Database Reference
In-Depth Information
Another example where you should increase this setting is when you set the database mirroring with Failover
Cluster instance of SQL Server as one of database mirroring partners. The cluster failover process usually takes longer
than 10 seconds, and keeping the default
PARTNER TIMEOUT
setting can trigger unnecessary database mirroring
failovers. You should set the
PARTNER FAILOVER
value to be greater than the typical cluster failover time.
You should factor database mirroring into the transaction log maintenance routine. Database mirroring requires
the database to be in the FULL recovery model. Moreover, SQL Server defers log truncation until all records from
VLF are sent to the mirror server. While in most cases this does not introduce any issues with log management
under normal circumstances, this is not the case when the mirror server is offline. The send queue will grow and the
transaction log will not be able to truncate until the mirror server is online again and log records are transmitted over
the network. This is another reason why the send queue must be constantly monitored.
■
Tip
Consider dropping database mirroring and recreating it later in the case of prolonged mirror server downtime.
The database on the mirror server stays in RESTORING state; therefore, clients are unable to access it. However,
it is possible to create a read-only database snapshot on the mirror server accessing it for reporting purposes. This
snapshot represents the database as of the last checkpoint on the primary server.
■
Coverage of database snapshots is beyond the scope of the topic. you can read more about it at:
Note
■
Important
Using database snapshots on the mirror server requires you to obtain a sQl server license for that server.
One of the very useful features of database mirroring is automatic page repair. In the case of synchronous
database mirroring, when SQL Server detects that a data page is corrupted, it replaces this corrupted page with a fresh
copy of the page from another server. This is an asynchronous process, and a query that accessed the corrupted page
and triggered a page repair could be interrupted and receive an error until the page is repaired in background.
■
Note
you can read more about automatic page repair at:
http://technet.microsoft.com/en-us/library/bb677167.aspx
.
Database mirroring supports the situation where the mirror server is running a newer version of SQL Server.
For example, you can have the principal server running SQL Server 2008 and the mirror server running SQL Server 2012.
This is an extremely useful feature, which allows you to upgrade SQL Server
almost
transparently to your users. You
can perform an in-place upgrade of the mirror server, failover, and upgrade the former principal server.
■
Important
Keep in mind that it is impossible to fail back to the older version of sQl server after failover.
remember to update all statistics in the database with the
sp_updatestats
stored procedure after an upgrade.
