Carrying on from my earlier blog post about the hacked version of GrandPerspective and how to use it on your Time Machine backups: It turns out there was something I didn't appreciate correctly. The hack here is that if a file is hard-linked, it's filtered out, as if it didn't exist at all.

So this means that my earlier statement, that this version of GrandPerspective gives you a better idea how large your backup folder is, was totally wrong (and has been cleverly expunged). Even more important, it raises the question: "So what's the good of that?"

It's good for two reasons. First, a big file or folder whose contents are changing faster than the frequency of your Time Machine backups (snapshots) will not be a hard link, it will show up in the GrandPerspective display, and so it will be easy to spot. The efficacy of this depends somewhat on your not making snapshots too often, or on your pruning your backups. But I do prune my backups (using Delete Backup to remove entire snapshots at a time), so it is efficacious.

Second - and this is really the clincher - a lot depends on when you use this version of GrandPerspective. A really good time to use it is immediately after making a snapshot. Point it at the "Latest" backup folder, and you will instantly learn what was copied into your backup folder in this most recent snapshot. What you learn may surprise you, and may help you take some control of your backups.

Here's what happened to me today. I made a snapshot and was amazed to have Time Machine tell me that it was copying 3GB. Why? I ran GrandPerspective to find out, and saw that this was all a bunch of MP3 files in my iTunes Library. But these files weren't new! No, but they were changed. I had recently assigned them an album, to help clump them together on my iPod.

This is just the sort of thing that's wrong with the whole Time Machine concept. Here we are, backing up 3GB of songs, duplicating the existing 3GB of songs within the backup folder, just because I made one tiny little change. Each of these songs now appeared in two versions in the backup folder - unnecessarily.

Time Machine doesn't give you an obvious way out of this situation, but with a little luck, you can mess with its head. Here's what I did. I entered the Time Machine interface, found the files in question, and told Time Machine to delete all copies of them. Now each song appeared in no versions in the backup folder. Then I exited the Time Machine interface, and immediately made a new snapshot. Now each song appeared in one version in the backup folder - which is just what I wanted.