sunit-swidth parameters

I am under the impression that mkfs.xfs is able to obtain details from the Linux md subsystem, such that it will automatically create the optimal sunit and swidth parameters, to suit an md RAID array on which the filesystem is being created.

Please correct me if I am wrong.

If this is the case, then I am a little confused by the following:

mkfs.xfs -f -l logdev=/dev/md1,size=10000b /dev/md5

meta-data=/dev/md5               isize=256    agcount=32, agsize=7630656 blks
         =                       sectsz=4096  attr=0
data     =                       bsize=4096   blocks=244179840, imaxpct=25
         =                       sunit=64     swidth=128 blks, unwritten=1
naming   =version 2              bsize=4096
log      =/dev/md1               bsize=4096   blocks=10000, version=1
         =                       sectsz=512   sunit=0 blks
realtime =none                   extsz=524288 blocks=0, rtextents=0

This is the output where /dev/md5 is a 3 disk RAID5, with a chunk size of 128kB.

After reading the man page and looking at some examples in the mailing list archives, I would have thought that the best sizes would have been sunit=32 and swidth=64, or am I wrong?

I ask, as I am about to resize this array by adding another drive and am trying to work out the new values for these parameters, to pass to mount - the values I have come up with are sunit=256 and swidth=768.

Thanks,

Richard