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Chapter 27: Database Mirroring
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■ (^) The principal server is the main production SQL Server to which clients connect.
This server contains the database that you want to create as a duplicate, hot
standby, or mirror copy. The initial full database backup is taken on this server.
■ The mirror server is the hot standby SQL Server. If the principal server fails, the
mirror server becomes the new principal server. This server must meet the mini-
mum performance requirements during a short-term crisis. If your business does
not allow any performance degradation, the mirror server should be similar (same
CPU and memory confi guration) to the principal server.
The principal server and mirror server are often referred to as partners in a data-
base mirroring session.
■ (^) The witness server is an optional separate SQL Server and is required only when
automatic failover is required. The witness server helps create a quorum to ensure
that only one SQL Server (either the principal server or the mirror server) owns the
database accessible by the clients. The witness controls automatic failure to the
mirror if the principal becomes unavailable.
One witness server can be used for multiple database mirroring sessions, each for different databases and different
partners.
Transaction log records are synchronously or asynchronously transferred from the principal
server to the mirror server, based on the transaction safety level that you select for the
database mirroring session. Database Mirroring has two transaction safety levels:
safety full and safety off, as detailed in Table 27-1. The failover options are
described in the “Role Switching” section later in this chapter.
TABLE 27-1 Database Mirroring Safety Levels and Failover Options
Safety Level Operating Mode Failover Mode
Witness
Server Required
FULL Synchronous Database Mirroring Automatic and Manual Yes
FULL Synchronous Database Mirroring Manual and Forced No
OFF Asynchronous Database Mirroring Forced N/A
Safety full is often referred to as synchronous database mirroring or high-safety mode.
In this mode, the mirror database is in sync with the principal database all the time, and
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