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Re: Verify filesystem is aligned to stripes

To: xfs@xxxxxxxxxxx
Subject: Re: Verify filesystem is aligned to stripes
From: Spelic <spelic@xxxxxxxxxxxxx>
Date: Fri, 26 Nov 2010 14:15:35 +0100
In-reply-to: <20101126122218.GH12187@dastard>
References: <4CED5BFC.8000906@xxxxxxxxxxxxx> <20101125054607.GM13830@dastard> <4CEE0995.9030900@xxxxxxxxxxxxxxxxx> <20101125101537.GD12187@dastard> <4CEEE9BC.2030401@xxxxxxxxxxxxxxxxx> <20101126091622.264830fa@xxxxxxxxxxxxxx> <20101126122218.GH12187@dastard>
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On 11/26/2010 01:22 PM, Dave Chinner wrote:

FWIW, for workloads that do random, small IO, XFS works best when you
_turn off_ aligned allocation and just let it spray the IO at the
disks. This works best if you are using RAID 0/1/10. All the numbers
I've been posting are with aligned allocation turned off (i.e. no
sunit/swidth set).

I think I also noticed this...

The thing is, for large sequential I/O it seems to me it's indifferent if XFS is aligned or not, because the resulting file will anyway be sequential, and if you have raid10 or even parity raid with a large stripe cache there won't be any reads anyway. Ok maybe with alignment you could avoid reads on the first stripe (not sure, it might read anyway if the RAID reacts fast, before enough output is sent to it), but that's the only one.

So when is alignment to be turned on?

Thanks for the info

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