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Re: Calculating swidth On A RAID6 Volume

To: xfs@xxxxxxxxxxx
Subject: Re: Calculating swidth On A RAID6 Volume
From: Michael Weissenbacher <mw@xxxxxxxxxxxx>
Date: Tue, 20 Jul 2010 16:13:29 +0200
In-reply-to: <201007201522.39455@xxxxxx>
References: <3C5E029826A0704DB5998577FCFF46F0094DA1AD69@dagobah> <201007201522.39455@xxxxxx>
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.10) Gecko/20100527 Thunderbird/3.0.5
Michael,
>> 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.
> 
IIRC sunit and swidth are both specified in units of 512 bytes (don't
ask me why) so a sunit value of 512 would mean 256KB stripe size which
is correct :-)

If i am right, the FAQ should be corrected (hopefully some dev can
confirm/deny it)

cheers,
Michael

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