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Message-ID: <4C89668E.6010800@sauce.co.nz>
Date: Fri, 10 Sep 2010 10:58:22 +1200
From: Richard Scobie <richard@sauce.co.nz>
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Subject: XFS over LVM over md RAID
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Using the latest, stable versions of LVM2 and xfsprogs and the 2.6.35.4 
kernel, I am setting up lvm on a 16 drive, 256k chunk md RAID6, which 
has been used to date with XFS directly on the RAID.

mkfs.xfs directly on the RAID gives:

meta-data=/dev/md8               isize=256    agcount=32, 
agsize=106814656 blks
          =                       sectsz=4096  attr=2
data     =                       bsize=4096   blocks=3418068864, imaxpct=5
          =                       sunit=64     swidth=896 blks
naming   =version 2              bsize=4096   ascii-ci=0

which gives the correct sunit and swidth values for the array.

Creating an lv which uses the entire array and mkfs.xfs on that, gives:

meta-data=/dev/vg_local/Storage  isize=256    agcount=13, 
agsize=268435455 blks
          =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=3418067968, imaxpct=5
          =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0

Limited testing using dd and bonnie++ shows no difference in write 
performance whether I use sunit=64/swidth=896 or sunit=0/swidth=0 on the lv.

My gut reaction is that I should be using 64/896 but maybe mkfs.xfs 
knows better?

Regards,

Richard

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