6.12.1 Switching to the backup HMC for the Power Enterprise Pool 6.13 Test scenario for HMC fallover For Enterprise Pool operations, querying operations can be run on the master or backup  HMC, but changing operations must run on the master HMC. If the master HMC fails, the  PowerHA SystemMirror actions are as follows: There are some prerequisites in PowerHA SystemMirror that must be met before switching to  the backup HMC when the master HMC fails: For more information about how to configure the backup HMC for the Power Enterprise  Pool, see IBM Knowledge Center and  Power Enterprise Pools on IBM Power Systems ,  REDP-5101. This section shows how PowerHA SystemMirror switches the HMC automatically when the  primary HMC fails. GLYPH<SM590000> For querying operations, PowerHA SystemMirror tries to switch to the backup HMC to  continue the operation, but does not set the backup HMC as the master. GLYPH<SM590000> For changing operations, PowerHA SystemMirror tries to set the backup HMC as the  master, and then continues the operation. Example 6-30 shows the command that  PowerHA SystemMirror performs to set the backup HMC as the master. This command is  triggered by PowerHA SystemMirror automatically. GLYPH<SM590000> Configure the master HMC and the backup HMC for your Power Enterprise Pool. GLYPH<SM590000> Ensure that both HMCs are configured in PowerHA SystemMirror. GLYPH<SM590000> Establish password-less communication between the PowerHA SystemMirror nodes to the  two HMCs. GLYPH<SM590000> Ensure reachability (pingable) from PowerHA SystemMirror nodes to the master and  backup HMCs. GLYPH<SM590000> Ensure that all of the servers that participate in the pool are connected to the two HMCs. GLYPH<SM590000> Ensure that the participating servers are in either the Standby state or the Operating state. Example 6-30 Setting the backup HMC as the master chcodpool -o setmaster -p <pool> --mc backup Chapter 6. Resource Optimized High Availability  249