Lazy update processing in a PowerHA SystemMirror cluster

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For LVM components under the control of PowerHA® SystemMirror®, you do not have to explicitly do anything to bring the other cluster nodes up to date. Instead, PowerHA SystemMirror can update LVM information on a node when it activates the volume group during a fallover.

In a cluster, PowerHA SystemMirror controls when volume groups are activated. PowerHA SystemMirror implements a function called lazy update. This function examines the volume group time stamp, which is maintained in both the volume group's VGDA, and the local ODM. AIX® updates both these time stamps whenever a change is made to the volume group. When PowerHA SystemMirror is going to vary on a volume group, it compares the copy of the time stamp in the local ODM with that in the VGDA. If the values differ, PowerHA SystemMirror will cause the local ODM information on the volume group to be refreshed from the information in the VGDA.

What this means for a PowerHA SystemMirror cluster administrator is that if a volume group under PowerHA SystemMirror control is updated directly (that is, without going through the C-SPOC facility of PowerHA SystemMirror), other nodes' information on that volume group will be updated when PowerHA SystemMirror has to bring the volume group online on those nodes, but not before.

The following figure illustrates how a lazy update works in a cluster. While it shows the AIX export and import functions being used, the actual operations performed will depend on the state of the volume group. As stated previuosly, this happens when PowerHA SystemMirror has to bring the volume group online on node B. If the C-SPOC facility of PowerHA SystemMirror is used to make changes to the volume group, an equivalent operation is performed automatically at the time of the change.
Figure 1. Lazy update processing example

Lazy update processing example
Note: PowerHA SystemMirror does not require lazy update processing for enhanced concurrent volume groups, as it keeps all cluster nodes updated with the LVM information.