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Re: ag selection

To: xfs@xxxxxxxxxxx, linux-xfs@xxxxxxxxxxx
Subject: Re: ag selection
From: Carlos Maiolino <cmaiolino@xxxxxxxxxx>
Date: Mon, 11 Nov 2013 15:53:14 -0200
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On Mon, Nov 11, 2013 at 06:25:13PM +0100, Bernd Schubert wrote:
> Hi all,
> 
> for streaming writes onto a raid6 the current round-robin ag
> selection seems does not seem to be optimal. Writing 4 files from 4
> threads into a single directory we get 900 MB/s, writing 4 files in
> 4 different directories we only get 700 MB/s (12 disks with with hw
> megaraid-sas). The current round-robin scheme seems to be optimized
> for linear raid0? With small AGs one could also argue, that choosing
> AGs which are not far away from each other (in respect to the number
> of blocks) also adds more parallel disk access for small and medium
> sized files.
> 
> Any objections against a patch to improve the AG selection?
> 

I wouldn't say this it is optimized specifically for raid 0 environments but I
lack some knowledge on this choice. The mainly reason for the round-robing IIRC,
was to avoid lock contention in a single AG. spreading different files along the
whole disk, and also making it able to allocate them contiguously along the 
disk.

But, I'm not sure what kind of optimization you have in mind and I believe
another engineers will also need some extra information about what optimization
you have in mind, what kind of tests you're doing (Direct I/O, buffered,
pre-allocation), etc.. You'll also need to post filesystem configurations like
FS aligment (su, sw options), etc.

For different write patterns, you might also want to take a look at the
rotor_step procfs option, and some other options dedicated to streaming writes,
that might help you in this case.

Just my $0.02

> 
> Thanks,
> Bernd
> 
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-- 
Carlos

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