How Time Machine manages backup archives

Time Machine lets you remove backup snapshots manually, which could lead to trouble.

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If you use Time Machine for a full backup with deep archives from macOS, you can manually delete any of the hourly, daily, and weekly snapshots that the app makes and retains. If you launch the Time Machine app, navigate to a Finder window and then a date, you can then right-click and select Delete Backup.

Generally, you should let Time Machine take care of this. It automatically deletes older snapshots over time, which in turn removes only files that were changed at intermediate points (between weeks or days or hours) from the previous backup, but retains all the recent revisions. Ultimately, for files that don’t change very often, you may be left with just a single copy of the file as it also appears on your live disk drive.

Macworld reader Spider writes in asking a question related to a recent Mac911 column about how Time Machine backs up files, wondering about deleting these snapshots. “How does a user know when deleting a snapshot might be deleting the last hard link to a particular file?”

It’s a fantastic question and requires a little noggin time to sort out interconnected issues. (My first thought as to the answer was wrong, in fact.)

  • The first time Time Machine creates a backup on a target drive or for a target computer, it copies all files as a baseline.
  • Subsequent backups only include changed files, and use (as noted in the above article) the special hard link style of file pointer to make each snapshot look and act on disk as if it’s a full backup.
  • Automatic snapshot deletion ensures at least one copy, always the most recent copy, of a file remains.
  • Deleting snapshots manually removes a combination of hard links, which don’t affect the original file, and updated versions, which do.

It’s possible to delete all the snapshots between the initial backup and the point at which you’re going to town removing things, and have no intermediate versions nor the latest version. You can also select a file, click the gear (action) icon, and choose to Delete All Backups, which explicitly removes all copies of the file from the Time Machine archive. (You can read this column, too, about how Time Machine retains files deleted from your live working system.)

Apple provides surprisingly little documentation about all this, however, so my general suggestion is that you let Time Machine cull snapshots automatically, which it does as a drive fills up. If you find that you want to use a drive for Time Machine and other purposes, I’d recommend partitioning it and allocating as much space as you want for a deep archive.

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