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.
Reduce the size of the transaction logs
To recover from a situation where the transaction logs grow to an unacceptable limit, 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 grow to an unacceptable limit, 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.
You can also delete the inactive transactions from a transaction log file by using the Truncate method.
Important After you manually truncate the transaction log files, you must create a full database backup before you create a transaction log backup.
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.
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.
- 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.