Snapshots aren’t backups

Most backup utilities now make snapshots of volumes they’re backing up, and Time Machine goes further by using snapshots in its backup process, and creating backups as snapshots. How then should we include snapshots in our backup plans? Could we rely on snapshots rather than conventional backups?

Each snapshot contains a complete set of the file system metadata for that volume at the time the snapshot was made, and all the extents required to reinstate that volume. Although tied to that volume, the snapshot is stored alongside the current volume metadata, in the same APFS container, in the same disk partition.

Extents list the storage blocks containing the data that composes every file that existed within that volume at the time the snapshot was made. Many of those will be the same as in the current volume, but the remainder will refer to data that has been deleted since the snapshot was made, but is being retained to enable the volume to be rolled back to its previous state. Those old extents can only be removed when that snapshot is deleted, which thus frees all those storage blocks at the same time. This is illustrated in the diagram.

This shows the same file in a snapshot and the current volume. Extents for the data of the earlier version of that file contained in the snapshot are shown at the top, and consist of blocks EA, EB, EC and ED. After that snapshot was made, the file was edited and then consists of the blocks shown at the bottom, EA, FB, FC and ED.

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