Newbie gmirror questions
jmc-freebsd2 at milibyte.co.uk
Fri Jan 15 23:34:55 UTC 2010
I'm about to upgrade to more disk space and I'm tempted use this as an
opportunity to get two disks and implement gmirror. Before I go ahead
there's a few aspects of mirroring I'm not sure about and would
appreciate some advice.
I'm using grub for multi booting. Does this introduce any problems if I
want to boot into Windows or Linux on one of the other partitions?
The gmirror manpage describes the procedure for handling kernel dumps
using the prefer balance algorithm in the early stages of booting and
then switching to round-robin in the /etc/rc.local script. It then goes
on to say that "If on the next boot a component with a higher priority
will be available, the prefer algorithm will choose to read from it and
savecore(8) will find nothing". Does this only arise if I've made some
change to the configuration of the mirror between the dump and the
reboot or is there some instances when the priority automatically
Some of the articles I've read about gmirror suggest setting the balance
to round-robin while others just leave this at the default setting of
split. Am I right in assuming that round-robin would give better
performance, and does it make much noticeable difference in real terms.
In particular am I likely to see a reduction in performance using
gmirror compared with what I would get with just a normal single disk.
Finally, recent articles say to set kern.geom.debugflags to 17 when
creating a mirror on a mounted drive while older articles say to set it
to 16. Although I'll probably be creating the mirror on my disks before
copying my system onto them so I don't really need to worry about
setting this flag but I'm curious to know the difference between using
the two values.
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