How Time Machine manages backup archives

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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