Re: Calculating swidth On A RAID6 Volume

From: Michael Monnerie <michael.monnerie@is.it-management.at>
To: xfs@oss.sgi.com
Subject: Re: Calculating swidth On A RAID6 Volume
Date: Tue, 20 Jul 2010 15:22:39 +0200	[thread overview]
Message-ID: <201007201522.39455@zmi.at> (raw)
In-Reply-To: <3C5E029826A0704DB5998577FCFF46F0094DA1AD69@dagobah>

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On Dienstag, 20. Juli 2010 Andrew Debenham wrote:
> I'm am new to XFS and am in the process of trying to tune an XFS file
>  system.  I spent some time searching through this mailing list's
>  archives but wasn't able to find anything that matched the question
>  that I have (I apologize in advance if this issue has already been
>  addressed).

Thanks for reminding me, wanted to write a FAQ entry, done now:
http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance

> I have a 3ware 9650SE-12M installed and have a RAID6 unit with 11 x
>  1.82 TB SATA drives and stripe size of 256KB.  I found an earlier
>  thread that discussed calculating the values for sunit and swidth
>  for RAID5, which stated that the swidth would be "sunit x (# disks
>  in stripe - 1)".  So, finally the question - would the swidth for a
>  RAID6 volume be "sunit x (# disks in stripe -2)?
> 
> If that is the case, then here is what I would expect my values to
>  be: sunit = 512
> swidth = 4608

I think sunit should be as big as the RAID controllers stripe size, as 
it describes the smallest I/O XFS should do. So sunit=512 would make it 
possible that XFS writes a single sector, while the RAID controller 
needs to read/write 256KB with your stripe size, resulting in 
performance degradation. But I'm no XFS dev, so maybe wait for a 
clearing from someone who has deeper XFS knowledge than me.

sunit=262144
swidth=9

Would be my suggestion for your setup.

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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