Sql-server – Transaction Log maintenance on Mirror database

mirroringsql serversql-server-2008transaction-log

SQL Server Version: 2008 R2 Enterprise SP2

I am trying to get a handle on our SQL Server maintenance and I came across something I think is incorrect. We have a single production instance with 3 databases each being mirrored offsite to a DR instance.

While looking at the DR instance I noticed that the LDF files were huge, over 35GB for the heavily used databases.

I understand that this is probably due to the mirror databases being in Full recovery mode and that the logs have never been backed up, they will just continue to grow until running out of drive space.

We are doing log backups on the principal database, and my question is what are the gotchas with doing a log backup on a mirror?

At least one full database backup from the mirror has to be completed before doing a log backup, in that case are there special options that need to be used because it is a mirror?

Again this is transaction log maintenance recommendations on the MIRROR database.

Thanks for any input

Best Answer

We are doing log backups on the principal database, and my question is what are the gotchas with doing a log backup on a mirror?

You can't do a log backup on the mirror database.

At least one full database backup from the mirror has to be completed before doing a log backup, in that case are there special options that need to be used because it is a mirror?

You can't do a full database backup on the mirror database either.

Take this for instance: I have Server1 which houses the principal database AdventureWorks2012, and I have Server2 which contains the mirror. Here's what happens when I try to run backups on the mirror database (on Server2):

use master;
go

backup database AdventureWorks2012
to disk = 'c:\sqlserver\AW_mirror.bak';
go

Msg 954, Level 14, State 1, Line 2
The database "AdventureWorks2012" cannot be opened. It is acting as a mirror database.
Msg 3013, Level 16, State 1, Line 2
BACKUP DATABASE is terminating abnormally.

backup log AdventureWorks2012
to disk = 'c:\sqlserver\AW_mirror.trn';
go

Msg 954, Level 14, State 1, Line 1
The database "AdventureWorks2012" cannot be opened. It is acting as a mirror database.
Msg 3013, Level 16, State 1, Line 1
BACKUP LOG is terminating abnormally.

Take a look at this Database Mirroring FAQ by Robert Davis. I will quote him regarding this operation and the mirror database transaction log maintenance:

When you backup the log on the principal, the virtual log files (individual units within the log file) are marked as re-writable. The same VLF’s are marked as re-writable in the mirror log file as well. The VLF status is mirrored on the database.

So there you have it. If you have transaction log backups on the principal then similar log reuse behavior will be mirrored to its partner database.

Related Solutions

Sql-server – Transaction log growing on a mirrored principal

Mirror, Synchronized / Restoring... is the typical state for the mirror instance.

You can determine why the log file is growing by issuing the following command in a SQL Server Management Studio query window:

SELECT name, LOG_REUSE_WAIT_DESC 
FROM sys.databases 
WHERE name='your_database_here';

This will show several possible choices, such as:

NOTHING
CHECKPOINT
LOG_BACKUP
ACTIVE_BACKUP_OR_RESTORE
ACTIVE_TRANSACTION
DATABASE_MIRRORING
REPLICATION
DATABASE_SNAPSHOT_CREATION
LOG_SCAN
OTHER_TRANSIENT

You may need to perform at least one transaction log backup, among other actions depending on the LOG_REUSE_WAIT_DESCRIPTION shown in the query above. For instance, there may be a long-running transaction that is forcing the log files to not be re-used.

Sql-server – Transaction Log and mirroring – looking for dumbest explanation possible

The transaction log is a important method to restore your database in a specific time. If you have a large database > 500 GB and if you must restore your database from a full backup, this will cost very much time. Also if you full backup your database everytime, think about how long this backup could take.

A very easy concept for SQL Server can be: Set Recover Model of your Database Full

Create a Maintenance Plan(1) in SQL Server:

Do FullBackup every Week maybe in D:\yourbackup\FullDBBackup.bak
Do Differential Backup every two days in D:\yourbackup\DiffBackup.bak
Do every 2 houers Backup your Transaction log in D:\Yourbackup\Tranlogbackup.trn

Create a Maintenance Plan(2) in SQL Server:

Delete all older Files 8 days from D:\yourBackup*.bak
Delete all older Files 3 Days from D:\yourBackup*.trn

In this case, you are able to recover your Database in a specifc time, very fast very easy. SQL Server will automaticly manage your "Backup" files, older files will be deleted after your specific time range.

I would suggest that you read about SQL Server Transaction Log here:

http://www.sqlservercentral.com/articles/Design+and+Theory/63350/

For using the Maintenance Plans in SQL Server just ask BING / google :D

you should build a small test db and test this before you go in production

Best Answer

Related Solutions

Sql-server – Transaction log growing on a mirrored principal

Sql-server – Transaction Log and mirroring – looking for dumbest explanation possible

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