59728: Acronis Backup 12.5: version 12 archive does not decrease in size after cleanup

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Symptoms

  1. You have an archive in 'version 12' format and a backup plan with retention rules set up.
  2. Retention rules are executed successfully, but archive size does not get smaller.

Cause

Version 12 mode uses a new generation archive format. After the first full backup, all other backups are saved to this archive in incremental mode. Physically all data is located inside one file as opposed to the version 11 format where each backup is stored in a separate .tib file.

When a certain backup expires due to the pre-defined retention rules, the backup algorithm just marks these outdated backup blocks as “free” ones. The initial archive still occupies the same space on the storage as before, however all newer backups will be written to the “free” blocks first, and only if all the “free” blocks are filled, the total size of the archive will be increased.

This approach allows keeping the archive size as small as possible and prevents it from excessive growing. Also, the implementation of this backup scheme significantly saves time and resources for managing the backups inside the archive because the “free” blocks marking is almost an instant operation.

The version 12 archive total size includes the size of the “used” blocks and the size of the “free” blocks.

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