Newbie gmirror questions

Mike Clarke jmc-freebsd2 at
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.

Mike Clarke

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