Database File Initialization

THIS TOPIC APPLIES TO: yesSQL ServernoAzure SQL DatabasenoAzure SQL Data Warehouse noParallel Data Warehouse

Data and log files are initialized to overwrite any existing data left on the disk from previously deleted files. Data and log files are first initialized by zeroing the files (filling with zeros) when you perform one of the following operations:

  • Create a database.

  • Add data or log files, to an existing database.

  • Increase the size of an existing file (including autogrow operations).

  • Restore a database or filegroup.

File initialization causes these operations to take longer. However, when data is written to the files for the first time, the operating system does not have to fill the files with zeros.

Instant File Initialization (IFI)

In SQL Server, data files can be initialized instantaneously to avoid zeroing operations. Instant file initialization allows for fast execution of the previously mentioned file operations. Instant file initialization reclaims used disk space without filling that space with zeros. Instead, disk content is overwritten as new data is written to the files. Log files cannot be initialized instantaneously.

Note

Instant file initialization is available only on Microsoft Windows XP Professional or Windows Server 2003 or later versions.

Important

Instant file initialization is available only for data files. Log files will always be zeroed when being created, or growing in size.

Instant file initialization is only available if the SQL Server service startup account has been granted SE_MANAGE_VOLUME_NAME. Members of the Windows Administrator group have this right and can grant it to other users by adding them to the Perform Volume Maintenance Tasks security policy.

Some feature usage, such as Transparent Data Encryption (TDE), can prevent Instant File Initialization.

To grant an account the Perform volume maintenance tasks permission:

  1. On the computer where the backup file will be created, open the Local Security Policy application (secpol.msc).

  2. In the left pane, expand Local Policies, and then click User Rights Assignment.

  3. In the right pane, double-click Perform volume maintenance tasks.

  4. Click Add User or Group and add any user accounts that are used for backups.

  5. Click Apply, and then close all Local Security Policy dialog boxes.

Security Considerations

Because the deleted disk content is overwritten only as new data is written to the files, the deleted content might be accessed by an unauthorized principal, until some other data writes on that specific area of the data file. While the database file is attached to the instance of SQL Server, this information disclosure risk is reduced by the discretionary access control list (DACL) on the file. This DACL allows file access only to the SQL Server service account and the local administrator. However, when the file is detached, it may be accessed by a user or service that does not have SE_MANAGE_VOLUME_NAME. A similar consideration exists when the database is backed up: if the backup file is not protected with an appropriate DACL, the deleted content can become available to an unauthorized user or service.

If SQL Server is installed in a secure environment, the performance benefits of enabling IFI can outweigh the security risk and hence the reason for this recommendation.

If the potential for disclosing deleted content is a concern, you should take one or both of the following actions:

  • Always make sure that any detached data files and backup files have restrictive DACLs.

  • Disable instant file initialization for the instance of SQL Server by revoking SE_MANAGE_VOLUME_NAME from the SQL Server service startup account.

Note

Disabling instant file initialization only affects files that are created or increased in size after the user right is revoked.

See Also

CREATE DATABASE (SQL Server Transact-SQL)