When does Time Machine decide to delete its snapshots?

[pedzsan]

I’ve been hunting missing disk space for the past week or so. It would magically reappear and then disappear again. I falsely accused Dropbox of being the culprit but it was Time Machine.

To get to the gist: Time Machine makes snapshots in APFS and I guess it decides to delete them after some length of time? Or perhaps some other conditions? I thought it would delete them once I attached my Time Machine disk and let it dump everything off but that isn’t the case. It seems more complex than that.

Does anyone know the details of when Time Machine decides to free up the disk space?

p.s. I know I can go into Disk Utility and free them up myself but I’d rather just know better what is going on.

 

[MarkC426]

On my machine it keeps them for 3 days.
But this is what is shown in Techtool Pro, whether it's different just in MacOS I don't know, as there is no visual indication.
There is a panel in TTP where you can manage it.
Also note if you delete say 50gb of data, that will be in a snapshot.

 

[pedzsan]

Also note if you delete say 50gb of data, that will be in a snapshot.

That is what was happening to me. I’d pull 500G in from the net and a disk, go through the data, and then delete it. I assumed I could ”rinse and repeat” and go through a different 500G but I couldn’t. Well… I should say I didn’t because df told me I had no space. But I wonder if I would have done it anyway if something would have triggered and freed the snapshot.

 

[MarkC426]

It should free the space after 3 days, unless manually deleted.
Plus that's a lot of data to look at......

 

[BigBlur]

Hmm, my local snapshots are only saved for 24 hours...and that jives with what Apple says (see link below). 3 days doesn't sound normal.

About Time Machine local snapshots - Apple Support

Time Machine lets you restore files from local snapshots of the files on your Mac, even when your Time Machine backup disk isn't available.

support.apple.com

 

[MarkC426]

Apologies....you are correct @BigBlur.
My TM snapshots do go after 24 hours.....?

As I have TechTool Pro, it creates it's own snapshots also, which are set to 3 days (but can be changed to suit).

Deleting data does keep a snapshot though for about 3 days before freeing up the space, but again, this may be related to me having TTP installed and being set to 3 days.

 

[pedzsan]

FYI: I had one snapshot created around 7 a.m. on the 8th and it was deleted today on the 12th around 6 p.m.

 

[DrCC]

I think snapshots are kept until the Time Machine disk space is full, then Time Machine will automatically start deleting the oldest. To keep more snapshots and go back further, the general rule of thumb is to have the Time Machine drive at least twice the size or the internal SSD.

 

[MarkC426]

I think snapshots are kept until the Time Machine disk space is full, then Time Machine will automatically start deleting the oldest. To keep more snapshots and go back further, the general rule of thumb is to have the Time Machine drive at least twice the size or the internal SSD.

I think your getting mixed up with TM incremental backups and 'snapshots'.
My current TM has backups from every day back to June 2021, which I can access at any time.
My snapshots are only kept for 3 days.

Snaphots are saved locally on the boot disk.