Time Machine thinks I made 850 thousand changes in an hour?

Question

My laptop has been nearly idle all day.  Edited a couple of Notes, two images, and downloaded maybe a dozen e-mails.  When Time Machine started a backup, it claimed to have found more than 850 thousand changes.  Previous backup on a different disk was only about an hour earlier.  Previous backup on the same disk was about four hours earlier.  (By the time-stamp in the directory name; I don't know whether that is the start time or the finish time.)

I know the O.S. does some things in the background, but 850K seems crazy, even if it did WiFi backups of two iPhones and an iPad.

Answer 1

This could be caused by events such as software updates, rescanning/reindexing of photos and other data, etc. Although 850.000 does sound like a lot in that case.

It could also be caused by something simple, but rather annoying, namely that perhaps your system clock was changed inadvertently.

I would check the system date and time, and let Time Machine do its work. If you do not experience any further problems with it, I would just accept it as is.

Answer 2

According to a so-called "senior advisor" at Apple (who had to find it in an article he admitted to not being very informative), these changes are not necessarily changed files but could be cached items or parts of changes, maybe even single line of code.

Even so, it's hard to believe there could be more than 650,000 in less than ninety minutes.

Answer 3

By default, time machine does backup pretty much everything. I found the culprit to be folders containing caches or regularly changing files of little relevance.

Which exactly depends on what applications exactly you are using, but here are some common ones to exclude using the "Options..." button.

• /Library/Caches
• ~/Library/Caches
• ~/Desktop because I subscribe to the mantra that no important files should be placed on the desktop; YMMV
• ~/Downloads because I can always redownload anything in it (hopefully)

Additionally you might want to add folders with local copies of cloud data, for example gaming clients:

• ~/Library/Application Support/GOG.com
• ~/Library/Application Support/Steam

If multiple users are using the same machine, consider adding the equivalent ~ folders for all of them, for example:

• /Users/someone/Library/Caches
• /Users/another/Library/Caches
• /Users/whoever/Library/Caches

If you want to include hidden folders that cannot be picked in the dialog, remember that you can press Command Shift . to make them visible in the dialog. For example:

• ~/.cache

And if you are a developer you will often have local build and cache folders for your project, for example:

• ~/Projects/some_python_app/build
• ~/Projects/some_python_app/venv

Sadly there is no option to ignore subfolders matching a name no matter where they are. For example, I would like to be able to ignore all folders names build, __pycache__ and venv.

Other things in Library one might want to exclude: HTTPStorages & Cookies.  And note that another way to add exclusions is to drag them from Finder into the exclusions list.

Answer 4

The easiest way to see what was in a Time Machine backup is to use the thirs party application Backup Loupe

This shows exactly what has changed between the backups.

e.g.