## 6.12.1 Switching to the backup HMC for the Power Enterprise Pool 

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: 

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. 

chcodpool -o setmaster -p <pool> --mc backup 

There are some prerequisites in PowerHA SystemMirror that must be met before switching to the backup HMC when the master HMC fails: 

GLYPH<SM590000> Configure the master HMC and the backup HMC for your Power Enterprise Pool. 

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. 

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. 

## 6.13 Test scenario for HMC fallover 

This section shows how PowerHA SystemMirror switches the HMC automatically when the primary HMC fails. 

