Time Machine thinning

TidBITS Talk
1

I just noticed that Time Machine (APFS, on Ventura) doesn’t do thinning until it runs low on space. I had hourly backups all the way back to January 1st, rather than just the last 24 hours.

When did this start?

2

Must be a bug. Looking at my TM volume (as of January 24, 2025, 19:10), I see:

  • Hourlies going back to January 23, 19:04 - about 24 hours
  • Dailies going back to December 26 (30 days)
  • Weeklies going back to when I put the drive in service (May 11, 2022)

This is a 2018 Mac mini running macOS Sonoma 14.6.1

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3

I wonder if it is related to me having multiple Time Machines configured.

There’s 3 configured right now. Two are attached and alternating, the other isn’t mounted.

4

I have had TM backups disappear just by updating the OS, restoring a boot volume, etc. Like just the other day when I finally updated to Sequoia. Lost count.

5

Right now on a drive I added on December 9th, I have:

  • Weekly backups 12/9 to 12/30
  • Daily backups 12/30 to 1/9
  • Hourly backups 1/9 to now

But on the backup drive that ran low enough on space that it had to purge, it now has:

  • Weekly backups 8/15 to 12/30 (computer was off 12/27 - 12/29)
  • Daily backups 12/30 to 1/25
  • Hourly backups 1/26 to now

So that drive is now thinning as expected.

What’s weird to me is that the first drive was thinning, else there’d be hourly backups before 1/9. Last macOS update was on 12/13.

I’d say it is something like “Time Machine doesn’t apply thinning rules to the entire backup drive every time, it thins using a window. For example, it looks at the previous 24 hours and removes all but one, looks at the week 30 days ago and removes all but one, etc. So if you miss the window, it doesn’t thin that period.”… but that doesn’t explain it, because if that was true, the first drive should have thinned yesterday.

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