TimeMachine - how to delete old backups and regain space?

[Leon1das]

New to TimeMachine.
I partitioned my external 1TB SSD to several partitions - and keeping 150Gb for TimeMachine.
I dont do automatic backups - as SSD is not constantly plugged in - I just use manual backups.
So here is the issue.
Backup 1 takes 120Gb out of 150Gb allocated to TM.
When I want Backup 2 - I am said I have 30Gb available space.
Why?
I thought - TM will overwrite 1st backup (or at least ask me if I want to) and move ahead.

OK, never mind. I will delete Backup 1 to make space for Backup 2.
I open TM partition in Finder and manually delete Backup 1.
But even then - TM still shows only 30Gb of free space - and my only solution is open Disk Utility - delete TM disk partition and create it again.

Is there any faster way deleting/rewriting older backup to regain space on TM?
Happy to learn if I am doing something wrong.
Many thanks

 

[jdb8167]

You should probably ask this in the general MacOS forum(s).

 

[Brian33]

So here is the issue.
Backup 1 takes 120Gb out of 150Gb allocated to TM.
When I want Backup 2 - I am said I have 30Gb available space.
Why?
I thought - TM will overwrite 1st backup (or at least ask me if I want to) and move ahead.

I think you are mis-understanding how Time Machine works. After the first backup, TM will only backup files that have changed since the previous backup. 30GB is enough space for more backups (unless you often change multi-GB files). Don't delete Backup 1 -- just let TM do it's thing!

There are two important things about TM:

(1) only files that have actually changed since the last backup are actually copied anew,

and despite point (1),

(2) each "backup" (technically, each snapshot) effectively contains all of the files that existed on the system at that date and time.

Point (2) is done through some filesystem magic: if a certain file hasn't changed between backups, then multiple snapshots all "point to" a single copy of that file. Only when all of the snapshots that point to that file are deleted will the (one copy of that) file be deleted.

Of course, after awhile, TM will run out of space on the backup disk. When that happens, TM will automatically delete one or more of the oldest snapshot(s) to make enough free space for the new backup. But this only deletes files that were unique to thoses first backups -- files that are "pointed to" by subsequent backups are not deleted by this action.

The beauty of Time Machine is that you can restore your entire system to any date/time in the past for which you still have a snapshot on the disk. The more GB TM has to work with, the older your oldest snapshot can be, and the more choices you have to restore from. System Preferences-->Time Machine will tell you how old your oldest snapshot is. With my setup I'm able to restore any file (or the entire system) from any point up to 9 months ago!

 

[Leon1das]

Terrific explanation - thank you for the time for writing this.