How Amanda Estimates Backup Size When Compression is Enabled

This article is for Amanda Enterprise (AE).

Amanda estimates the size of the backup for backup level 0, previous backup level, and previous backup level -1 as the first phase of the backup run. The estimate phase is key to the backup plan constructed by Amanda's process - planner.

Amanda also supports server or client-side compression of backups for each Source. Any compression algorithm that is supported by the server or client can be used.

Compression of backup images makes estimating backup size more complex. Amanda estimates the size of uncompressed backup images and uses historical information for the compression ratio. Amanda server stores the compression ratios achieved for the last three backup runs for each source in its database. The estimated backup size is multiplied by the historical compression ratio and used for backup plan computation.

If there is no historical information, a compression ratio of 0.5 is used.

The accuracy of the historical compression ratio improves with each backup run if the compression configuration remains the same.