All Time Machine backups take too long

2018 Mac mini running macOS 12.6.1. I have an 8TB USB drive connected to the machine with two volumes on APFS. One volume I use to Time Machine backup my laptop over the network, and that seems to work just fine. The other volume is a local Time Machine volume for the mini. This is a fairly new disk and it was all working fine for a couple of weeks, but now the local backup takes forever and seems to find hundreds of thousands of files to backup. It takes over 24 hours to complete. Then the next time Time Machine starts, it all happens again, hundreds of thousands of changes found and it runs for 24+ hours. It's almost like the mini doesn't recognize that it only has to do an incremental backup. I know for sure there aren't all that many changes between each backup run. I've double checked the Excludes From Backup options and that all looks reasonable (it excludes both the local and remote TM volumes, iCloud, OneDrive, and Dropbox folders). I can't find anything else that would include so much changes. It estimates the size of a full backup at 1.68TB which appears to be the entire Macintosh HD (303.64GB available of 2TB). I can't find any other settings that would cause TM to do a full backup every single time and I've rebooted the machine at least once to see if that fixes the problem, to no avail.

I can't upgrade this machine to Ventura yet due to compatibility issues. Any ideas what's going on or what to look for?

Mac mini 2018 or later

Posted on Dec 2, 2022 5:29 PM

Top-ranking reply

pumpichank Author

Level 1

27 points

Posted on Dec 31, 2022 4:02 PM

Following up with the resolution and information I've found.

Thanks for the BackupLoupe recommendation. It's a great tool and ultimately provided me with exactly the information I needed to figure out what was going on. Aside: I've had some trouble with BL getting stuck when indexing aborted TM backups, but the company/developer behind it is responsive and we're trying to work out the resolution of that problem.

For my TM performance problems, BL not only helped me identify old cruft that I didn't need (like Application Support files for apps I no longer use or have installed), but it also helped me understand why it was mysteriously trying to back up huge amounts of small files. Not only did this consume too much of my TM partition, but it also is what I think caused the backup to take so long. It turns out that I use the Backblaze online backup service, and that has a cache folder that contains copies of files during upload, log files, etc. Once I excluded /Library/Backblaze.bzpkg from my TM backups, performance returned to what I expected and seems to be operating well now. I confirmed with Backblaze that it's essentially okay to exclude this directory from TM. It'll cause a little extra work to recover things if I lose /Library/Backblaze.bzpkg and need to do a Backblaze restore, but it's fairly easily recovered and it doesn't compromise my backups.

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All Time Machine backups take too long

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