XFS over LVM over md RAID

[Richard Scobie]

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