An interesting question came up on the MacGroup iBBS today (and if you're a MacGroup member, you really owe it to yourself to sign up for the iBBS). Someone using Time machine to back up their Mac was wondering where all the space went on his disk drive. He has a 500 GB disk that under half used, yet his 1 TB Time Machine drive is at 924 GB used. Fortunately, he also mentioned a key piece of information – he runs Windows as a virtual machine using VMWare's Fusion product.
Virtual machines are a great way to run Windows, Linux, and now another copy of Mac OS X as a secondary machine while you're running your current machine – no need to reboot or re-partition you hard drive. You do lose some speed this way, but if you're running Office or some other non-realtime, intensive programming, this can be a great way to run a program that's written for some other OS on your Mac.
Instead of installing to a physical hard drive, virtual machines (VMs) install to virtual disks – which are basically large file, sort of like a .DMG or an .ISO file is a mountable disk image. When you look on disk, you see a single file that represents your VM, but in reality that's a container. You can right-click on a VM and if you Show Package Contents you'll see something like this:
I've highlighted the virtual disk – the file is pretty large, as you can see.
So what's all this got to do with the full-up Time Machine disk? Well, when you start up your virtual machine, you open this file. Windows, or what the guest OS is, will start writing log files and other system files even if you don't do anything else. So this file gets updated. Every hour, Time Machine looks at files that have been updated and copies them out to its backup, keeping as many older versions as it can on hand until it's forced to start removing files to make room.
So if you were to leave Windows up for a day, with a 20 GB file, and let Time Machine back it up every hours, you'd be looking at around 500 GB used up for just this file. Older copies of other files might get removed to make room.
That's why you need to exclude these files from your Time Machine backups – they're just too large. You can do this by opening up Time Machine's preferences and then clicking on "Options…" to bring up a list of disks, directories, and files that you want excluded. Parallels I believe excludes their VM directory automatically during install.
You may have other files like this Microsoft Entourage has a single database file where it stores all emails. Get one email an hour, and you get a new copy of this file. If you have a lot of email, suddenly you have a lot of disk eaten up on your Time Machine backups. So you should exclude that file as well.
So if you exclude these files, what happens if you need them back? Well, hopefully, you aren't relying solely on Time Machine for your backups. If you read calvin's articles on Fridays, he explains about the different backup types, and when to use what. Suffice to say here that you should be taking regular clone backups of disk in addition to Time Machine backups. You can only restore files from a Time Machine backup, you can't boot. But you can boot a clone and be up rather quickly (if maybe a bit behind in some files – that you can restore from your Time Machine backup).