Usually, backups are kept only for a certain time period (retention time) or there is a policy to keep only the last X backups in the backup chain. Backup archives are managed on a daily, weekly, etc. basis. The main limitation of the Legacy mode backup archive is that you cannot delete a random backup from the backup chain since it may have dependencies on it from subsequent backups. This is where Always Incremental backup archive can help.
Always Incremental mode uses a new generation archive format which may contain several backups from a number of virtual machines. 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 Legacy archive format where each backup is stored in a separate .tib file. Therefore, unlike the Legacy mode archive, it is possible to delete a random backup from Always Incremental archive even if it has dependencies.
When a certain backup expires due to the pre-defined retention rules (for example to “delete the backups if they are older than 2 days”), the backup algorithm just marks these outdated backup blocks as “free” ones.
The blocks of expired backups with dependencies (which are needed to restore the newer backups) are not marked as “free” to ensure the archive consistency. Everyday, the archive should contain data that is not older than two days in order to restore the backup (retention time). This is the basic rule of the Always Incremental archive. All excessive data in the archive is marked for deletion, i.e. as “free” space. 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. Thus, the limitations of the Legacy archive mode are no longer true for Always Incremental archive.
The Always Incremental archive total size includes the size of the “used” blocks and the size of the “free” blocks. Usually, the size of the Always Incremental archive does not grow indefinitely and stays within the total size of the backups you want to keep.