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Thursday, 31 March 2016

Tail log Backup in SQL Server

Tail log Backup in SQL Server

In most cases, under the full or bulk-logged recovery models, SQL Server 2005 and later versions require that you back up the tail of the log to capture the log records that have not yet been backed up
A log backup taken of the tail of the log just before a restore operation is called a tail-log backup.
SQL Server 2005 and later versions usually require that you take a tail-log backup before you start to restore a database. The tail-log backup prevents work loss and keeps the log chain intact. When you are recovering a database to the point of a failure, the tail-log backup is the last backup of interest in the recovery plan. If you cannot back up the tail of the log, you can recover a database only to the end of the last backup that was created before the failure.
Not all restore scenarios require a tail-log backup. You do not have to have a tail-log backup if the recovery point is contained in an earlier log backup, or if you are moving or replacing (overwriting) the database and do not need to restore it to a point of time after the most recent backup. Also, if the log files are damaged and a tail-log backup cannot be created, you must restore the database without using a tail-log backup. Any transactions committed after the latest log backup are lost.
We recommend that you take a tail-log backup in the following situations:
  • If the database is online and you plan to perform a restore operation on the database, before starting the restore operation, back up the tail of the log using WITH NORECOVERY:
    BACKUP LOG database_name TO <backup_device> WITH NORECOVERY
    To avoid an error, the NORECOVERY option is necessary.

  • If the database is offline and does not start.
    Try to take a tail-log backup. Because no transactions can occur at this time, using WITH NORECOVERY is optional. If the database is damaged, use WITH CONTINUE_AFTER_ERROR, as follows:
    BACKUP LOG database_name TO <backup_device> WITH CONTINUE_AFTER_ERROR
    If the database is damaged, for example, if the database does not start, a tail-log backup succeeds only if the log files are undamaged, the database is in a state that supports tail-log backups, and the database does not contain any bulk-logged changes.

Use NORECOVERY whenever you intend to continue with a restore operation on the database. NORECOVERY takes the database into the restoring state. This guarantees that the database does not change after the tail-log backup.
The log is truncated unless the NO_TRUNCATE option or COPY_ONLY option is also specified.
Important noteImportant
We recommend that you avoid using NO_TRUNCATE, except when the database is damaged.
Use CONTINUE_AFTER_ERROR only if you are backing up the tail of a damaged database.
When you use back up the tail of the log on a damaged database, some of the metadata ordinarily captured in log backups might be unavailable. For more information, see "Tail-Log Backups with Incomplete Backup Metadata," later in this topic.