Maintenance Mode allows you to temporarily stop alert generation and the collection of some performance metrics for the time period that your SQL Server instance is offline. Using Maintenance Mode allows you to decrease alert notification spam and the collection of unnecessary data.
It is possible to enable or disable maintenance mode on several SQL Server instances, see About Mass Maintenance Mode.
Uses for putting a SQL Server instance into Maintenance Mode
Enable Maintenance Mode before you begin any regularly scheduled maintenance or testing on a monitored SQL Server instance. You can also enable Maintenance Mode when you need to bring a SQL Server instance offline for unplanned diagnostics.
Be aware that the total number of monitored SQL Server instances allowed by your license includes monitored instances that are in Maintenance Mode.
Bring the SQL Server back online in SQL Diagnostic Manager
Disabling Maintenance Mode allows SQL Diagnostic Manager to resume performance statistics collection and alert generation for the selected SQL Server instance. Before you enable Maintenance Mode, make sure you complete all maintenance and testing on the target SQL Server instance.
If you have a SQL Server instance in Maintenance Mode and are ready to bring it back online in SQL Diagnostic Manager, click Maintenance Mode on the Server Overview tab.