Is it possible to change sunit of log section post-mkfs

Michael Weissenbacher
2015-11-03T12:19:07Z

Hi!
I have a XFS file system which lies on a 10-disk RAID-6 device that was
created with Chunk Size = 1MiB.
On mkfs.xfs time this was - as far as i know - specified with "-d
su=1m,sw=8".

xfs_info shows the following:
meta-data=/dev/sdb1 isize=256 agcount=15,
agsize=268435200 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=3905945088, imaxpct=5
= sunit=256 swidth=2048 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal bsize=4096 blocks=521728, version=2
= sectsz=512 sunit=8 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0

Interestingly, the sunit value of the log seems to be incorrect - as it
should be 256 too, like the sunit value of the data. I am pretty sure
the reason is that the log sunit cannot be 256 blks (=1024KiB) and
because of this mkfs.xfs did fall back to the default of 8 blks
(=32KiB). I found evidence of this in the following thread:
http://oss.sgi.com/archives/xfs/2012-06/msg00431.html

What i want to achieve is to set the log sunit to the maximum possible
of 64 blks (=256KiB).

- Is that even possible without doing mkfs.xfs (and losing all data)?
- Would it be an improvement performance-wise?
- Would changing to an external log help?

tia,
Michael