LVM performance tuning with the lvmo command

You can use the lvmo command to manage the number of LVM pbufs on a per volume group basis.

The tunable parameters for the lvmo command are the following:

pv_pbuf_count: The number of pbufs that will be added when a physical volume is added to the volume group.
max_vg_pbuf_count: The maximum number of pbufs that can be allocated for the volume group. For this value to take effect, the volume group must be varied off and varied on again.
global_pbuf_count: The minimum number of pbufs that will be added when a physical volume is added to any volume group. To change this value, use the ioo command.
aio_cache_pbuf_count: The current total number of pbufs that are available for the aio_cache logical volume in the volume group. The maximum number of aio_cache_pbuf_count that can be allocated to the volume group is specified by the max_vg_pbuf_count parameter.

In the following example, the lvmo -a command displays the current values for the tunable parameters in the rootvg volume group.

# lvmo -a

vgname = rootvg
pv_pbuf_count = 256
total_vg_pbufs = 768
max_vg_pbuf_count = 8192
pervg_blocked_io_count = 0
global_pbuf_count = 256
global_blocked_io_count = 20
aio_cache_pbuf_count = 512

If you want to display the current values for another volume group, use the following command:

lvmo -v <vg_name> -a

To set the value for a tunable with the lvmo command, use the equal sign, as in the following example:

Note: In the following example, the pv_pbuf_count tunable is set to 257 in the redvg volume group.

# lvmo -v redvg -o pv_pbuf_count=257

vgname = redvg
pv_pbuf_count = 257
total_vg_pbufs = 257
max_vg_pbuf_count = 263168
pervg_blocked_io_count = 0
global_pbuf_count = 256
global_blocked_io_count = 20

Note: If you increase the pbuf value too much, you might see a degradation in performance or unexpected system behavior.