FreeBSD HOWTO: Software Mirror System Disk

[Danny Howard]

[ From http://dannyman.toldme.com/2005/01/24/freebsd-howto-gmirror-system/ ]

A new feature of FreeBSD 5.3 is the ability to set up a software mirror 
of your system disk. This allows you to boot off either of a pair of 
hard disks, which will then function as a RAID1, which will ensure 
system uptime in the face of a single disk failure.

As the documentation is a bit sketchy, here’s a quick cheat sheet for 
setting this up with gmirror:
(This crib sheet assumes you have a pair of identical IDE (in my case, 
SATA) drives identified as ad4 and ad6.)

   1. Install FreeBSD on to ad4.
   2. Reboot with the Install CD.
   3. Enter Fixit mode, using Install CD disc2 as the “live filesystem”
   4. # *chroot /dist*
      # *mount_devfs devfs /dev*
      # *gmirror label -v -b round-robin gm0 /dev/ad4*
      # *gmirror insert gm0 /dev/ad6*
      # *mount /dev/mirror/gm0s1a /mnt*
      # *echo ‘geom_mirror_load="YES"‘ >> /mnt/boot/loader.conf*
      # *echo ’swapoff="YES"‘ >> /mnt/etc/rc.conf*
   5. Edit /mnt/etc/fstab to convert ad4 -> mirror/gm0
   6. Reboot

Thanks to the few dozen people who have come before me, and posted 
crucial hints to the mailing lists. Thanks also to Ralf S. Engelschall 
who has a far more verbose explanation of how to do this sort of thing 
with mis-matched disks. [See http://people.freebsd.org/~rse/mirror/ ]
<http://people.freebsd.org/%7Erse/mirror/>
You should definately look over the gmirror man page, and review the 
output of *"gmirror list gm0"* when swapping out drives. You can disable 
automatic rebuild, etc. It is quite nice.

Sincerely,
-danny