If a database with no mirroring definitions is started on a database server (for example, S1) started with -xp on, and the mirroring definitions are then made, S1 would be the initial primary server for the database.
Alternatively, when you first set up a database mirroring system and there are no state information files, and the copies of the database and transaction log are identical, both servers are eligible to act as the primary. In this situation, the server names are compared, and the server with the lower name acts as primary. For example, the name server1 is lower than server2, so server1 becomes the primary server.
For the initial startup, both servers must be running and connected for them to agree on their roles; the presence of an arbiter is not enough since the prior state information recorded in the state information files does not exist.
During a normal startup, the following inputs affect which server becomes the primary server:
the contents of the state information files
the transaction log position on each database server
the designation of a preferred primary server
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