System Management Guide:
Operating System and Devices

Split a Mirrored Disk from a Volume Group

Beginning with AIX 5.2, snapshot support helps you protect the consistency of your mirrored volume groups from potential disk failure. Using the snapshot feature, you can split off a mirrored disk or disks to use as a reliable (from the standpoint of the LVM metadata) point-in-time backup of a volume group, and, when needed, reliably reintegrate the split disks into the volume group. In the following procedure, you first split off a mirrored disk from a volume group and then you merge the split-off disk into the original volume group. To further ensure the reliability of your snapshot, file systems must be unmounted and applications that use raw logical volumes must be in a known state (a state from which the application can recover if you need to use the backup).

A volume group cannot be split if any one of the following is true:

• A disk is already missing.
• The last non-stale partition would be on the split-off volume group.
• Any stale partitions exist in the volume group, unless you use the force flag (-f) with the splitvg command.

Furthermore, the snapshot feature (specifically, the splitvg command) cannot be used in enhanced or classic concurrent mode. The split-off volume group cannot be made concurrent or enhanced concurrent and there are limitations to the changes allowed for both the split-off and the original volume group. For details, read the chvg command description in AIX 5L Version 5.2 Commands Reference.

1. Ensure that the volume group is fully mirrored and that the mirror exists on a disk or set of disks that contains only this set of mirrors.
2. To enable snapshot support, split off the original volume group (origVG) to another disk or set of disks, using the following command:

   splitvg origVG

   At this point, you now have a reliable point-in-time backup of the original volume group. Be aware, however, that you cannot change the allocation on the split-off volume group.
3. Reactivate the split-off disk and merge it into the original volume group using the following command:

   joinvg origVG

   At this point, the split-off volume group is now reintegrated with the original volume group.