Re: RAID stripe size (was: 5.1 RAID5 write performance)

[Eric Haszlakiewicz <erh%nimenees.com@localhost>]

On Mon, Jun 06, 2011 at 10:38:07AM -0700, Brian Buhrow wrote:
>       Hello.  I like this explanation.  Can you help clarify by giving a
> theoretical example?
> -thanks
> -Brian

If sectPerSU is the per-component stripe size, and let's say you have 4 disks
(components), then I think the total size of a stripe will be 4*sectPerSU.

If, for example, you create your filesystem with blocksize 32k, you'll
want your sectPerSU to be 16 (i.e. 16 * 512 = 8k).

eric
-- 
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> On Jun 6,  9:50am, Thor Lancelot Simon wrote:
> } Subject: Re: RAID stripe size (was: 5.1 RAID5 write performance)
> } On Mon, Jun 06, 2011 at 03:24:15PM +0200, Edgar Fu? wrote:
> } > > Ah, yes, the old 
> rmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmwrmw
>  cycle.  Gets me every time.
> } > OK, I've fixed that (before doing the tests I reported the last two days).
> } > 
> } > So, what's the advantage of a larger sectPerSU?
> } > It appears to me that the raidctl manpage should note that
> } > -- the stripe size should match fsbsize
> } 
> } Wait.  I didn't notice this until just now.  This is not right.
> } 
> } The filesystem block size (or, where this is not possible, the maximum
> } cluster size the filesystem will write) should be equal to sectPerSU
> } times the number of data (not parity) disks.  The sectPerSU value is
> } the *per-component* stripe size, not the total amount of data written
> } across all components in one stripe.
> } 
> } Thor
> >-- End of excerpt from Thor Lancelot Simon
>