Close, but not quite:

First, use extendvg <VG> <hdisk> to add the disk(s) to the VG. You can only mirror to disks already present in the VG.

Second: use the command you suggested. Note, however, that you have to watch out for some gotchas:

You may need to change the quorum of the VG. The quorum is the number of PVs needed to be available for a varyonvg to succeed. Additional restrictions apply if the VG is part of a resource group in a HA-cluster, but i suggest you do NOT try to configure such a cluster if you have no firm grasp of AIX concepts. Do yourself a favor and hire an expert for such a task or - if you have enough time for this - first learn AIX/HACMP concepts to the point where you are familiar with these. It's like driving a car: in principle not too complicated, but try to do it without training and you are likely to cause some damage. There are a lot of valuable resources for this (i suggest reading the freely available "IBM Redbooks", which can be downloaded in PDF-format) but you need to get familiar with these first, everything else is asking for trouble.

Basically, the LVM works like this: you have "physical volumes" (PVs), which are logical disks. Each PV is member of one and only one VG. Upon adding the disk to the VG (by which it becomes a PV) it is chopped into small pieces (PPs - "physical partitions") and these pieces can be added to "logical volumes" (LVs). Onto LVs filesystems or all other sorts of things can be put: swap devices, JFSlog devices, ... .

In principle these LVs consist of "logical partitions" (LPs), which are the same size as PPs. Every LP is represented by one PP. In case of mirroring you have a 2:1 ratio and every LP is represented by 2 (or even 3) different PPs, each with the same content. Mirroring is done on LV level and you can have mirrored alongside unmirrored LVs within the same VG. A mirrorvg command just executed the mirror process for each LV automatically.

Also in principle, PPs are anonymous and you do not have to care from which disk (PV) they come from. However, the selection process for allocation as well as mirroring can be influenced (i.e. allow/forbid all copies of a LP to be on the same disk) in the LV properties ("Intra-policy" and "Inter-policy", quite a poor choice of naming).

Now, this was a (very very short) introduction to LVM concepts, but it probably left more questions than it provided answers. For LVM too there is a redbook which i suggest you read.

I hope this helps.

bakunin