What Time Machine leaves behind
April 4, 2008
I mentioned earlier that while I like Time Machine a lot, depending on it as your sole backup is rather fraught with peril.
This post on macosxhints gives another good reason — apparently some applications (including notably, VMWare) silently exclude data files from Time Machine.
You can see which files from the Terminal:
Turns out that Apple’s doing the right thing here and using the metadata store. This means we can find the files with the Spotlight engine. To do this from the command line, do this:
sudo mdfind “com_apple_backup_excludeItem = ‘com.apple.backupd'”
(note: that command will require you to enter your password)
The macosxhints article also gives several variations of an Applescript you can use for this task. The list of files returned does not include files or folders that you’ve manually excluded — these are files that are excluded without your knowledge.
Checking, sure enough, my VMWare virtual machines are excluded, along with what appears to be my iTunes library file (probably because iTunes is running at the moment).
Note that this makes perfect sense, given the way that VMWare and Time Machine work — I wouldn’t want Time Machine to save a 20 gig virtual machine every hour while I was using VMWare — but I would be very, very unhappy if I had to restore from backups and found that my virtual machines were gone.
The long and short of this is that while Time Machine is wonderful for certain things — and far better than not doing backups at all — it’s still an supplement for a good clone backup, just not a replacement for it.