Setting up a mirror through geom on a GPT labeled disk

Synchronise the disklayout

gpart backup ada0 > /tmp/ada0.gpt
gpart restore -F /dev/ada1 < /tmp/ada0.gpt

Check that everything went as expected.

gpart show

Add the geom kernel module as loaded on startup

echo 'geom_mirror_load="YES"' >> /boot/loader.conf

Add bootcode to the second disk

gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 /dev/ada1

Reboot into single-user mode. Then load the geom kernel module

gmirror load

Set up the mirror

gmirror label -vb round-robin p1 /dev/ada0p1
gmirror label -vb round-robin p2 /dev/ada0p2
gmirror label -vb round-robin p3 /dev/ada0p3

Add the mirrored disk

gmirror insert p1 /dev/ada1p1
gmirror insert p2 /dev/ada1p2
gmirror insert p3 /dev/ada1p3

Wait for the disks to synchronise

gmirror status

Modify the fstab on the new device

mount /dev/mirror/p2 /mnt
cp /mnt/etc/fstab /mnt/etc/fstab.old
ee /mnt/etc/fstab

Reboot into multi-user mode

To backout from this, using the second disk as main disk do the following with the assumption that first disk is disconnected, thus there’s only one physical disk left now named ada0:

Boot into single-user mode. Then load the geom kernel module

gmirror load

Deactivate the now degraded mirror

gmirror deactivate p1 /dev/ada0p1
gmirror deactivate p2 /dev/ada0p2
gmirror deactivate p3 /dev/ada0p3

Clear metadata from the disks

gmirror clear /dev/ada0p1
gmirror clear /dev/ada0p2
gmirror clear /dev/ada0p3

fsck the the disk in order to be able to mount it

fsck /dev/ada0

Edit the fstab. ee can be run without having to mount /usr

mount /dev/ada0p2 /mnt
cp /mnt/etc/fstab /mnt/etc/fstab.mirrored
ee /mnt/etc/fstab

Reboot into multi-user mode