Export (0) Print
Expand All
11 out of 12 rated this helpful - Rate this topic

Tail-Log Backups

This topic is relevant only for databases that are using the full or bulk-logged recovery models.

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. For more information, see "Restoring Without Using a Tail-Log Backup" later in this topic.

Like any log backup, a tail-log backup is taken by using the BACKUP LOG statement. 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

    NoteNote

    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.

The following table summarizes these options.

BACKUP LOG option

Comments

NORECOVERY

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.

CONTINUE_AFTER_ERROR

Use CONTINUE_AFTER_ERROR only if you are backing up the tail of a damaged database.

NoteNote
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.

To create a transaction log backup when the database is damaged

Tail log backups capture the tail of the log even if the database is offline, damaged, or missing data files. This might cause incomplete metadata from the restore information commands and msdb. However, only the metadata is incomplete; the captured log is complete and usable.

If a tail-log backup has incomplete metadata, in the backupset table, has_incomplete_metadata is set to 1. Also, in the output of RESTORE HEADERONLY, HasIncompleteMetadata is set to 1.

If the metadata in a tail-log backup is incomplete, the backupfilegroup table will be missing most of the information about filegroups at the time of the tail-log backup. Most of the backupfilegroup table columns are NULL; the only meaningful columns are as follows:

  • backup_set_id

  • filegroup_id

  • type

  • type_desc

  • is_readonly

Restore scenarios in which a tail-log backup is unnecessary include the following:

Updated content

Updated the "Backing Up the Tail of the Log" section to correct information about taking a tail-log backup if the database is offline and does not start.

Did you find this helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

ADD
Show:
© 2014 Microsoft. All rights reserved.