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List:       linux-raid
Subject:    Re: ICH7R "strip" size
From:       Mark Hahn <hahn () physics ! mcmaster ! ca>
Date:       2006-07-27 5:18:56
Message-ID: Pine.LNX.4.64.0607270113120.19846 () coffee ! psychology ! mcmaster ! ca
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> My question: What are the chunk and usable storage sizes per stripe for four 
> discs in RAID-0 on an ICH7R configured for a 128KB "strip"?

raid0 always has 100% usable; configuring it is deciding how much 
concurrency you want.

if your writes are 64K and you have 4 disks, your max concurrency would
be at 64K per disk (stripe size in MD), or 256K for a whole stripe.
if you want max bandwidth, but concurrency of 1, you want the 64k write
to busy all the disks, for a 16K stripe size (64K whole-stripe).

with raid5, the main thing additional factor is to try to arrange 
"blind" whole-stripe writes if possible.  that is, you pay the
read-modify-write penalty unless you write a whole stripe at once:
n-1 * stripe size if you can manage it aligned, or at least several
whole stripes to amortize the reads necessary on the edges...
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