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Creating Large Volume Groups

Here's an example of the limitation prior to AIX 4.3. In AIX 4.2.1, in order to use a 4.5-GB disk, the PP size can be no less than 8 MB. 4500 MB / 1016 PP/PV = 4.4, rounding up to the next power of 2 is 8 MB. A 9.1-GB disk can have no less than a 16-MB PP (9100 / 1016 = 8.9).

Here's an example of creating a large volume group with a small PP size in AIX 4.3 and above:

You have 16 x 9.1-GB disks configured into two RAID 5 arrays (60816 useable MB each), and you need a PP size of 8 MB. 8 x 1016 is less than 60816, so normally this wouldn't work. However, by modifying the factor, it will work. The factor is a tradeoff between the number of disks and the size of the disks. You can have many small disks or a few large disks.

We need to find a factor so that:

factor x 1016 x PP size>= 60816
factor = 60816 / (8 x 1016) = 7.4 (round up to next power of 2)
So factor is 8.
Check: 8 x 8 x 1016 = 65024, 65024 > 60816

The end result will be:

root@unxm:lsvg  raidvg8A
VOLUME GROUP:   raidvg8A    VG IDENTIFIER:  000921765f1d266c
VG STATE:       active      PP SIZE:        8 megabyte(s)
VG PERMISSION:  read/write  TOTAL PPs:      15204 (121632 megabytes)
MAX LVs:        256         FREE PPs:       10881 (87048 megabytes)
LVs:            28          USED PPs:       4323 (34584 megabytes)
OPEN LVs:       28          QUORUM:         1
TOTAL PVs:      2           VG DESCRIPTORS: 3
STALE PVs:      0           STALE PPs:      0
ACTIVE PVs:     2           AUTO ON:        no
MAX PPs per PV: 8128        MAX PVs:        4

Notice that:

MAX PVs = 32 / factor
Total PVs is 2, the two RAID arrays
MAX PPs per PV = factor x 1016
PP size is 8 MB, the smallest allocatable unit of disk space

So we have reduced the number of disks by factor of 8, but increased their size by factor of 8.

Notes: You must create the VG from the command line and use the -t option.

The other way to avoid this situation is to use a very large physical partition size, i.e. 128 MB.

AIX now has a "big" volume group, use the -B flag, this allows 128 PVs and 512 LVs and with the -G flag a VG can have 1024 PVs.

It is possible to convert a volume group to a big volume group from a "normal" volume group. Use this if you have an existing VG with a small PP size and you want to add a large disk or RAID array to it.

This example is not limited to SSA disks, large SCSI disks would have to be configured the same.

See man mkvg for a complete description.