conf/190152: [patch] [rc] gmirror savecore support

[Paul J Murphy]

http://www.freebsd.org/cgi/query-pr.cgi?pr=conf/190152

Hi geom-people, a friendly heads-up on the above PR.  I'm seeking
validation of my belief that there's no changes necessary to
/etc/rc.d/dumpon, only /etc/rc.d/savecore.  The kernel will always
deterministically pick a single physical device within the mirror, and
we just need to use "prefer" to force the same determinism when
savecore runs, right?  (With the obvious caveat that failed and
replaced devices, or other changes to the devices and their priorities
that make up the mirror, will change the results of the determinism.)

Any feedback on my patch in conf/190152 is welcome.  Its aim is to at
least cover the common and simple cases with gmirrored system disks.

See also http://www.freebsd.org/cgi/query-pr.cgi?pr=docs/178818 which
initially was attempting the fix via dumpon alone.


Thanks,

Paul.