The Hill backup system was described in a prior posting. It’s main purpose is to provide roll-back to a known system state in case of complete loss in Ungar. For this reason, it uses dump to provide an block-accurate disk image, including hard links and sparse files and does not modify touch times during backup.
The backups are incremental with the current pattern of a zero level at start of epoch, and four level 4′s at the start of each month and four weekly or semi-weekly level 8′s.
System backups based on snapshots incidentally serve other purposes, for instance, to recover user files that have been corrupted or deleted. Past snapshots are saved with an accuracy that coarsens with receding time.
It is often unacknowledged that no backup scheme fits all needs. For instance, archive and source-control contrast to snapshots. Snapshots are meant to be mechanical and precise. Archive is permanent, deliberate, and indexed; Source control keeps versions, including deleted files, and possibly renaming and file moves, and has collaboration mechanisms.