Backup compression (SQL Server)
Applies to: SQL Server (all supported versions)
This article describes the compression of SQL Server backups, including restrictions, performance trade-off of compressing backups, the configuration of backup compression, and the compression ratio. Backup compression is supported on SQL Server editions: Enterprise, Standard, and Developer. Every edition of SQL Server 2008 and later can restore a compressed backup.
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 article.
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.
Performance impact of compressing backups
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).
Beginning with SQL Server 2022 (16.x), you can use Integrated offloading & acceleration to compress backups and offload the CPU resources for the backup.
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.
Calculate the compression ratio of a compressed backup
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:
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. For example, if data is encrypted at the column level with Always Encrypted, or with other application-level encryption, compressing backups might not reduce the size significantly.
For more information related to compressing databases encrypted with Transparent Data Encryption (TDE), see Backup compression with TDE.
Whether the database is compressed.
If the database is compressed, compressing backups might not reduce their size by much, if at all.
Backup compression with TDE
Starting with SQL Server 2016 (13.x), setting
MAXTRANSFERSIZE larger than 65536 (64 KB) enables an optimized compression algorithm for Transparent Data Encryption (TDE) encrypted databases that first decrypts a page, compresses it, and then encrypts it again. If
MAXTRANSFERSIZE is not specified, or if
MAXTRANSFERSIZE = 65536 (64 KB) is used, backup compression with TDE encrypted databases directly compresses the encrypted pages, and may not yield good compression ratios. For more information, see Backup Compression for TDE-enabled Databases.
Starting with SQL Server 2019 (15.x) CU5, setting
MAXTRANSFERSIZE is no longer required to enable this optimized compression algorithm with TDE. If the backup command is specified
WITH COMPRESSION or the backup compression default server configuration is set to 1,
MAXTRANSFERSIZE will automatically be increased to 128K to enable the optimized algorithm. If
MAXTRANSFERSIZE is specified on the backup command with a value > 64K, the provided value will be honored. In other words, SQL Server will never automatically decrease the value, it will only increase it. If you need to back up a TDE encrypted database with
MAXTRANSFERSIZE = 65536, you must specify
WITH NO_COMPRESSION or ensure that the backup compression default server configuration is set to 0.
For more information, see BACKUP (Transact-SQL).
Allocation of space for the backup file
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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