Re: [linux-lvm] Volume alignment over RAID

[Doug Ledford <dledford@redhat.com>]

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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@redhat.com>
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