Time Machine confusion

Gordon

Hello! I'm a bit confused about the rolling backups that Time Machine does..

My understanding is this:

Day 1 - Mirror all content from Mac onto Time Machine
Day 2 - Copy only the amendments from Day 1 Mac onto Time Machine
Day 3 - Copy only the amendments from Day 2 Mac onto Time Machine
Day 4 - etc

So Day 1 is a large file (same as all used space on my Mac) and subsequent updates are a lot smaller as less files would be created/deleted than the initial Day 1 backup.

Is that correct so far?! If so, what happens when the disk is full? I was under the impression that the oldest backup would auto-delete to save space on the drive for latest updates. If that's the case - then isn't the original content being deleted and only the updates remain?

Day 1 - Contains - Files A,B,C,D,E,F,G,H,I
Day 2 - Contains - Files J,K,L
Day 3 - Contains - Files M,N
Day 4 - Contains - Files O

When the disk gets full and Day 1 is the oldest backup, that would get deleted, so wouldn't I just be left with Files J,K,L,M,N & O?

Or have I got this all arse-ended?!

(Oh, and I've no idea why you can't tell Time Machine to only back up once a week or so, as it seems to do an hourly backup when the disk is connected.)

Sad Professor

First part is correct.

If so, what happens when the disk is full? I was under the impression that the oldest backup would auto-delete to save space on the drive for latest updates. If that's the case - then isn't the original content being deleted and only the updates remain?

Day 1 - Contains - Files A,B,C,D,E,F,G,H,I
Day 2 - Contains - Files J,K,L
Day 3 - Contains - Files M,N
Day 4 - Contains - Files O

When the disk gets full and Day 1 is the oldest backup, that would get deleted, so wouldn't I just be left with Files J,K,L,M,N & O?

No, Time Machine is much smarter than that. As I understand it, when TM makes a backup it is doing two thing: it is taking a "snapshot" of your hard drive and it is incrementally copying any new files or files that have been changed since the last snapshot was taken. The same file may exist across multiple "snapshots" but is only copied once. For example, File B may have been present during all 4 of your backups, while File A might have been deleted just prior to your second backup.

When TM runs out of space and has to delete the oldest backup, it deletes the snapshot but not the files that were copied during that backup. It will only delete a file when it is not longer associated with a snapshot. So lets you run out of space. The Day 1 snapshot gets deleted along with File A because it only existed in that snapshot (you deleted it prior to Day 2), while Files B-I would remain because they also exist in other snapshots.

Hope that makes sense.

Colonel Panic

Basically, Time Machine tracks file changes using the same kernel subsystem as Spotlight. Each time it fires, it'll back up changed files and manage a series of hard links that tie the files together on your Time Machine disk.

So when old backups are deleted, it's changed files at that backup. It will never delete files that you did not yourself delete.

So the only way it would delete files A through I would be if you deleted them and that backup eventually expired.

Gordon

Cool, cheers all I got confused when I started deleting old backups to make space for other content on the hard drive. I saw that each date's TM folder was roughly the total size of my Mac hard drive. But I've got loads of TM date folders, if added up, the total size of the TM folders would be much more than the disk capacity, so it must work differently than a regular folder..

Thanks for the info

Sad Professor

Yeah I think they are called hardlinks. It looks like are several copies of the file, but there's really only one. It's a feature of Unix.