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Database snapshots with Always On availability groups (SQL Server)

Applies to: SQL Server

You can create a database snapshot on a primary or secondary database in an availability group. The replica role must be either PRIMARY or SECONDARY, and can't be in the RESOLVING state.

Note

Creating database snapshots on any database adds CPU and I/O overhead because of copy-on-write activity. On database replicas, this overhead can reduce redo throughput and affect other operations, especially as the number of snapshots increases.

You should create database snapshots when the database synchronization state is SYNCHRONIZING or SYNCHRONIZED. However, you can still create database snapshots when the database synchronization state is NOT SYNCHRONIZING.

A database snapshot on a secondary replica continues to work if the replica is DISCONNECTED from the primary replica.

Some Always On availability groups conditions cause both the source database and its database snapshots to restart, temporarily disconnecting users. These conditions are as follows:

  • The primary replica changes roles. This change can happen because the current primary replica goes offline and comes back online on the same server instance, or because the availability group fails over.

  • The database enters the secondary role.

If the availability replica that hosts database snapshots fails over, the database snapshots remain on the server instance where you created them. You can continue to use the snapshots after the failover. If performance is a concern in your environment, create database snapshots only on secondary databases hosted by a secondary replica that is configured for manual failover mode.

If you ever manually fail over the availability group to this secondary replica, you can create a new set of database snapshots on another secondary replica, redirect clients to the new database snapshots, and drop all of the database snapshots from the now primary databases.

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