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10.5: One possible cause of huge Time Machine backups
Authored by: iangoldby on Jul 16, '09 03:02:28PM

This is a different cause, but someone else might be reading here because some of their Time Machine backups take an excessively long time to complete, so I'll share it.

I've noticed that occasionally Time Machine will take an excessively long time to do a backup, and for most of the time the Time Machine control panel says that it is just preparing the backup. The amount of data to backup once it gets going is often rather small.

I think this happens when the Mac suffers a hard reset - i.e. it was shut down by a kernel panic or by holding down the power button after it became unresponsive. My theory is that Time Machine works by monitoring all changes to the filing system, so when it comes to do a backup it already knows what files have changed without having to compare the current files with the last backup. But if the Mac suffers a hard reset then it reckons it might have missed some changes. So it has to work out what files to backup the long way by comparing all files with the backup, and that takes a very long time.

So it's annoying but probably nothing to worry about and won't happen often.

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10.5: One possible cause of huge Time Machine backups
Authored by: palahala on Jul 16, '09 03:57:20PM

Your theory is almost correct. Your Mac keeps track of all changed directories (not all files), and Time Machine uses this information to only scan the directories for which it knows something changed. Indeed, if your Mac for any reason thinks that this information is no longer correct, then it will declare all the old information invalid. Time Machine then needs to compare your backup disk with your Mac's disk.

See also some very nice articles on Ars Technica and Earthling Soft.

Note that the logfile actually indicates this situation. See Time Machine Buddy to easily view that log.

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