[linux-lvm] Volume alignment over RAID

[Luca Berra]

On Thu, May 20, 2010 at 11:48:31PM -0700, Linda A. Walsh wrote:
> Luca Berra wrote:
>>> I'm using a RAID 'chunk' size of 64k as suggested by the RAID documentation
>>> and am using 6 disks to create a RAID6, giving 4 units of data/stripe.  Does
>> I suppose by raid you mean md, so i wonder what documentation you were
>> looking at?
> ---
> 	Well, doc in 2 different raid controllers LSI and rocket raid both
> suggest 64K as a unit size (forget, their exact term).
>
>> I think 64k might be small as a chunk size, depending on your array size
>> you probably want a bigger size.
> ---
> 	Really?  What are the trade offs?  Array size well 6 disks and 4 of data.
ok, i trew the stone ..
First we have to consider usage scenarios, i.e. average read and average
write size, large reads benefit from larger chunks, small writes with
too large chunks would still result on whole stripe Read-Modify-Write.

there were people on linux-raid ml doing benchmarks, and iirc using
chunks between 256k and 1m gave better average results

-- 
Luca Berra -- bluca at comedia.it
         Communication Media & Services S.r.l.
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