You split (lvsplit) the mirror, mount the second copy somewhere (/mnt/backup in our example below), take the copy, then put it back (lvmerge, etc).

One risk is that the mirroring (silently?) doesn't get put back, so you are in effect running without it.

Checking for mirroring happiness: We supply a script chk-for-stale-VGs (VG == volume group), which could be run out of 'cron' to give a clean bill of health.

Putting things right when busted: We sometimes find ourselves in a "unmirrored and needing fixing" situation. We fix it (carefully) by hand:

Do a bdf | grep /lvol to see if there are any /mnt/backup partitions mounted.

If so, umount them; e.g.

sudo umount /mnt/backup/._disc2
sudo umount /mnt/backup/._disc3

Use lvdisplay to find the guys with "Mirror copies: 0"; e.g.

lvdisplay /dev/vg03/lvol1
lvdisplay /dev/vg02/lvol1
lvdisplay /dev/vg01/lvol1

Now knowing the ones that need merging back...

sudo /usr/sbin/lvmerge /dev/vg02/lvol1b /dev/vg02/lvol1
sudo /usr/sbin/lvmerge /dev/vg01/lvol1b /dev/vg01/lvol1

NB: the last step is not one you want to get wrong :-)