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XFS over LVM over md RAID

To: xfs@xxxxxxxxxxx
Subject: XFS over LVM over md RAID
From: Richard Scobie <richard@xxxxxxxxxxx>
Date: Fri, 10 Sep 2010 10:58:22 +1200
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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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