[linux-lvm] Volume alignment over RAID

Doug Ledford dledford at redhat.com
Thu May 27 16:40:57 UTC 2010

On 05/22/2010 03:23 AM, Luca Berra wrote:
> 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).

Hardware raid and software raid are two entirely different things when
it comes to optimization.

>>> 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

That was me.  The best results are with 256 or 512k chunk sizes.  Above
512k you don't get any more benefit.

Doug Ledford <dledford at redhat.com>
              GPG KeyID: CFBFF194

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