Manually failover all databases in an SQL Server Database Mirroring configuration

How to manually failover an SQL Server database mirroring session? A manual SQL Server failover can only be initiated from the principal server, and the mirrored database must be synchronized (that is, when the database is in the SYNCHRONIZED state). In this article I'll show you how to manual failover mirrored SQL Server databases using Transact-SQL (T-SQL) in SQL Server 2017+.

Performing such a failover, using T-SQL, comes in handy when you are installing Windows Updates and SQL Server Cumulative Updates, and you need to reboot the database server. It is recommended to failover the databases on your principal SQL Server to the slave before you reboot the server.

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Query the current state of each database in the mirroring set-up

The following T-SQL query shows the current state of each database in the mirroring configuration.

-- ==============================================================
-- query that shows the current state of each database in the mirroring set-up
-- ==============================================================
  SELECT   db_name(sd.[database_id])              AS [Database Name]
          ,sd.mirroring_state                  AS [Mirror State]
          ,sd.mirroring_state_desc             AS [Mirror State Desc] 
          -- 1 = Principal, 2 = Mirror
          ,sd.mirroring_role              AS [Mirror Role]
          ,sd.mirroring_role_desc              AS [Mirror Role Desc]
          ,sd.mirroring_partner_name           AS [Partner Name]
          ,sd.mirroring_safety_level_desc      AS [Safety Level]
          ,sd.mirroring_witness_name           AS [Witness]
          ,sd.mirroring_connection_timeout AS [Timeout(sec)]
    FROM sys.database_mirroring AS sd
    WHERE mirroring_guid IS NOT null
    ORDER BY [Database Name];

When executed against the principal, the Partner Name column identifies the partner, or mirror, server. The Mirror State must not be null or zero.

Everything looks OK?

Perform a failover for all SQL Server databases in a mirroring set-up

You can execute the following T-SQL query to initiate the failover, and to manually fail over database mirroring.

--
-- Perform a failover for all databases in a mirroring set-up
--

declare @databasename nvarchar(255)
declare @alldatabases cursor

-- Only select principal databases (mirroring_role).
set @alldatabases = cursor for 
        select d.name
        from sys.databases d, sys.database_mirroring m
        where m.database_id = d.database_id
        and m.mirroring_role_desc = 'PRINCIPAL'

-- Execute the failover.
open @alldatabases
fetch next from @alldatabases into @databasename
while @@FETCH_STATUS = 0
begin
        print @databasename
        exec('alter database [' + @databasename + '] set partner failover')
        fetch next from @alldatabases into @databasename                                                                                                                                                          
end                                                                                                                                                                                                                
close @alldatabases                                                                                                                                                                                               
deallocate @alldatabases

Once executed and ready, your former principal server is now the partner SQL Server, and can be safely rebooted.

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SQL Server Mirroring connectivity flow diagram

In a SQL Server Mirroring setup, both the Principal (master/primary) and Mirror (secondary) database servers have a connection with each other and a connection with the Witness server.

When the Principal server loses its connection with the Witness server, or misses a "ping", an automatic failover is initiated and the Mirror becomes the Principal database server for databases. See Database Mirroring (SQL Server) on Microsoft technical documentation for more information.

Check mirroring failover progress

You can use the following T-SQL to check the progress of your failover:

SELECT COUNT(d.name)
	FROM sys.databases d, sys.database_mirroring m
	WHERE m.database_id = d.database_id
	AND m.mirroring_role_desc = 'PRINCIPAL'

This query must be executed against the current PRINCIPAL SQL Server, not the one you are failing over to.

Check SQL Server mirroring state

Use the this T-SQL to check the current mirroring state of all databases (except 'model', 'master', 'tempdb', 'distribution', 'msdb'):

SELECT d.name, m.mirroring_state
  FROM sys.databases d, sys.database_mirroring m
  WHERE d.name NOT IN ('model', 'master', 'tempdb', 'distribution', 'msdb')
  AND m.mirroring_state IS NOT NULL

You should see for mirroring_state = 4 for all databases on your principal server, as that's SYNCHRONIZED.

mirroring_state	State of the mirror database and of the database mirroring session
0	Suspended
1	Disconnected from the other partner
2	Synchronizing
3	Pending Failover
4	Synchronized
5	The partners are not synchronized. Failover is not possible now.
6	The partners are synchronized. Failover is potentially possible. For information about the requirements for failover see, Database Mirroring Operating Modes.
NULL	Database is inaccessible or is not mirrored.

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