Hello,
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.
An alternative method is that the customer uses an Availability Group Listener for SQL which will give a virtual network name (VNN), virtual IP (VIP), this VIP or VNN can be used to configure the SEPM database. The Availability Group Listener will ensure that it connects to the correct SQL database.
Reference:
SEPM support for SQL database mirroring
http://www.symantec.com/docs/TECH197242
Secondly, check these Articles:
Does the SEPM support use of Microsoft SQL database mirroring?
http://www.symantec.com/docs/TECH197215
Best practices for upgrading to the latest version of Symantec Endpoint Protection 12.1.x
http://www.symantec.com/docs/TECH163700
Hope that helps!!