How Amanda estimates backup size when compression is enabled

This article is for Amanda Enterprise (AE)

Amanda estimates the size of 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 process – planner.

Amanda also supports server or client side compression of backups for each DLE (Disk List Entry). Any compression algorithm that is supported by the server or client can be used.

Compression of backup images makes the 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 last 3 backup runs for each DLE 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 historical compression ratio improves with each backup run if the compression configuration remains the same.