View the Size of the Sparse File of a Database Snapshot (Transact-SQL)

Applies to: SQL Server

This topic describes how to use Transact-SQL to verify that a SQL Server database file is a sparse file and to find out its actual and maximum sizes. Sparse files, which are a feature of the NTFS file system, are used by SQL Server database snapshots.

Note

During database snapshot creation, sparse files are created by using the file names in the CREATE DATABASE statement. These file names are stored in sys.master_files in the physical_name column. In sys.database_files (whether in the source database or in a snapshot), the physical_name column always contains the names of the source database files.

Verify that a Database File is a Sparse File

1. On the instance of SQL Server:

   Select the is_sparse column from either sys.database_files in the database snapshot or from sys.master_files. The value indicates whether the file is a sparse file, as follows:

   1 = File is a sparse file.

   0 = File is not a sparse file.

Find Out the Actual Size of a Sparse File

Note

Sparse files grow in 64-kilobyte (KB) increments; thus, the size of a sparse file on disk is always a multiple of 64 KB.

To view the number of bytes that each sparse file of a snapshot is currently using on disk, query the size_on_disk_bytes column of the SQL Server sys.dm_io_virtual_file_stats dynamic management view.

To view the disk space used by a sparse file, right-click the file in Microsoft Windows, click Properties, and look at the Size on disk value.

Find Out the Maximum Size of a Sparse File

The maximum size to which a sparse can grow is the size of the corresponding source database file at the time of the snapshot creation. To learn this size, you can use one of the following alternatives:

• Using Windows Command Prompt:

  1. Use Windows dir commands.

  2. Select the sparse file, open the file Properties dialog box in Windows, and look at the Size value.

• On the instance of SQL Server:

  Select the size column from either sys.database_files in the database snapshot or from sys.master_files. The value of size column reflects the maximum space, in SQL pages, that the snapshot can ever use; this value is equivalent to the Windows Size field, except that it is represented in terms of the number of SQL pages in the file; the size in bytes is:

  ( number_of_pages * 8192)

Example

The following script will show the size on disk in kilobytes for each sparse file. The script will also show the maximum size in megabytes to which a sparse file can grow. Execute the Transact-SQL script in SQL Server Management Studio.

SELECT  DB_NAME(sd.source_database_id) AS [SourceDatabase], 
		sd.name AS [Snapshot],
		mf.name AS [Filename], 
		size_on_disk_bytes/1024 AS [size_on_disk (KB)],
		mf2.size/128 AS [MaximumSize (MB)]
FROM sys.master_files mf
JOIN sys.databases sd
	ON mf.database_id = sd.database_id
JOIN sys.master_files mf2
	ON sd.source_database_id = mf2.database_id
	AND mf.file_id = mf2.file_id
CROSS APPLY sys.dm_io_virtual_file_stats(sd.database_id, mf.file_id)
WHERE mf.is_sparse = 1
AND mf2.is_sparse = 0
ORDER BY 1;

See Also

Database Snapshots (SQL Server)
sys.fn_virtualfilestats (Transact-SQL)
sys.database_files (Transact-SQL)
sys.master_files (Transact-SQL)