Fail-over techniques in database mirroring. There are different techniques for database mirroring.
- Automatic fail over
- Manual fail over
- Forced fail over
The following table summarizes which forms of failover are
supported under each of the operating modes.
High performance
|
High-safety mode
without a witness
|
High-safety mode with
a witness
|
|
Automatic failover
|
No
|
No
|
Yes
|
Manual failover
|
No
|
Yes
|
Yes
|
Forced service
|
Yes
|
Yes
|
No
|
We are now looking into Forced Service for database mirroring. This option is available against high safety but automatic
failover is not available. In the event
of failure of the principal, the database service becomes unavailable. You need
manual intervention to make the database service available.
This option is available only under all the following
conditions:
·
The principal server is down.
·
WITNESS is set to OFF or is connected to the mirror server.
Forcing
service suspends the session and starts a new recovery fork. The effect of
forcing service is similar to removing mirroring and recovering the former
principal database. However, forcing service facilitates resynchronizing the
databases (with possible data loss) when mirroring resumes.
To force service in a database mirroring
session when Principal is OFF
1.
Connect to the mirror server.
2.
Issue the following statement:
ALTER DATABASE <database_name> SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS
The
mirror server immediately transitions to principal server, and mirroring is
suspended.
·
In the figure, the original principal server, Partner_A, becomes unavailable to the mirror server, Partner_B, causing the mirror database to be disconnected. After
ensuring thatPartner_A is
not available to clients, the database administrator forces service, with
possible data loss, on Partner_B. Partner_B becomes the principal server and runs with the database exposed (that is, unmirrored). At this point, clients can reconnect
to Partner_B.
To verify that you need to go for following steps:
1. Configure Mirroring without Witness. You can see Principal and Mirror run healthy.
1. Configure Mirroring without Witness. You can see Principal and Mirror run healthy.
2. lets Disconnect principal and look at the mirror status, Mirror shows Disconnected and in recovery.
4. Mirror database become principal and you can access it now.
5. Start Original principal server again and both databases goes into suspended mode
6. Go to Properties of principal database and press resume to
get database back to online but message shows possible data lost.
6. Go to Properties of principal database and press resume to get database back to online. look at the final status here. Mirror has been active as principal and original principal working as mirror server.
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