Transaction log truncation for Exchange Server

About Microsoft Exchange Server log

Before committing a transaction to a database file, Exchange logs it to a transaction log file. To track which of the logged transactions have been committed to the database, Exchange uses checkpoint files. Once the transactions are committed to the database and tracked by the checkpoint files, the log files are no longer needed by the database.

If log files are not deleted, they will eventually consume all the available disk space and the Exchange databases will be taken offline until the log files are purged from the disk. Using circular logging is not a best practice for a production environment. When circular logging is enabled, Exchange overwrites the first log file after its data has been committed to the database, and you can recover data only up until the last backup.

We recommend that you delete the log files after backing up an Exchange server, because log files are backed up along with other files. Therefore, after a recovery you will be able to roll the database back or forward.

For more information about transaction logging see http://technet.microsoft.com/en-us/library/bb331958.aspx.

Log truncation by using the Enable VSS Full backup option

The easiest method of log truncation is to use the Enable VSS Full backup backup option (Options > Default backup and recovery options > Default backup options > Volume Shadow Copy Service > Enable VSS Full backup). It is recommended in most cases.

If enabling this option is undesirable (for example, you need to keep logs of another VSS-aware application running on the machine), follow the recommendations below.

Log truncation of offline databases

After normal shutdown the database state is considered consistent and the database files are self-contained. This means that you can delete all the log files of the database or the storage group.

To delete transaction log files:

1. Dismount the database (in Exchange 2010) or all databases of the storage group (in Exchange 2003/2007). For more information, see:
   • Exchange 2010: http://technet.microsoft.com/en-us/library/bb123903
   • Exchange 2007: http://technet.microsoft.com/en-us/library/bb124936(v=exchg.80)
   • Exchange 2003: http://technet.microsoft.com/en-us/library/aa996179(v=exchg.65)
2. Delete all the log files of the database or the storage group.
3. Mount the dismounted database or databases.

   For more information, see:

   • Exchange 2010: http://technet.microsoft.com/en-us/library/bb123587.aspx
   • Exchange 2007: http://technet.microsoft.com/en-us/library/aa998871(v=exchg.80).aspx
   • Exchange 2003: http://technet.microsoft.com/en-us/library/aa995829(v=exchg.65)

Log truncation of online databases

This method is good for the databases that are in constant use and cannot be dismounted. If a database is in use, you can safely delete only those transaction log files whose data has been committed to the database. Do not delete log files whose data has not been committed to the database, they are essential to recover the database consistency from unexpected shutdown.

To delete the committed transaction logs

1. Determine which logs have been committed to the database by using the Eseutil tool:
   1. Execute the eseutil /mk <path to checkpoint file> command, where the <path to checkpoint file> is a path to the checkpoint file of the required database or the storage group.
   2. Look at the Checkpoint field in the output. For example, you should see something like this:

      CheckPoint: (0x60B, 7DF, 1C9)

   The first number 0x60B is the hexadecimal log generation number of the current log file. This means that all the log files with lesser numbers have been committed to the database.

2. Delete all the log files whose numbers are less than the number of the current log file. For example, you can safely delete Enn0000060A.log, Enn00000609.log and the lesser files.

Log truncation after a backup

You can automate the above truncation procedure by using a script. If you add the script to the Post-backup command, the logs will be truncated immediately after a backup.

This method assumes that you have scripting skills and are familiar with Acronis Backup command-line utility (acrocmd). For detailed information about acrocmd see the Command-Line Reference.

The script should contain the following steps:

1. Mount the volumes containing the necessary database files by using the mount command.

   Template:

   acrocmd mount --loc=<path> --credentials=<user name>,<password> --arc=<archive name> --volume=<volume numbers> --letter=<letters>

   Example:

   acrocmd mount --loc=\\bkpsrv\backups --credentials=user1,pass1 --arc=my_arc --volume=1-1 --letter=Z

2. In the mounted volumes, determine which logs have been committed to the database by using the Eseutil tool. The procedure is described in step 1 of "Log truncation of online databases" above.
3. In the respective online database or storage group, delete all the log files whose numbers are less than the number of the current log file in the backup.
4. Unmount the mounted volumes by using the umount command.