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Chapter 27: Database Mirroring
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■ (^) By default, it does not support reporting. If you want to use the mirror database
for reporting purposes, you can create a database snapshot (refer to Chapter 24,
“Database Snapshots,” for details) on the mirror database and use the database
snapshot for reporting purposes. Alternatively, you can use AlwaysOn Availability
Groups to create read-only Secondary Replicas. See the “High Availability/lwaysOn”
section for more details.
If you use the mirror database for reporting purposes, you need to fully license the mirror server. On the other hand, if
the mirror server is used as a hot standby or passive server, (as per Microsoft SQL Server 2012 Pricing and Licensing
guide) a license is not required, provided that the number of processors in the passive server is equal to or less than
those of the active server. The passive server can take the duties of the active server for 30 days. Afterward, it must
be licensed accordingly.
■ (^) It provides options for no data loss for committed transactions.
■ (^) It can provide an almost instantaneous database failover solution by using an
optional server called a witness.
■ (^) There is no real distance limitation between the principal and mirror servers.
■ (^) It can be implemented without exotic hardware and may be signifi cantly cheaper
than other high availability solutions such as SQL Server failover clustering. SQL
Server failover clustering provides high availability for the entire SQL Server
instance, whereas database mirroring provides high availability only at the
database level. If you plan to use AlwaysOn Availability Groups, Windows Server
Failover Clustering (WSFC) is required.
■ (^) It can complement existing log shipping and failover clustering implementations.
■ (^) If your applications use ADO.NET or the SQL Server Native Client to connect to a
database, if a failure occurs, the applications can automatically redirect the clients
to the mirror database.
However, database mirroring has a few drawbacks:
■ (^) Only user databases in full recovery model can be used for database mirroring.
The Simple or bulk-logged recovery model cannot be used. This also means
that database mirroring can break if the recovery model for a mirrored database is
changed from the full to simple or bulk-logged recovery model.
■ (^) System databases (master, model, msdb, and tempdb) cannot be mirrored.
■ (^) Database mirroring does not support FILESTEAM. This means that databases with
FILESTREAM fi legroup cannot be mirrored, nor can you create a FILESTREAM fi le-
group on a principal database.
■ (^) Database mirroring does not support cross-database transactions or distributed
transactions.
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