When Time Machine Volume Fills Up...

Chaszmyr

macrumors 601
Original poster
Aug 9, 2002
4,263
76
I've done a lot of searching and haven't been able to find an answer to this question, so maybe it's just the case that no one really knows yet, but here goes...


Time Machine says when the target backup volume runs out of space it overwrites files based on age. My question is, is there any way to keep it from overwriting files that only have one copy? For example, let's say I have a file that has not been changed for 2 years, so only one copy of it exists on the backup volume. Would Time Machine overwrite that file to make room for new copies of files that receive frequent changes?

I don't see anything that suggests that wouldn't happen, but if it does, it sure seems like Time Machine wouldn't actually make for a very efficient backup...
 

Decrepit

macrumors 65816
Sep 6, 2007
1,146
0
Foothills to the Rocky Mountains
I've done a lot of searching and haven't been able to find an answer to this question, so maybe it's just the case that no one really knows yet, but here goes...


Time Machine says when the target backup volume runs out of space it overwrites files based on age. My question is, is there any way to keep it from overwriting files that only have one copy? For example, let's say I have a file that has not been changed for 2 years, so only one copy of it exists on the backup volume. Would Time Machine overwrite that file to make room for new copies of files that receive frequent changes?

I don't see anything that suggests that wouldn't happen, but if it does, it sure seems like Time Machine wouldn't actually make for a very efficient backup...
I was wondering the same thing. Here's hoping you get a response shortly.
 

SDDave2007

macrumors regular
Apr 12, 2007
197
1
If you think about what Time Machine is, and what its supposed to do.. It only makes sense to over-write copies of the oldest files.... BUT... the oldest files that have newer copies backed up.

So if your Time Machine image has a copy of XYZ.APP that is 2 years old, and 40 versions of ABC.TXT I would think that it would never touch XYZ.APP until it had eaten at least 39 versions of ABC.TXT first. Basically reducing the number of "generations" of available restore points.

This way Time Machine can still do its job to some extent... and that is restore you computer to an image that existed in the past... and that image would need to have a copy of XYZ.APP in it.

So the question then becomes.... what does Time Machine do when the backup device is full and there is only ONE generation of files stored?
 

Decrepit

macrumors 65816
Sep 6, 2007
1,146
0
Foothills to the Rocky Mountains
If you think about what Time Machine is, and what its supposed to do.. It only makes sense to over-write copies of the oldest files.... BUT... the oldest files that have newer copies backed up.

So if your Time Machine image has a copy of XYZ.APP that is 2 years old, and 40 versions of ABC.TXT I would think that it would never touch XYZ.APP until it had eaten at least 39 versions of ABC.TXT first. Basically reducing the number of "generations" of available restore points.

This way Time Machine can still do its job to some extent... and that is restore you computer to an image that existed in the past... and that image would need to have a copy of XYZ.APP in it.

So the question then becomes.... what does Time Machine do when the backup device is full and there is only ONE generation of files stored?
There are enough people generating enough versions of stuff that we should hear about this in a couple of weeks. :)
 

hh83917

macrumors 6502
Jun 30, 2005
296
65
I think Time Machine is not really a "backup" program. It's more like an ongoing trash can where things that you've deleted "not too long ago" (depend on your Time Machine drive space) can get pick up again when you've decided not to delete it. It makes a clone copy of your drive (like e.g Carbon Copy Cloner) and continue to make incremental copies of changed files. I don't think it will overwrite file that only have one copy. It will be more likely that Time Machine will ask you if you want to delete the oldest version of the file that you've edited often in order for it to make incremental copies of that often changed file. Correct me if I'm wrong.

Therefore, Time Machine will not make backup softwares like SuperDuper and CCC obsolete because it is still best to make cloned copies of your drive for storage, if you really have things that are worth protecting.
 

Chaszmyr

macrumors 601
Original poster
Aug 9, 2002
4,263
76
I've been thinking more about the quote on Apple's site...

Backing up to a full disk.
One day, no matter how large your backup drive is, it will run out of space. And Time Machine has an action plan. It alerts you that it will start deleting previous backups, oldest first. Before it deletes any backup, Time Machine copies files that might be needed to fully restore your disk for every remaining backup. (Moral of the story: The larger the drive, the farther back in time you can back up.)
This sounds to me as if Time Machine at very least tries to maintain the current version of everything on your hard disk (which I guess would depend on the Time Machine volume being as large or larger than the total contents of your main drive... god only knows what it would do otherwise), which would be fine by me. Still, I'd really like confirmation of this.
 

Doctor Q

Administrator
Staff member
Sep 19, 2002
37,712
3,819
Los Angeles
Because of the way Time Machine works, if a file has never changed, its data is stored only once on the backup drive but the file is treated as "part of" every snapshot. If Time Machine deletes the oldest snapshot, that file remains as part of the second-oldest snapshot, and you've lost nothing.

I'll use letter suffixes to represent file revisions in this example:
Monday snapshot: File 1, File 2, File 3
Tuesday snapshot: File 1, File 2a, File 3
Wednesday snapshot: File 1, File 2a, File 3a
Thursday snapshot: File 1, File 2a, File 3b
Friday snapshot: File 1, File 2b, File 3c​

If Time Machine deletes the Monday snapshot, you then have:
Tuesday snapshot: File 1, File 2a, File 3
Wednesday snapshot: File 1, File 2a, File 3a
Thursday snapshot: File 1, File 2a, File 3b
Friday snapshot: File 1, File 2b, File 3c​
You lose the oldest version of File 2, but you don't lose the single version of File 1.
 

Virgil-TB2

macrumors 65816
Aug 3, 2007
1,143
1
If you think about what Time Machine is, and what its supposed to do.. ...
You should read the details on how time Machine works. Your thoughts make sense in regards a typical back-up situation, but Time Machine works a bit differently. It stores only the changes in files, not copies of the files themselves (after the first backup).

If a file never changed, then it is a unique copy and would not be purged no matter how old it is. If it is changed, all the changes (and of course the original file), are kept.
 

CavemanUK

macrumors 6502
Jun 29, 2006
437
8
Rhyl, North Wales
Because of the way Time Machine works, if a file has never changed, its data is stored only once on the backup drive but the file is treated as "part of" every snapshot. If Time Machine deletes the oldest snapshot, that file remains as part of the second-oldest snapshot, and you've lost nothing.

I'll use letter suffixes to represent file revisions in this example:
Monday snapshot: File 1, File 2, File 3
Tuesday snapshot: File 1, File 2a, File 3
Wednesday snapshot: File 1, File 2a, File 3a
Thursday snapshot: File 1, File 2a, File 3b
Friday snapshot: File 1, File 2b, File 3c​

If Time Machine deletes the Monday snapshot, you then have:
Tuesday snapshot: File 1, File 2a, File 3
Wednesday snapshot: File 1, File 2a, File 3a
Thursday snapshot: File 1, File 2a, File 3b
Friday snapshot: File 1, File 2b, File 3c​
You lose the oldest version of File 2, but you don't lose the single version of File 1.
Agreed.. this is exactly how Time Machine works... Its also worth pointing out that File 1 only ever takes up 1 space on your backup drive. The file system uses "hard links" so that the same file is referenced by each backup. So if your oldest backup goes, thats fine.

Obviously if you were to delete that file from your current machine, then over time that file would dissappear. In this case, you should be making an external backup of files before deleting them if they are important.
 

micsaund

macrumors 6502
May 31, 2004
364
0
Colorado, USA
If you still have a file on your active drive, TM will have a complete copy of it in the backups. Worst case, your external drive is the exact same size as your internal drive, and you get an effective mirror 1:1 copy. If you have more space on the external, then you get additional copies of the various versions of the file.

Time Machine is basically a fancy wrapper around a long-time Unix tool called snapshot (one variant of which I've been running on my Mac for a while to do my own "time machine" backups to an external drive). It uses extensive hard linking on the file system to save space. And no, it canNOT backup partial files - if one byte of a file is changed, the entire file is duplicated. Links don't support block-level changes -- you'd need ZFS or similar to do that.

Mike
 

SilentPanda

Moderator emeritus
Oct 8, 2002
9,993
30
The Bamboo Forest
So the question then becomes.... what does Time Machine do when the backup device is full and there is only ONE generation of files stored?
The only way this could happen is if your Time Machine drive was smaller than your System drive.

I suppose somebody could set up their iPod Shuffle as a Time Machine backup drive and test it out really fast... :p
 
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