Time Machine is Mac OS X’s backup solution. It supports incremental backups. Which is really the way to go. But I avoided it because of the email program I use: MacSOUP. Like most email programs it keeps individual messages in large files. I currently have 2.3 gigabytes of email. The actual implementation MacSOUP uses is to have a file for each mailbox.
My largest mailbox is 600 megabytes. The file grows every time I get email, dozens of times per day. This will cause a problems for me because my backup hard drive is the same size as my main drive. This means the drive will fill up very fast and I will have few incremental snap shots to choose from.
Apple got around this problem for Mail.app by keeping email messages in their own separate files. So Time Machine’s file based incremental backup fits nicely with that scheme.
I recently learned about the new bundle version of Apple’s sparse disk image format. Using this image format can really help keep Time Machine incremental backups from filling up the back up hard drive.
First some definitions. A disk image is a file on your computer that you can mount as a hard drive. It is becoming very common for software to be released on disk images. Like real hard drives, disk images have a maximum amount of data they can hold. A sparse disk image keeps the image file small and it will grow as you increase the amount data stored on the image. A sparse disk image bundle uses many 8 megabyte files to contain the image data. The small files are created as needed and writes to the disk image only modify the individual files that have changes.
This fits very nicely with Time Machine. MacSOUP can use it’s large files to store email and Time Machine has it’s small files for incremental backup.
Tidbits has a nice tutorial on how to create these sparse disk image bundles.
After copying all of my MacSOUP data to the mounted disk image, I made an alias of the MacSOUP settings file. The alias file resides in my documents folder. I then added that alias to my dock. When I click the alias, the system is smart enough to mount the disk image if needed and then open the file which starts MacSOUP.