When Time Machine Volume Fills Up...

Discussion in 'macOS' started by Chaszmyr, Oct 30, 2007.

  1. Chaszmyr macrumors 601

    Chaszmyr

    Joined:
    Aug 9, 2002
    #1
    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...
     
  2. Decrepit macrumors 65816

    Decrepit

    Joined:
    Sep 6, 2007
    Location:
    Foothills to the Rocky Mountains
    #2
    I was wondering the same thing. Here's hoping you get a response shortly.
     
  3. SDDave2007 macrumors regular

    Joined:
    Apr 12, 2007
    #3
    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?
     
  4. Decrepit macrumors 65816

    Decrepit

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    #4
    There are enough people generating enough versions of stuff that we should hear about this in a couple of weeks. :)
     
  5. hh83917 macrumors 6502

    Joined:
    Jun 30, 2005
    #5
    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.
     
  6. Chaszmyr thread starter macrumors 601

    Chaszmyr

    Joined:
    Aug 9, 2002
    #6
    I've been thinking more about the quote on Apple's site...

    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.
     
  7. Doctor Q Administrator

    Doctor Q

    Staff Member

    Joined:
    Sep 19, 2002
    Location:
    Los Angeles
    #7
    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.
     
  8. Virgil-TB2 macrumors 65816

    Virgil-TB2

    Joined:
    Aug 3, 2007
    #8
    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.
     
  9. CavemanUK macrumors 6502

    CavemanUK

    Joined:
    Jun 29, 2006
    Location:
    Rhyl, North Wales
    #9
    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.
     
  10. micsaund macrumors 6502

    micsaund

    Joined:
    May 31, 2004
    Location:
    Colorado, USA
    #10
    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
     
  11. SilentPanda Moderator emeritus

    SilentPanda

    Joined:
    Oct 8, 2002
    Location:
    The Bamboo Forest
    #11
    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
     
  12. Sweetfeld28 macrumors 65816

    Sweetfeld28

    Joined:
    Feb 10, 2003
    Location:
    Buckeye Country, O-H
    #12
    I would think that it would delete the oldest of a duplicate file, and not delete the oldest single files. :confused:
     

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