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

[Michael Weissenbacher <mw@dermichi.com>]

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

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