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When Time Machine says "I will delete the oldest files first", does it mean this literally? Here is a theoretical example:

  • Source Drive: 300 GB, consisting of 1 280 GB file and a 1 GB file.
  • Backup Drive: 300 GB

The initial backup will backup both files, using 281 GB.

If I modify the 1 GB file 21 times, what will Time Machine do when I run out of room on the backup drive?

  • Delete the original 280 GB because it is the oldest file? or
  • delete the oldest version of the file I have modified 21 times?

I hope it would delete the oldest version of the file I have modified 21 times, but I want to be sure.

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It's not file level, but backup date level. That's what it says, backups being a complete version of your data:

The oldest backups are deleted when your disk becomes full.

If you look into the computer-named subdirectory of the directory Backups.backupdb of your Time Machine volume, you'll see several folders named with a timestamp as name. These are your available backup versions. Each of these is the full content of your system volume (minus the excluded files) and can be selected independently for restoration e.g. during OS X setup.

To conserve disk space, Apple uses file and directory hard links, so if a file doesn't change between backups, the new file system entry points to the same data on the disk.

If Time Machine decides to delete older backups to free some disk spaces, it deletes one (or more) of these complete versions of your data. And here's the cool thing: If it deletes an old backup, but you didn't change a particular file in the mean time, it simply reduces the link counter by 1, and nothing actually is deleted (as there never was any data duplication on the backup volume). So if you still have the file (i.e. there are more recent backups that point to the same data), the file is retained, even though some hard links to it are removed.

This means, of course, that in your example, Time Machine will continue to delete old backups until some disk space is actually freed. If you still have the 280GB file, it just won't be deleted.

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Ah, got it. I guess. All programming and no play makes @slhck a dull boy. I forgot about the fact that you of course just need a recent copy of the file to have it retained. Still, in the example above, if both files are still present on the current system, and if the backup disk gets full, would that show the "Backup disk full" warning? I've just never seen it. – slhck Nov 21 '11 at 18:43
@slhck I haven't seen it either -- it's the harsh fate of a laptop user backing up to an external hard disk with ten times the capacity... – Daniel Beck Nov 21 '11 at 18:44

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