PresSTORE supports three different backup levels. Here is a brief description what these can be used for:
The full backup
The full backup saves all files selected for backup and writes them to the media. This is the traditional, complete backup of all files.
In bigger systems, a full backup may take a long time and thus it must be limited in time, e.g. it runs over a weekend but shall stop on Monday morning.
A full backup also starts a new backup cycle, as all files are saved once more.
The incremental backup
The incremental backup is designed to save all files that are not in the last performed backup of the backup plan.
This is used in various ways:
- Incremental backups will save all files that have been modified since the last backup. This operation can be understood as an update of the latest full backup, actualizing the current backup cycle.
- An Incremental backup will also save all files which are not yet included in the current backup. E.g. in case a full backup broke because of an error or because it was limited in time, the increment will complete the backup. So if the complete backup takes a total time of one weekend and three additional nights to complete, schedule a full backup for the weekend with time limit and add time limited incremental backups for the subsequent nights. This way, even a lengthy backup operation does not need to block resources during the work time.
- Performing only incremental backups (and never starting a new cycle with a full backup) seems to work like a charm, but there are disadvantages: the complete size of the backup keeps growing since all the tapes are required to restore a complete state of the file system being backed up. A complete restore is a lengthy procedure. Old tapes never get checked since they are never re-involved in the backup process. Finally, the index on disk that PresSTORE uses to maintain the contents of the tapes grows too. Therefore, it is highly to run a full backup from time to time.
The synthetic backup
A synthetic backup is actually more of a reorganization procedure than a true backup. It creates a new full backup based on the most recently backed up state of the file system and starts a new backup cycle. To save this state, files are read from existing tapes rather than from the file system. This procedure requires at least two tape drives, one to read and one to write.
Since data is newly written, the aging of tapes can be avoided. Furthermore, of the files that have been saved several times in several version during the lifetime of the last full backup cycle, only the latest version is carried over into the new backup cycle. Hence, the entire size of the backup cycle is reduced. Synthetic backups are useful when the data to back up is pulled off very slow lines (and a true full backup would take too much time or involve too many resources), for example, when performing remote backups.