3.6.0 MxDB for SQL Server Installation and Administration Guide (5697-7088, December 2007)

Chapter 2: Introduction 7

reenable it. The backup instance will then be available for the other

primary instances.

SQL Monitors and Failover

When you add a SQL instance to a Virtual SQL Server, MxDB for SQL

Server creates a service monitor for that instance. The monitor is active on

the node currently hosting the Virtual SQL Server and periodically

determines whether the SQL service is up.

If the monitor detects that the SQL service is down, MxDB for SQL Server

can attempt to restart the service. (When you create the monitor, you can

specify the number of times that MxDB for SQL Server should try to

restart the service.) If the attempts are not successful, by default the

Virtual SQL Server will fail over to a backup node.

You can customize the failover behavior for each SQL service monitor.

For example, you can use the Policy configuration option to specify

whether the Virtual SQL Server should fail back to the original node or

remain on the backup node after the problem causing the failover has

been corrected.

What Happens During a Failover?

Failovers can be planned (for example, upgrading the operating system)

or unplanned (such as a crash of a primary node). MxDB for SQL Server

takes these actions during a failover:

• Removes the IP address for the Virtual SQL Server from the original

node and then adds it to the backup node.

• Updates registry keys and then starts SQL Server and SQL Agent on

the backup node.

Clients using the IP address of the Virtual SQL Server will now access the

databases on the PSFS filesystem via the SQL instance on the backup

node.

After a Virtual SQL Server fails over, by default it remains on the backup

node. (Optionally, you can configure the Virtual SQL Server to

automatically fail back to the original node.)