A database mirroring session consists of a relationship between the partner
servers when they mirror a database from one partner to another. A given
partner server may have the principal role for one database and a mirror role
for a different database.
In addition to the two partner servers (principal and mirror) a database
mirroring session may have an optional third server, called the witness. The
witness server's role is to enable automatic failover. When database mirroring
is used for high availability, if a principal server suddenly fails, if the
mirror server has confirmation from the witness, it can automatically take on
the role of principal and make its database available within a few seconds.
Some important items to note about database mirroring:
• The principal database must be in the FULL recovery model. Log records that
result from bulk-logged operations cannot be sent to the mirror database.
• The mirror database must be initialized from a restore of the principal
database with NORECOVERY, followed by restores in sequence of principal
transaction log backups.
• The mirror database must have the same name as the principal database.
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