Microsoft SQL database mirroring will work with the Symantec Endpoint Protection Manager (SEPM)12.1.

SEPM Database on SQL server with Mirroring is Supported with SEP 12.1.

When the SEPM Database connection wizard is run, specify an IP address or machine name hosting the database. As long as that IP address is available, the SEPM will connect to it. However, the SEPM cannot natively “failover” between two databases. Thus if you implement database mirroring, the content of the DB will be available, but you will have to re-run the wizard to reconfigure the database IP address though you may only have to stop and restart the SEPM services to get the SEPM pointed to the proper address.