SQLServer2012镜像主库挂掉如何切换到镜像备库
主节点的主库服务挂了，无法启动。
强行启动从库。
在备库执行：
USE master;
ALTER DATABASE ec SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS;
之后会恢复两分钟，备机的数据库就能用了。
数据库日志：
The mirrored database "ec" is changing roles from "MIRROR" to "PRINCIPAL" because the mirroring session or availability group failed over due to FORCE_SERVICE_ALLOW_DATA_LOSS. This is an informational message only. No user action is required.
Starting up database 'ec'.
1 transactions rolled forward in database 'ec' (7:0). This is an informational message only. No user action is required.
0 transactions rolled back in database 'ec' (7:0). This is an informational message only. No user action is required.
CHECKDB for database 'ec' finished without errors on 2016-09-01 15:33:43.907 (local time). This is an informational message only; no user action is required.
如果主机恢复了就切回去。
--原来的主服务器恢复,可以继续工作,需要重新设定镜像，第一次可能会报个错
在备库执行：
USE master;
ALTER DATABASE ec SET PARTNER RESUME; --恢复镜像
ALTER DATABASE ec SET PARTNER FAILOVER; --切换主备
主备切换完成。
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