HADR setup in a DB2 pureScale environment

Asymmetric standby members

Because only one member on the standby replays logs, consider configuring a standby member with more CPU power and memory to serve as the preferred replay member. Similarly, consider which member on the primary cluster has the most CPU and memory, so that you can select it to be the preferred replay member if the current primary become the standby after a role switch. In both cases, you designate the preferred replay member by issuing the START HADR command from that member, with either AS PRIMARYor AS STANDBYoption. The preferred replay member designation is persistent. It remains in place until changed by the next START HADR command.

Because the member topologies on the primary and standby must be the same, if you add members on the primary, you must also add members on the standby. If you have resource constraints, such as hardware constraints, you can configure the new standby members as logical members that share hosts. If the standby takes over the primary role, this new primary will not be as powerful as the old primary.

Standby cluster caching facilities

The cluster caching facility (CF) does not have to be the same on the primary and standby clusters. The standby makes minimal use of the CF because only one member performs the replay, so it is possible to have only one CF on the standby cluster. If, however, the standby takes over as the new primary, you should add a CF to help ensure that your DB2 pureScale environment is highly available. Adding that secondary CF requires an outage because you cannot add it without stopping the DB2 pureScale instance.

Member subsetting

You can use member subsetting to specify a subset of members to use for a database. The subset is stored internally as a list of member numbers. The database then maps the members to host names for the client to connect to. If this database uses HADR, you can only specify the subset list on the primary database. The subset member list is replicated to the standby.