When you want to make multiple backups of your files – to keep a number of versions, for example – you may worry about these backups filling up your hard disk. With Personal Backup, there’s a useful feature that lets you make multiple backups without using much space at all. It’s not voodoo, but it relies on an interesting technical trick.<\/p>\n
Lets say you want to keep ten backups of your important work files. You back them up several times a day, and you want to make sure that, if something happens, you can go back to an earlier version. Set up your backup script in Personal Backup with the Backup Options like this:<\/p>\n
When Personal Backup runs the first time, it will copy all your files. The second time around, it will create a new folder, and all your files will be in that folder (including those changed since the first backup, but without those deleted since then). However, it won’t take up much more space than the first backup: in fact, the only difference will be those files that you changed or added.<\/p>\n
Let’s be even clearer: the first backup is, say, 100 MB. The second, because of some new files, is 105 MB. However, the second backup actually only takes up an additional 5 MB on your hard disk.<\/p>\n
The trick that Personal Backup uses is called “hard links.” A hard link is similar to an alias on Mac OS X, but it is, in some ways, the original file. In other words, if you were to delete the first version of a file, the hard link that is the second version of the file will still be there, and it will be the file itself<\/i>. You can create as many hard links as you want to a file, and as long as one remains, the file is not deleted.<\/p>\n
Hard links take up no space, but they appear to take up space. In the above example, you’ll find that the first backup folder is 100 MB and the second 105 MB, but that’s only because the Mac OS X Finder interprets hard links as actual files when calculating disk space used. However, you’ll find that you still have free space; more than you should if you add up all of your backup folders.<\/p>\n
This is a hard concept to grasp, but what it means is that you can keep many backups of your files, and they will only take up more space when you add or change files. Personal Backup even includes an option to keep as many copies as possible until the destination disk is full: in that case, Personal Backup will keep adding new backups until there’s no more room, at which time it will delete the oldest backup(s) so new ones can be added. In this manner, you can keep dozens, even hundreds of backups of your most important files on a hard disk without worrying about filling up the disk.<\/p>\n