DBCC SHRINKFILE (Transact-SQL)

Artikkeli
12/03/2008

Shrinks the size of the specified data or log file for the current database or empties a file by moving the data from the specified file to other files in the same filegroup, allowing the file to be removed from the database. You can shrink a file to a size that is less than the size specified when it was created. This resets the minimum file size to the new value.

Topic link icon Transact-SQL Syntax Conventions

Syntax

DBCC SHRINKFILE 
(
    { file_name | file_id } 
    { [ , EMPTYFILE ] 
    | [ [ , target_size ] [ , { NOTRUNCATE | TRUNCATEONLY } ] ]
    }
)
[ WITH NO_INFOMSGS ]

Arguments

file_name
Is the logical name of the file to be shrunk.

file_id
Is the identification (ID) number of the file to be shrunk. To obtain a file ID, use the FILE_IDEX system function or query the sys.database_files catalog view in the current database.

target_size
Is the size for the file in megabytes, expressed as an integer. If not specified, DBCC SHRINKFILE reduces the size to the default file size. The default size is the size specified when the file was created, or the last size set by using ALTER DATABASE.

If target_size is specified, DBCC SHRINKFILE tries to shrink the file to the specified size. Used pages in the part of the file to be freed are relocated to available free space in the part of the file retained. For example, if there is a 10-MB data file, a DBCC SHRINKFILE operations with a target_size of 8 causes all used pages in the last 2 MB of the file to be reallocated into any unallocated pages in the first 8 MB of the file. DBCC SHRINKFILE does not shrink a file past the size needed to store the data in the file. For example, if 7 MB of a 10-MB data file is used, a DBCC SHRINKFILE statement with a target_size of 6 shrinks the file to only 7 MB, not 6 MB.

EMPTYFILE
Migrates all data from the specified file to other files in the same filegroup. Because the Database Engine no longer allows data to be placed in the empty file, the file can be removed by using the ALTER DATABASE statement.

NOTRUNCATE
Moves allocated pages from the end of a data file to unallocated pages in the front of the file with or without specifying target_percent. The free space at the end of the file is not returned to the operating system, and the physical size of the file does not change. Therefore, when NOTRUNCATE is specified, the file appears not to shrink.

NOTRUNCATE is applicable only to data files. The log files are not affected.

TRUNCATEONLY
Releases all free space at the end of the file to the operating system but does not perform any page movement inside the file. The data file is shrunk only to the last allocated extent.

target_size is ignored if specified with TRUNCATEONLY.

TRUNCATEONLY is applicable only to data files.

WITH NO_INFOMSGS
Suppresses all informational messages.

Result Sets

The following table describes the columns in the result set.

Column name	Description
DbId	Database identification number of the file the Database Engine tried to shrink.
FileId	The file identification number of the file the Database Engine tried to shrink.
CurrentSize	Number of 8-KB pages the file currently occupies.
MinimumSize	Number of 8-KB pages the file could occupy, at minimum. This corresponds to the minimum size or originally created size of a file.
UsedPages	Number of 8-KB pages currently used by the file.
EstimatedPages	Number of 8-KB pages that the Database Engine estimates the file could be shrunk down to.

Remarks

DBCC SHRINKFILE applies to the files in the current database. For more information about how to change the current database, see USE (Transact-SQL)

DBCC SHRINKFILE operations can be stopped at any point in the process, and any completed work is retained.

Note

In SQL Server 2005, when a DBCC SHRINKFILE operation fails an error is raised. In earlier versions of SQL Server, this operation failed without raising an error.

The database being shrunk does not have to be in single-user mode; other users can be working in the database when the file is shrunk. You do not have to run the instance of SQL Server 2005 in single-user mode to shrink the system databases.

Shrinking a Log File

For log files, the SQL Server 2005 Database Engine uses target_size to calculate the target size for the whole log; therefore, target_size is the amount of free space in the log after the shrink operation. Target size for the whole log is then translated to target size for each log file. DBCC SHRINKFILE tries to shrink each physical log file to its target size immediately. However, if part of the logical log resides in the virtual logs beyond the target size, the Database Engine frees as much space as possible, and then issues an informational message. The message describes what actions are required to move the logical log out of the virtual logs at the end of the file. After the actions are performed, DBCC SHRINKFILE can be used to free the remaining space. For more information, see Shrinking the Transaction Log.

Because a log file can only be shrunk to a virtual log file boundary, shrinking a log file to a size smaller than the size of a virtual log file might not be possible, even if it is not being used. The size of the virtual log file is chosen dynamically by the Database Engine when log files are created or extended. For more information about virtual log files, see Transaction Log Physical Architecture.

Best Practices

Consider the following information when you plan to shrink a file:

A shrink operation is most effective after an operation that creates lots of unused space, such as a 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 repeatedly and notice that the database size grows again, this indicates that the space that was shrunk is required for regular operations. In these cases, repeatedly shrinking the database is a wasted operation.
A shrink operation does not preserve the fragmentation state of indexes in the database, and generally increases fragmentation to a degree. This is another reason not to repeatedly shrink the database.

Troubleshooting

This section describes how to diagnose and correct issues that can occur when running the DBCC SHRINKFILE command.

The File Does Not Shrink

If the shrink operation runs without error, but the file does not appear to have changed in size, verify that the file has adequate free space to remove by performing one of the following operations:

Run the following query.

SELECT name ,size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS int)/128.0 AS AvailableSpaceInMB
FROM sys.database_files;

Run the DBCC SQLPERF command to return the space used in the transaction log.

If insufficient free space is available, the shrink operation cannot reduce the file size any further.

Typically it is the log file that appears not to shrink. This is usually the result of a log file that has not be truncated. You can truncate the log by setting the database recovery model to SIMPLE, or by backing up the log and then running the DBCC SHRINKFILE operation again. For more information, see Transaction Log Truncation and Shrinking the Transaction Log.

The Shrink Operation Is Blocked

It is possible for shrink operations to be blocked by a transaction that is running under a row versioning-based isolation level. For example, if a large delete operation running under a row versioning-based isolation level is in progress when a DBCC SHRINK DATABASE operation is executed, the shrink operation will wait for the delete operation to complete before shrinking the files. When this happens, DBCC SHRINKFILE and DBCC SHRINKDATABASE operations print out an informational message (5202 for SHRINKDATABASE and 5203 for SHRINKFILE) to the SQL Server error log every five minutes in the first hour and then every hour after that. For example, if the error log contains the following error message:

DBCC SHRINKFILE for file ID 1 is waiting for the snapshot 
transaction with timestamp 15 and other snapshot transactions linked to 
timestamp 15 or with timestamps older than 109 to finish.

This means that the shrink operation is blocked by snapshot transactions that have timestamps older than 109, which is the last transaction that the shrink operation completed. It also indicates that the transaction_sequence_num, or first_snapshot_sequence_num columns in the sys.dm_tran_active_snapshot_database_transactions dynamic management view contains a value of 15. If either the transaction_sequence_num, or first_snapshot_sequence_num columns in the view contains a number that is less than the last transaction completed by a shrink operation (109), the shrink operation will wait for those transactions to finish.

To resolve the problem, you can do one of the following tasks:

Terminate the transaction that is blocking the shrink operation.
Terminate the shrink operation. If the shrink operation is terminated, any completed work is retained.
Do nothing and allow the shrink operation to wait until the blocking transaction completes.

For more information about the SQL Server error log, see Viewing the SQL Server Error Log.

Permissions

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

Examples

A. Shrinking a data file to a specified target size

The following example shrinks the size of a data file named DataFile1 in the UserDB user database to 7 MB.

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

B. Shrinking a log file to a specified target size

The following example shrinks the log file in the AdventureWorks database to 1 MB. To allow the DBCC SHRINKFILE command to shrink the file, the file is first truncated by setting the database recovery model to SIMPLE.

USE AdventureWorks;
GO
-- Truncate the log by changing the database recovery model to SIMPLE.
ALTER DATABASE AdventureWorks
SET RECOVERY SIMPLE;
GO
-- Shrink the truncated log file to 1 MB.
DBCC SHRINKFILE (AdventureWorks_Log, 1);
GO
-- Reset the database recovery model.
ALTER DATABASE AdventureWorks
SET RECOVERY FULL;
GO

C. Truncating a data file

The following example truncates the primary data file in the AdventureWorks database. The sys.database_files catalog view is queried to obtain the file_id of the data file.

USE AdventureWorks;
GO
SELECT file_id, name
FROM sys.database_files;
GO
DBCC SHRINKFILE (1, TRUNCATEONLY);

D. Emptying a file

The following example demonstrates the procedure for emptying a file so that it can be removed from the database. For the purposes of this example, a data file is first created and it is assumed that the file contains data.

USE AdventureWorks;
GO
-- Create a data file and assume it contains data.
ALTER DATABASE AdventureWorks 
ADD FILE (
    NAME = Test1data,
    FILENAME = 'C:\t1data.ndf',
    SIZE = 5MB
    );
GO
-- Empty the data file.
DBCC SHRINKFILE (Test1data, EMPTYFILE);
GO
-- Remove the data file from the database.
ALTER DATABASE AdventureWorks
REMOVE FILE Test1data;
GO

Change History

Release	History
14 April 2006	New content: Added the section "The File Does Not Shrink". Added resolution tasks to the section "The Shrink Operation Is Blocked". Added examples B, C, and D. Updated content: Clarified the definitions of TRUNCATEONLY and NOTRUNCATE.

14 April 2006

New content:

Added the section "The File Does Not Shrink".
Added resolution tasks to the section "The Shrink Operation Is Blocked".
Added examples B, C, and D.

Updated content:

Clarified the definitions of TRUNCATEONLY and NOTRUNCATE.

Jaa