Recover from a full transaction log in a SQL Server database

This article describes the steps that you must follow when the transaction logs become too large. Full transaction logs can make your Microsoft SQL Server database unusable. This article describes how to truncate and shrink the transaction logs, and how to prevent the transaction logs from growing unexpectedly.

Reduce the transaction log size

To recover from a situation where the transaction logs are full, you must reduce the size of the transaction logs. To do this, you must truncate the inactive transactions in your transaction log and shrink the transaction log file.

Note The transaction logs are very important to maintain the transactional integrity of the database. Therefore, you must not delete the transaction log files even after you make a backup of your database and the transaction logs.

Truncate the inactive transactions in your transaction log

When the transaction logs are full, you must immediately back up your transaction log file. While the backup of your transaction log files is created, SQL Server automatically truncates the inactive part of the transaction log. The inactive part of the transaction log file contains the completed transactions, and therefore, the transaction log file is no longer used by SQL Server during the recovery process. SQL Server reuses this truncated, inactive space in the transaction log instead of permitting the transaction log to continue to grow and to use more space.

For additional information about the issues that you must consider when you make a backup of the transaction logs and the issues that you must consider when you restore the transaction log backups, visit the following topics in SQL Server Books Online:

• Transaction log backups
• Transaction log backup and restore

You can also delete the inactive transactions from a transaction log file by using the Truncate method. For additional information about truncating the transaction logs, see the "Truncating the transaction log" topic in SQL Server Books Online.

Important After you manually truncate the transaction log files, you must create a full database backup before you create a transaction log backup.

For additional information about the issues that may occur when you truncate the transaction log files, click the following article number to view the article in the Microsoft Knowledge Base:

Shrink the transaction log file

The backup operation or the Truncate method does not reduce the log file size. To reduce the size of the transaction log file, you must shrink the transaction log file. To shrink a transaction log file to the requested size and to remove the unused pages, you must use the DBCC SHRINKFILE operation. The DBCC SHRINKFILE Transact-SQL statement can only shrink the inactive part inside the log file.

Note The DBCC SHRINKFILE Transact-SQL statement cannot truncate the log and shrink the used space inside the log file on its own.

For more information about shrinking the transaction log files, see the following topics in SQL Server Books Online:

• Shrinking the transaction log
• DBCC SHRINKFILE

For additional information about how to shrink the transaction log files in SQL Server 2000, click the following article number to view the article in the Microsoft Knowledge Base: For additional information about the problems that may occur when you shrink the transaction log files, click the following article numbers to view the articles in the Microsoft Knowledge Base:

Prevent the transaction log files from growing unexpectedly

To prevent the transaction log files from growing unexpectedly, consider using one of the following methods:

• Set the size of the transaction log files to a large value to avoid the automatic expansion of the transaction log files.
• Configure the automatic expansion of transaction log files by using memory units instead of a percentage after you thoroughly evaluate the optimum memory size.
  
  For additional information about the issues to consider when you configure the autogrow option, click the following article number to view the article in the Microsoft Knowledge Base:
  315512  (http://support.microsoft.com/kb/315512/ ) Considerations for autogrow and autoshrink configuration
• Change the recovery model. If a disaster or data corruption occurs, you must recover your database so that the data consistency and the transactional integrity of the database are maintained. Based on how critical the data in your database is, you can use one of the following recovery models to determine how your data is backed up and what your exposure to the data loss is:
  • Simple recovery model
  • Full recovery model
  • Bulk-logged recovery model
  By using the simple recovery model, you can recover your database to the most recent backup of your database. By using the full recovery model or the bulk-logged recovery model, you can recover your database to the point when the failure occurred by restoring your database with the transaction log file backups.
  
  By default, in SQL Server 2000 and in SQL Server 2005, the recovery model for a SQL Server database is set to the Full recovery model. With the full recovery model, regular backups of the transaction log are used to prevent the transaction log file size from growing out of proportion to the database size. However, if the regular backups of the transaction log are not performed, the transaction log file grows to fill the disk, and you may not be able to perform any data modification operations on the SQL Server database.
  
  You can change the recovery model from full to simple if you do not want to use the transaction log files during a disaster recovery operation.
• Back up the transaction log files regularly to delete the inactive transactions in your transaction log.
• Design the transactions to be small.
• Make sure that no uncommitted transactions continue to run for an indefinite time.
• Schedule the Update Statistics option to occur daily.
• To defragment the indexes to benefit the workload performance in your production environment, use the DBCC INDEXDEFRAG Transact-SQL statement instead of the DBCC DBREINDEX Transact-SQL statement. If you run the DBCC DBREINDEX statement, the transaction log may expand significantly when your SQL Server database is in Full recovery mode. Additionally, the DBCC INDEXDEGRAG statement does not hold the locks for a long time, unlike the DBCC DBREINDEX statement.
  
  For additional information about defragmenting the indexes in SQL Server 2000, see the following Microsoft website: 
  Microsoft SQL Server 2000 index defragmentation best practices (http://technet.microsoft.com/en-us/library/cc966523.aspx)
  If you must run the DBCC DBREINDEX statement as a job that is a part of the database maintenance plan, you must break up the job into multiple jobs. Additionally, you must take the frequent backups for the transaction logs between the execution of the jobs.

More information about transaction log files

In SQL Server 2000 and in SQL Server 2005, each database contains at least one data file and one transaction log file. SQL Server stores the data physically in the data file. The transaction log file stores the details of all the modifications that you perform on your SQL Server database and the details of the transactions that performed each modification. Because the transactional integrity is considered a fundamental and intrinsic characteristic of SQL Server, logging the details of the transactions cannot be turned off in SQL Server.

The transaction log file is logically divided into smaller segments that are referred to as virtual log files. In SQL Server 2000, you can configure the transaction log file to expand as needed. The transaction log expansion can be governed by the user or can be configured to use all the available disk space. Any modifications that SQL Server makes to the size of the transaction log file, such as truncating the transaction log files or growing the transaction log files, are performed in units of virtual log files.

If the transaction log file that corresponds to a SQL Server database is filled and if you have set the option for the transaction log files to grow automatically, the transaction log file grows in units of virtual log files. Sometimes, the transaction log file may become very large and you may run out of disk space. When a transaction log file grows until the log file uses all the available disk space and cannot expand any more, you can no longer perform any data modification operations on your database. Additionally, SQL Server may mark your database as suspect because of the lack of space for the transaction log expansion.

For additional information about the scenarios that may cause the transaction log file to grow unexpectedly, click the following article number to view the article in the Microsoft Knowledge Base:

For additional information about troubleshooting the additional disk space requirement during the recovery process, see the "Insufficient disk space" topic in SQL Server Books Online. For additional information about the transaction log architecture, see the following topics in SQL Server Books Online:

• Transaction log architecture
• Transaction log logical architecture
• Transaction log physical architecture

For more information about the recovery models in SQL Server 2000, see the following topics in SQL Server Books Online:

• Selecting a recovery model
• Simple recovery
• Full recovery
• Bulk-logged recovery
• Switching recovery models