SQL Server, Database mirroring

--principle and mirror role(hot standby server).
--All database mirroring sessions support only one principal server and one mirror server
--High-safety mode with automatic failover requires a third server instance, known as a witness

role switching
roling upgrade

Unlike replication, which works at the logical level, database mirroring works at the level of the physical log record. Beginning in SQL Server 2008, the principal server compresses the stream of transaction log records before sending it to the mirror server. This log compression occurs in all mirroring sessions

operation mode:
high-safety==synchronous ,
high-performance== asynchronous