Snapshots from other apps

Most Macs already carry snapshots DeltaSnap did not create. Time Machine leaves local snapshots behind between backups, and Carbon Copy Cloner, SuperDuper, and Arq each take their own before they copy. DeltaSnap treats these as ordinary history on any tracked APFS volume: you can mount them, index them, compare them, tag them, and restore individual files from them.

Reading a snapshot is a read-only operation on a frozen filesystem, so whose snapshot it is makes no difference to what it can tell you. A Time Machine local snapshot from this morning is often the most useful comparison point on the disk, and it costs nothing extra because it already exists.

What works on an external snapshot

DeltaSnap never rotates, expires, or thins another app's snapshots. Count retention, natural lifespans, and the safety TTL apply only to snapshots DeltaSnap created. The one setting that can reach them is low-space pruning, and Protect Time Machine and other apps' snapshots now defaults to on, so it will not.

Apple's com.apple.os… system rollback snapshots are the exception to all of the above. They back macOS recovery, they are filtered out before anything in DeltaSnap sees them, and no DeltaSnap operation will touch them.

Volumes with the APFS Backup role, such as a Time Machine destination disk, stay display-only. DeltaSnap lists what is on them and does nothing else.

How they get indexed

Comparison and Version Control are index-backed, so an external snapshot has to be scanned once before it can serve file versions. DeltaSnap does not scan them in the background on their own schedule, because Time Machine alone would add an hourly full-volume scan. Instead they are indexed at the points where you have asked for them:

Compare two snapshots. The comparison scans both sides and keeps the result, so comparing a Time Machine snapshot is usually the quickest way to bring it into your history. The first comparison is slow; every later one is served from the index.

Index Snapshot. Right-click a row on the Snapshots tab. This works for any snapshot newer than the newest one already indexed.

Re-index Snapshots. Right-click the volume in the sidebar. This rebuilds the whole volume from its oldest snapshot and covers everything on it, external snapshots included. It is the way to bring older external snapshots into version history, and it is the slowest option because it rescans the volume's full timeline. Settings → Maintenance → Rebuild All Snapshot Indexes does the same across every volume.

Automatically, from the next DeltaSnap snapshot. Whenever DeltaSnap takes a snapshot, it first catches up on what other apps did in the meantime: any external snapshot created since the last index is queued, and indexes for external snapshots that have since been deleted are dropped. On a volume with an active schedule this keeps other apps' snapshots current without you doing anything.

When another app deletes its snapshot

Other tools manage their own snapshots and remove them on their own schedule. Time Machine thins local snapshots within about a day. DeltaSnap notices and drops the matching index, on its hourly reconciliation pass and again whenever it takes a snapshot, so version history does not accumulate entries pointing at snapshots that no longer exist.

Nothing is lost that was not already gone. The versions those snapshots served disappear with them, which is why an active DeltaSnap schedule is still worth having alongside another tool.

If a snapshot is mounted

A snapshot can only be mounted once. If you have one open in Finder, or Disk Utility or Time Machine has mounted it, DeltaSnap reads from that existing mount rather than trying to mount its own copy. If it cannot, it leaves the snapshot queued and picks it up on the next pass instead of reporting a problem.

Related

• Compare snapshots for what a comparison reports.
• Version Control for per-file timelines and restore.
• Schedules and retention for what DeltaSnap will and will not prune.