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Microsoft SQL Server 2008 R2 : Using Replication and Database Mirroring Together, Using Database Snapshots from a Mirror for Reporting

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6/28/2012 5:26:29 PM

Using Replication and Database Mirroring Together

SQL Server 2008 allows you to use combinations of options to achieve higher availability levels. A prime example would be to combine data replication with database mirroring to provide maximum availability of data, scalability to users, and fault tolerance via failover, potentially at each node in a replication topology. By starting with the publisher and perhaps the distributor, you make them both database mirror failover configurations. Figure 1 shows a possible data replication and database mirroring configuration (database mirroring of the publisher and database mirroring of the distributor).

Figure 1. Rolling out database mirroring failover within data replication for scalability, availability, and fault tolerance.

Using database mirroring and replication together is essentially the best of both worlds: you get the super-low latency of database mirroring for fault tolerance, and you get high availability (and scalability) of data through replication. The downside of this type of combined capability is that it requires additional servers (for mirroring of the databases). The upside is the increased scalability and resilience of your applications.

Using Database Snapshots from a Mirror for Reporting

A powerful configuration to help offload reporting workload is to use database snapshots with database mirroring. A database snapshot is a highly efficient feature of SQL Server 2008 that allows for the generation and use of a read-only, stable view of a database at a moment in time (hence, it’s called a snapshot). The database snapshot is also created without the overhead of creating a complete copy of the database or having completely redundant storage. A database snapshot is simply a reference point of the pages used in the database (that is defined in the system catalog). When pages are updated, a new page chain is started that contains the data pages changed since the database snapshot was taken, as illustrated in Figure 2.

Figure 2. Database snapshots and the original database share pages and are managed within the system catalog of SQL Server 2008.

As the original database diverges from a snapshot, the snapshot gets its own copy of original pages when they are modified. The copy-on-write technology used for database mirroring also enables a database snapshot. When a database snapshot is created on a database (a mirror database, in this case), all writes check the system catalog of changed pages first; if the snapshot is not there, the original page is copied (using copy-on-write) and is put in a place for reference by the database snapshot (because the snapshot must be kept intact). In this way, a database snapshot and the original database share the data pages that have not changed.

Unlike a mirror database, a database snapshot can be accessed by a reporting client in read-only mode, as shown in Figure 3. As long as the mirror server is communicating to the principal, reporting clients can access the snapshot database.

Figure 3. A database snapshot defined from a mirror server for reporting use.

If the principal fails over to the mirror server, the connections to the snapshot database are disconnected during the database restart process (which makes the mirror server the new principal server). It is possible to reconnect the reporting clients to the database snapshot after a failover is completed, but you must remember that now both the transactional clients and reporting clients are connected to a single SQL Server instance. This may not be acceptable from a performance point of view. Also, it is always a good idea to keep the number of snapshots to a minimum when creating them against a database mirror.

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