Setting size quotas for network Time Machine backups

Tech Support
1

I have a Raspberry Pi 3B+ with a 4TB external hard drive (AC powered) which is set up as a TM server for two Macs. The Pi is running Bullseye and the HD has one Ext4 partition. I would like to set a limit on how large each TM backup can get so that neither backup takes up more than a certain proportion of the available space.

I found a 10-year-old post that recommended sudo defaults write /Library/Preferences/com.apple.TimeMachine MaxSize [#], but I don’t know if it is only for local TM backup or also for network backup… Or if it would even work with more current versions of Mac OS. Can anyone confirm?

If not, I would ideally prefer a solution that did not involve repartitioning the drive and / or losing the existing backups.

2

I don’t think there is any way to do this.

3

A decade ago I set up a Mac mini server, and it was supposed to be able to limit the time machine backup size per system. However the limitation never worked. I would expect that since it didn’t work then it won’t work now. Apple really doesn’t care about server features.

Perhaps you could make an additional drive partition, so each Mac could have its own Time Machine partition on the server?

4

Time Machine’s UI allows you to set a usage limit. I just set up a backup to a shared folder in TM, and it allowed me to specify.

Back up to a shared folder with Time Machine on Mac - Apple Support.

5

Thanks for the replies.

I thought about that. I really don’t want to repartition the HD…

The page you linked to is only relevant when you back up one Mac to another Mac. I am backing up 2 Macs to a headless (and GUI-less) Raspberry Pi, and there is no checkbox to set size limits.

Some more searching led me to this page – which may be a good solution (if it works), but it does not have dummy-proof instructions. I will need some more Google-fu to figure out some of the steps. In particular, it mentions setting permissions for the TimeMachine.quota.plist file, which I don’t really know how to do in the command-line. Yet.

FWIW, all of this is theoretical at the moment. The HD in question is 4TB, and at the moment both TM backups combined take up less than 1TB of space. I imagine it will be a while before I run into a capacity issue.

7

Well there is always that. I TM’ed 3 Macs to my server and found that the backups would corrupt before the drive filled. (I don’t count Time Machine as one of my redundant backup destinations, because, frankly, I’ve never trusted it.)

2 Likes
8

Oh dear. Does TM give you any indication that the backups are corrupted?

FWIW, TM is my 3rd tier of backup — after SuperDuper clones and Backblaze.

9

+1

I have used TM on disks, other Macs, and NAS and in each case the most common error was a failure to delete old backups when the drive/partition became full. And I have done test restores and found my TM backup unusable. Time Machine is great if it works but I never rely on it.