How hard disk compression can affect backup and restore jobs on a Windows (NTFS) File System Partition

Hard disk compression is a Windows feature that frees up additional disk space on a hard drive when the option is enabled. Compression was used a lot more frequently in the past, because of the need for additional disk space and because of the cost of adding more hard disks to a system. The cost of hard disks has become much more economical since then and the amount of disk space available is usually more than adequate for applications to function properly.

Backup and Restore Performance:
 

NT File System (NTFS) compression can save disk space, but compressing data can adversely affect backup and restore performance. When backing up a compressed NTFS file, NTFS first decompresses the files, then the backup software copies the files to either tape or disk and the files will be re-compressed if either hardware or software compression is enabled within the backup application. The process will function in reverse order when performing a restore. NTFS compression is processor-intensive and the performance cost is more noticeable on servers which are processor-bound. Database or file servers with a lot of write traffic (i.e.: SharePoint Server,user home folders and roaming profiles which use a large number of read and write operations) will not perform optimally when data compression is utilized. Compressed files are also expanded before copying them over the network when performing remote backups, so NTFS compression does not save network bandwidth.
 

Database Applications:

Compressed volumes do not support transactional recovery of a database that requires sector-aligned writes. Storing databases on a compressed drive can cause Microsoft Jet (extensible storage engine) Databases to have write-through delays which can lead to data loss and Jet database corruption. The following list provides examples of Jet-related files such as DIT files, logs, checkpoints, and temporary databases that should not be stored on compressed volumes:

Compression can also generate an Out of Space error on pre-allocated files, interfering with successful recovery of a database. Because of this, database programs that use transaction logging and write to a database or log should have their files stored on an uncompressed volume.
 

Note:  Other third party, non-Microsoft database applications, such as IBM Lotus Domino and Oracle, may not function properly on a compressed hard disk. Check with the database vendor to confirm if they support disk compression and run test restores of the databases to make sure that the data backed up is recoverable.