Feature release packages can be installed in the High Availability CommServe Host setup, without shutting down the CommCell operations.
Procedure
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Initiate a maintenance failover to the standby CommServe host. 
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Install the feature release on the production CommServe host. 
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Switch CommCell operations back to the production CommServe host by resetting the maintenance failover. 
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Install the service pack on the standby CommServe host(s) and SQL client(s).