write time) that make a more optimized operation with mixed disks
array (ssd+hdd array) for raid 0,1,10,5,6,x+y

2011/1/18 John Robinson [off-list ref]:

On 18/01/2011 15:59, Igor Podlesny wrote:

quoted

   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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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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