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I keep seeing Time Machine doing a full backup of my ~80 GB hard disk. I see this a few times a week, when I first turn on my mac. During the day, I see Time Machine doing small incremental backups of ~100 MB, hourly.

Why does Time Machine does these big backups?

I also see the large backups in the logs here.

nanceki@Stephen-Nancekivells-iMac /v/log> bzcat system.log.* | grep backupd | grep Copied | grep GB| sort | less

Jul 23 08:46:32 Stephen-Nancekivells-iMac[7569]: Copied 0 bytes of 78.3 GB, 0 of 23051 items
Jul 23 08:47:31 Stephen-Nancekivells-iMac[7569]: Copied 5324 files (1.1 GB) from volume Macintosh HD. 
Jul 24 08:55:37 Stephen-Nancekivells-iMac[20310]: Copied 0 bytes of 78.4 GB, 0 of 23013 items
Jul 24 09:52:18 Stephen-Nancekivells-iMac[20310]: Copied 24172 files (78.4 GB) from volume Macintosh HD. 
Jul 28 13:43:25 Stephen-Nancekivells-iMac[98930]: Copied 0 bytes of 76.5 GB, 0 of 4557 items
Jul 28 15:43:43 Stephen-Nancekivells-iMac[98930]: Copied 20.4 MB of 76.5 GB, 2477 of 4557 items
Jul 28 17:43:59 Stephen-Nancekivells-iMac[98930]: Copied 305.8 MB of 76.5 GB, 5797 of 5797 items
Jul 28 19:45:03 Stephen-Nancekivells-iMac[98930]: Copied 1015.8 MB of 76.5 GB, 5797 of 5797 items
Jul 28 21:44:58 Stephen-Nancekivells-iMac[98930]: Copied 1017.8 MB of 76.5 GB, 5797 of 5797 items
Jul 28 23:45:22 Stephen-Nancekivells-iMac[98930]: Copied 1.4 GB of 76.5 GB, 5797 of 5797 items
Jul 29 01:45:40 Stephen-Nancekivells-iMac[98930]: Copied 1.8 GB of 76.5 GB, 5797 of 5797 items
Jul 29 03:46:08 Stephen-Nancekivells-iMac[98930]: Copied 2.4 GB of 76.5 GB, 5797 of 5797 items
Jul 29 05:46:26 Stephen-Nancekivells-iMac[98930]: Copied 2.8 GB of 76.5 GB, 5797 of 5797 items
Jul 29 07:46:47 Stephen-Nancekivells-iMac[98930]: Copied 3.4 GB of 76.5 GB, 5797 of 5797 items
Jul 30 03:50:41 Stephen-Nancekivells-iMac[98930]: Copied 4.1 GB of 76.5 GB, 5800 of 5800 items
Jul 30 05:51:07 Stephen-Nancekivells-iMac[98930]: Copied 4.4 GB of 76.5 GB, 5800 of 5800 items
Jul 30 07:51:20 Stephen-Nancekivells-iMac[98930]: Copied 4.8 GB of 76.5 GB, 5800 of 5800 items
Jul 30 09:02:00 Stephen-Nancekivells-iMac[98930]: Copied 5.4 GB of 76.5 GB, 5800 of 5800 items
Jul 30 09:55:30 Stephen-Nancekivells-iMac[98930]: Copied 6598 files (76.5 GB) from volume Macintosh HD. 
Jul 31 09:09:22 Stephen-Nancekivells-iMac[4793]: Copied 0 bytes of 76.5 GB, 0 of 4510 items
Jul 31 10:03:35 Stephen-Nancekivells-iMac[4793]: Copied 5615 files (76.5 GB) from volume Macintosh HD. 
share|improve this question
Can you tell if it actually adds 80GB to the backup disk? Or is it saying it needs to back up that much, but in the end only adds much less? – Arjan Jul 30 '12 at 7:04
How/where do you see the size of what Time Machine is backing up? In other words what is your source? – Pro Backup Jul 31 '12 at 19:17
I see the how much its backing up when clicking on the icon on the task bar. – Stephen Aug 1 '12 at 0:10
by grepping for "GB" aren't you excluding any log lines about smaller backups? – matt b Aug 13 '12 at 14:41
up vote 2 down vote accepted

You could try using BackupLoupe to inspect exactly what has been copied in each backup. I find I have to exclude things like my web browser cache to keep the size of the backups down.

share|improve this answer
Thanks. Thats shown me that some of my excluded files are not being excluded, and keep being backed up. Now to look into why thats happening. – Stephen Aug 1 '12 at 1:16

Time Machine doesn't normally do that. Open Console (under Applications/Utilities), select All Messages in the log list, and type "backupd" into the filter bar. If you scroll back up to the time of one of these full backups, there may be a log message explaining why it backed up so much.

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I suppose that Time Machine keeps those backups longer, while the incremental backups are kept just for the last day; so TM doesn't have to consolidate hourly backups into a single one to keep a long history. But this is just a guess.

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I think your guess is wrong. The reason is that all TM backups are in some sense full: Every backup contains the entire directory structure with all the files in it. Files that are unchanged from one backup to the next will be hardlinked together, and even directories which are unchanged (including all files in all subdirectories, recursively) will be hardlinked together. (Normally, you can't hardlink directories, but Apple made an exception for the sake of TM backups.) – Harald Hanche-Olsen Jul 30 '12 at 6:55

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