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Chapter 27: Database Mirroring
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Also from this page you can access the following from the links in the top-right side of the
page:■ (^) Start Failover Wizard: Opens the Failover Wizard that enables you to manually
failover your groups to make an existing secondary replica a primary replica in
the availability group.
■ View AlwaysOn Health States: Opens the Extended Events viewer. Here you can
review the AlwaysOn-related events fi red by the server.
■ View Cluster Quorum Information: Opens the Cluster Quorum Information
screen. This screen shows you members of the cluster, member type, member
state, and quorum vote counts.
Summary
Database mirroring is an inexpensive software-based solution to achieve high database
availability. Database mirroring works by transferring transaction log records from the
production database on one SQL Server to a mirror database on another SQL Server over
the network, either synchronously or asynchronously. Asynchronous database mirroring
provides the best performance but has potential for data loss and does not support auto-
matic failover. Synchronous database mirroring provides a higher level of data protection
than asynchronous mode and provides automatic and manual failover options at the cost of
reduced application performance.AlwaysOn Availability Groups is a robust high-availability solution that offers a scalable
and fl exible architecture for your databases. Availability groups work much like mirroring;
however, you can transfer and failover one or more databases together in a group. Through
the use of Windows Failover Cluster Service (WSFC), AlwaysOn Availability Groups can main-
tain high availability and scalability across your network.At fi rst glance, database mirroring or AlwaysOn Availability Groups may appear to be bet-
ter than log shipping, but it is not a replacement for log shipping. Each solution has unique
features, and depending on your business requirements, you may require selecting one
or both the solutions. For example, log shipping supports multiple copies of the produc-
tion databases, but database mirroring allows only one copy of the production database. If
you need multiple copies of the production database and the features of database mirror-
ing, you can implement database mirroring and log shipping, or you can set up AlwaysOn
Availability Groups. Database mirroring also complements existing failover clustering
implementations.As with any other solution, you should take a performance baseline of your environment
before and after confi guring database mirroring, and use your performance data to help
you determine whether you should use database mirroring or whether you should use syn-
chronous mode or asynchronous mode in your production environment.c27.indd 725c27.indd 725 7/31/2012 9:50:34 AM7/31/2012 9:50:34 AM
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