Редактиране

Споделяне чрез

Shrink a file

  • Статия

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

This article describes how to shrink a data or log file in SQL Server by using SQL Server Management Studio or Transact-SQL.

Shrinking data files recovers space by moving pages of data from the end of the file to unoccupied space closer to the front of the file. When enough free space is created at the end of the file, data pages at end of the file can be deallocated and returned to the file system.

Limitations

  • The primary data file can't be made smaller than the size of the primary file in the model database.

Recommendations

  • A shrink operation is most effective after an operation that creates a large amount of unused storage space, such as a large DELETE statement, truncate table, or a drop table operation.

  • Most databases require some free space to be available for regular day-to-day operations. If you shrink a database file repeatedly and notice that the database size grows again, this indicates that the free space is required for regular operations. In these cases, repeatedly shrinking the database file is a wasted operation. Autogrow events necessary to grow the database file a hinder performance.

  • Data that is moved to shrink a file can be scattered to any available location in the file. This causes index fragmentation and can slow the performance of queries that search a range of the index. To eliminate the fragmentation, consider rebuilding the indexes on the file after shrinking.

  • Unless you have a specific requirement, don't set the AUTO_SHRINK database option to ON.

Remarks

Shrink operations in progress can block other queries on the database, and can be blocked by queries already in progress. Introduced in SQL Server 2022 (16.x), shrink file operations have a WAIT_AT_LOW_PRIORITY option. This feature is a new additional option for DBCC SHRINKDATABASE and DBCC SHRINKFILE. If a new shrink operation in WAIT_AT_LOW_PRIORITY mode can't obtain the necessary locks due to a long-running query already in progress, the shrink operation will eventually time out after one minute and silently exit, preventing other queries from being blocked. WAIT_AT_LOW_PRIORITY applies to data files (.mdf & .ndf). It doesn't apply to transaction log files. For more information, see DBCC SHRINKFILE.

Permissions

Requires membership in the sysadmin fixed server role or the db_owner fixed database role.

Use SQL Server Management Studio (SSMS)

Shrink a data or log file using SSMS

  1. In Object Explorer, connect to an instance of the SQL Server Database Engine and then expand that instance.

  2. Expand Databases and then right-click the database that you want to shrink.

  3. Point to Tasks, point to Shrink, and then select Files.

    Database
    Displays the name of the selected database.

    File type
    Select the file type for the file. The available choices are Data and Log files. The default selection is Data. Selecting a different filegroup type changes the selections in the other fields accordingly.

    Filegroup
    Select a filegroup from the list of Filegroups associated with the selected File type above. Selecting a different filegroup changes the selections in the other fields accordingly.

    File name
    Select a file from the list of available files of the selected filegroup and file type.

    Location
    Displays the full path to the currently selected file. The path isn't editable, but it can be copied to the clipboard.

    Currently allocated space
    For data files, displays the current allocated space. For log files, displays the current allocated space computed from the output of DBCC SQLPERF(LOGSPACE).

    Available free space
    For data files, displays the current available free space computed from the output of DBCC SHOWFILESTATS(fileid). For log files, displays the current available free space computed from the output of DBCC SQLPERF(LOGSPACE).

    Release unused space
    Cause any unused space in the files to be released to the operating system and shrink the file to the last allocated extent, reducing the file size without moving any data. No attempt is made to relocate rows to unallocated pages.

    Reorganize pages before releasing unused space
    Equivalent to executing DBCC SHRINKFILE specifying the target file size. When this option is selected, the user must specify a target file size in the Shrink file to box.

    Shrink file to
    Specifies the target file size for the shrink operation. The size can't be less than the current allocated space or more than the total extents allocated to the file. Entering a value beyond the minimum or the maximum will revert to the min or the max once the focus is changed or when any of the buttons on the toolbar are selected.

    Empty file by migrating the data to other files in the same filegroup
    Migrate all data from the specified file. This option allows the file to be dropped using the ALTER DATABASE statement. This option is equivalent to executing DBCC SHRINKFILE with the EMPTYFILE option. EMPTYFILE is not supported in Azure SQL Database or Azure SQL Database Hyperscale.

  4. Select the file type and file name.

  5. Optionally, select the Release unused space check box.

    Selecting this option causes any unused space in the file to be released to the operating system and shrinks the file to the last allocated extent. This reduces the file size without moving any data.

  6. Optionally, select the Reorganize files before releasing unused space check box. If this is selected, the Shrink file to value must be specified. By default, the option is cleared.

    Selecting this option causes any unused space in the file to be released to the operating system and tries to relocate rows to unallocated pages.

  7. Optionally, enter the maximum percentage of free space to be left in the database file after the database has been shrunk. Permissible values are between 0 and 99. This option is only available when Reorganize files before releasing unused space is enabled.

  8. Optionally, select the Empty file by migrating the data to other files in the same filegroup check box.

    Selecting this option moves all data from the specified file to other files in the filegroup. The empty file can then be deleted. This option is the same as executing DBCC SHRINKFILE with the EMPTYFILE option.

  9. Select OK.

Use Transact-SQL

Shrink a data or log file using Transact-SQL

  1. Connect to the Database Engine.

  2. From the Standard bar, select New Query.

  3. Copy and paste the following example into the query window and select Execute. This example uses DBCC SHRINKFILE to shrink the size of a data file named DataFile1 in the UserDB database to 7 MB.

USE UserDB;
GO
DBCC SHRINKFILE (DataFile1, 7);
GO

Related content