Understanding concurrent access and PowerHA SystemMirror scripts
You should seldom, if ever, need to intervene in a concurrent access cluster. In a concurrent access environment, as in a non-concurrent environment, the PowerHA® SystemMirror® event scripts control the actions taken by a node and coordinate the interactions between the nodes. However, as a system administrator, you should monitor the status of the concurrent access volume groups when PowerHA SystemMirror events occur.
When intervening in a cluster, you must understand how nodes in a concurrent access environment control their interaction with shared LVM components. For example, the PowerHA SystemMirror node_up_local script may fail before varying on a volume group in concurrent mode. After fixing whatever problem caused the script to fail, you may need to manually vary on the volume group in concurrent access mode. The following sections describe the processing performed by these scripts.
Nodes join the cluster
A node joining a cluster calls the node_up_local script, which calls the cl_mode3 script to activate the concurrent capable volume group in concurrent access mode. If resource groups are processed in parallel, process_resources calls cl_mode3.
The cl_mode3 script calls the varyonvg command with the -c flag. For more information about this command and its flags, see Activating a volume group in concurrent access mode. If the concurrent capable volume group is defined on a RAID disk array device, the scripts use the convaryonvg command to vary on the concurrent volume groups in concurrent mode.
Nodes leave the cluster
Nodes leaving the cluster do not affect the concurrent access environment. They simply vary off the volume groups normally. The remaining nodes take no action to change the concurrent mode of the shared volume groups.
When a node has cluster services stopped with resource groups brought offline, it executes the node_down_local script, which calls the cl_deactivate_vgs script. This script uses the varyoffvg command to vary off the concurrent volume groups.