LVM is much simpler to use if you're not already familiar with VxVM volume manager. Note that MirrorDisk/UX is a purchased product so it must be installed on your system before the lvextend -m 1 command will work. To see if it exists:
swlist -l fileset|grep -i mirror
You will see LVM-MIRROR-RUN if the product has been installed.
If you are just using two disks, one primary and one mirror, then the commands are correct for the first set of commands. pvcreate forms a boot and header area, vgextend adds the disk to the current volume group and mkboot commands setup the second disk so it can also be bootable. The lvextend -m 1 does the actual mirroing. You repeat just the lvextend command for each lvol on the disk. It isn't necessary to specify the target disk for mirroring in this case. Using a target disk for mirroring is used with volume groups that have many unused disks to specify the desired location for the mirror. You can do the lvextends in a simple loop from the command line:
for LV in 1 2 3 4 5 6 7 8
do
lvextend -m 1 /dev/vg00/lvol$LV
done
Now the manual is probably a bit misleading with the explanation of a hot spare. The vgextend command will indeed add the 3rd disk to the group but it won't be of any use as it will have no data on it. Instead, you can change the lvextend command from -m 1 to -m 2 and specify the physical disks in the order you want to use. This is double mirroring and quite useful in taking backups. All 3 disks will be continuously synced with very low overhead and at any time, you can reduce the mirror count to split off one of the mirrors, then mount the separate disk under it's own set of mountpoints. NOTE: The maximum number of additional disk space that can be added to a volume group is fixed when the VG is created and cannot be changed. So if there aren't enough extents slots in the VG, an additional disk can't be added.
And resizing is completely transparent. If you increase the size of an lvol, all of the mirrors are extended automatically. Note that to extend a filesystem without un-mounting it, you'll need the extra product Online JFS. Without Online JFS, to extend a filesystem, you unmount it, extend the lvol with lvextend, then extend the filesystem with extendfs. Now you can remount the larger filesystem. It is recommended never to reduce a filesystem without a full backup. Note that root can use lvreduce ay any time which generally destroys the lvol.
Bill Hassell, sysadmin
