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Backup Compression (SQL Server)

This topic describes the compression of SQL Server backups, including restrictions, performance trade-off of compressing backups, the configuration of backup compression, and the compression ratio.

Note Note

For information which editions of SQL Server 2014 support backup compression, see Features Supported by the Editions of SQL Server 2014. Every edition of SQL Server 2008 and later can restore a compressed backup.

In this Topic:

  • Because a compressed backup is smaller than an uncompressed backup of the same data, compressing a backup typically requires less device I/O and therefore usually increases backup speed significantly.

    For more information, see Performance Impact of Compressing Backups, later in this topic.

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The following restrictions apply to compressed backups:

  • Compressed and uncompressed backups cannot co-exist in a media set.

  • Previous versions of SQL Server cannot read compressed backups.

  • NTbackups cannot share a tape with compressed SQL Server backups.

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By default, compression significantly increases CPU usage, and the additional CPU consumed by the compression process might adversely impact concurrent operations. Therefore, you might want to create low-priority compressed backups in a session whose CPU usage is limited by Resource Governor. For more information, see Use Resource Governor to Limit CPU Usage by Backup Compression (Transact-SQL).

To obtain a good picture of your backup I/O performance, you can isolate the backup I/O to or from devices by evaluating the following sorts of performance counters:

  • Windows I/O performance counters, such as the physical-disk counters

  • The Device Throughput Bytes/sec counter of the SQLServer:Backup Device object

  • The Backup/Restore Throughput/sec counter of the SQLServer:Databases object

For information about Windows counters, see Windows help. For information about how to work with SQL Server counters, see Use SQL Server Objects.

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To calculate the compression ratio of a backup, use the values for the backup in the backup_size and compressed_backup_size columns of the backupset history table, as follows:

backup_size:compressed_backup_size

For example, a 3:1 compression ratio indicates that you are saving about 66% on disk space. To query on these columns, you can use the following Transact-SQL statement:

SELECT backup_size/compressed_backup_size FROM msdb..backupset;

The compression ratio of a compressed backup depends on the data that has been compressed. A variety of factors can impact the compression ratio obtained. Major factors include:

  • The type of data.

    Character data compresses more than other types of data.

  • The consistency of the data among rows on a page.

    Typically, if a page contains several rows in which a field contains the same value, significant compression might occur for that value. In contrast, for a database that contains random data or that contains only one large row per page, a compressed backup would be almost as large as an uncompressed backup.

  • Whether the data is encrypted.

    Encrypted data compresses significantly less than equivalent unencrypted data. If transparent data encryption is used to encrypt an entire database, compressing backups might not reduce their size by much, if at all.

  • Whether the database is compressed.

    If the database is compressed, compressing backups might not reduce their size by much, if at all.

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For compressed backups, the size of the final backup file depends on how compressible the data is, and this is unknown before the backup operation finishes. Therefore, by default, when backing up a database using compression, the Database Engine uses a pre-allocation algorithm for the backup file. This algorithm pre-allocates a predefined percentage of the size of the database for the backup file. If more space is needed during the backup operation, the Database Engine grows the file. If the final size is less than the allocated space, at the end of the backup operation, the Database Engine shrinks the file to the actual final size of the backup.

To allow the backup file to grow only as needed to reach its final size, use trace flag 3042. Trace flag 3042 causes the backup operation to bypass the default backup compression pre-allocation algorithm. This trace flag is useful if you need to save on space by allocating only the actual size required for the compressed backup. However, using this trace flag might cause a slight performance penalty (a possible increase in the duration of the backup operation).

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