Backing up the tail

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Recovery models
Main backup types
Backing up the database files by copying
The transaction log
Transaction log restore sequence
Log sequence numbers
Truncating and shrinking the transaction log
Backing up the tail
Inside the transaction log
So, what's in a backup file?
Test: A full backup does not contain deleted data
Verifying backup files
Verifying backup files on a budget
Cumulative backups
Recovering individual tables
Backup and restore history details
Backup reads and writes
Speeding up backups
Backup speed details
Speeding up restores
Restore state affects speed too
Backup and restore rights
Log shipping
Log shipping in SQL Server 2000
Setting up log shipping using Enterprise Manager
Checking the set up
Log shipping in SQL Server 2005
Setting up log shipping using Management Studio
Checking the set up
Log shipping status report
Log shipping in SQL Backup
Using the CopyTool utility
3rd party backup applications
VDI versions
VDI errors
SQL Backup - beyond compression
Restoring a chain of transaction log backups
Restoring to the latest possible state
Backing up multiple databases
Backup retention
Making a copy of the backup file
Backup file naming conventions
Restoring the latest backup set
Network resilience
Integrated database verification
Database file relocation
Improved backup retention
High-availability group support
Common SQL Backup issues
Installation checklist
Setting up rights
Configuring service rights
Backup data
Hanging issues
Common backup and restore errors
Error 3201 - when performing a backup to a network share
Full database backup file is larger than database size
Error 3205 - Too many backup devices specified for backup or restore
Error 4305 - an earlier transaction log backup is required
Bringing a database that is in recovery or read-only mode online
Using bulk-logged recovery model but transaction log backup is still large
Error 14274 - unable to delete SQL Server Agent job
Error messages when restoring from different versions of SQL Server.
vdi error codes
Restore speed details
Help, my transaction log file is huge!
Mirror or log ship

The tail of the transaction log usually refers to the contents of the database's transaction log that has not been backed up.  Basically, every time you perform a transaction log backup, you are backing up the tail of the transaction log.

Then why all the fuss over this?  Well, the complication starts when the database's data files are no longer available, perhaps due to a media failure.  When this occurs, the next logical step is to back up the current transaction log, and apply this backup to the standby database.  You can back up the transaction log even though the data files are no longer available, using the NO_TRUNCATE option e.g.

BACKUP LOG AdventureWorks TO DISK = 'G:\Backups\AdventureWorks_log_tail.bak' WITH NO_TRUNCATE

You can then use the resulting log backup to bring the standby database to the state the database was in before the failure.

This is another good reason to place your transaction log files on different disks from the data files.  If they were on the same disks, a disk failure would prevent you from taking a backup of the transaction log.

Another complication is when your database is using the bulk-logged recovery model, and the current transaction log contains minimally logged transactions.  In this situation, a transaction log backup needs to store the modified data pages (extents).  If the data files are not available, you cannot back up the transaction log, even with the NO_TRUNCATE option.

Lastly, in SQL Server 2005 and above, every time you try to restore a database which already exists, is using the full or bulk-logged recovery models, and the transaction log contains active transactions, an error similar to the following is displayed:

Server: Msg 3159, Level 16, State 1, Line 1
The tail of the log for the database "AdventureWorks" has not been backed up. Use BACKUP LOG WITH NORECOVERY to backup the log if it contains work you do not want to lose. Use the WITH REPLACE or WITH STOPAT clause of the RESTORE statement to just overwrite the contents of the log.
Server: Msg 3013, Level 16, State 1, Line 1
RESTORE DATABASE is terminating abnormally.

This is just SQL Server's way of telling you that there are log records in the transaction log that have not been backed up. If the current transaction log can be discarded, you can use the REPLACE option to tell SQL Serer to ignore the current transaction log e.g.

RESTORE DATABASE AdventureWorks FROM DISK = 'G:\Backups\AdventureWorks_full.bak' WITH REPLACE

Document history
7/23/2008    Initial release.    
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