Monday, October 29, 2007

Verify your backups

Apple shipped Mac OS X 10.5 this weekend, and three of the features are Time Machine, dtrace, and improved CHUD tools. Time Machine, dtrace, CHUD tools. iPod, mobile phone, web browser. Time Machine, dtrace, CHUD tools.


To spell that out in long hand, it's very easy now to see how various features in the Operating System behave. And in the case of Time Machine, we see that it walks through the source file system, copying the files to the destination. When I last gave a talk to OxMUG on the subject of data availability, it was interesting to notice how the people who had smugly put their hands up to indicate that they performed regular backups became crestfallen when I asked the second question: and how many of you have tested that backup in the last month?


Time Machine is no different in this regard. It makes copies of files, and that's all it does. It doesn't check that what it wrote at the other end matches what it saw in the first place, just like most other backup software doesn't. If the Carbon library reports that a file was successfully written to the destination, then it happily carries on to the next file. Just like any other backup software, you need to satisfy yourself that the backup Time Machine created is actually useful for some purpose.

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