What is happening with System Data on MacOs 13.1 Ventura?

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What is happening with System Data on MacOs 13.1 Ventura?

Yesterday I cleaned out some old video projects on my iMac (2017, 27-inch). I started out with about 420-425 GB of free space on a 1 TB drive. I cleaned out about 120-150 GB of space, but after emptying out the Trash, I still had about the same amount of available space (420-425-ish GB). The space migrated to System Data because it increased the same amount.

I read a few forum posts here in the Community and found a tool that helped to find "orphaned" files and directories called OmniDiskSweeper (https://www.omnigroup.com/more) and ran it. After removing several files and directories (none of which were shown on my Mac's "Documents" in the Storage section) I had just under 600 GB available (about 597 GB). My System Data dropped to around 20-30 GB!!!

This morning I start up and decided to check it again, to double check if I had enough space for an upcoming project, only to find that I have even LESS space than I started with before this entire process. I even restarted just to see what would happen. As it stands, I currently have 397 GB available. My System Data is at 229 GB. Which means 200 GB moved from being available to System Data - 597-200 = 397 (my free space from yesterday to today). System Data - 29 + 200 = 229 ( my change in data from yesterday to today).

What is going on here?!

iMac 27″ 5K, macOS 13.1

Posted on Jan 19, 2023 06:18 AM

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Answer 1

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James Brickley

Level 5

5,614 points

Jan 19, 2023 06:55 AM in response to mikesway79

It's likely APFS snapshots related to Time Machine. There is a setting in Disk Utility to reveal the APFS snapshots.

Highlight Macintosh HD and View then Show APFS Snapshots.

The way snapshots work is they don't take any space until you modify the file system. When you deleted the data, the snapshots grew in size to allocate the deleted data. I don't think the OmniDiskSweeper had as large an impact as you think. I think that a Time Machine backup occurred and eventually cleared out the snapshots once that data was backed up and after 24 hours perhaps overnight the snapshots were cleared by macOS.

Some explanations can be found here:

https://eclecticlight.co/2022/12/30/free-space-on-an-apfs-volume-is-an-illusion/

https://eclecticlight.co/2019/09/20/the-problem-with-apfs-snapshots/

https://eclecticlight.co/2022/07/19/what-can-you-do-with-time-machine-backups-on-apfs/

You might want to consider an alternative backup solution to Time Machine such as Carbon Copy Cloner which also uses snapshots but you will have greater control over them. https://bombich.com/

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Answer 2

mikesway79 Author

Level 1

8 points

Jan 19, 2023 11:23 AM in response to James Brickley

Thanks for the help, James.

I don't know too much about file systems in-and-of themselves, other than formatting drives properly for the right system, but the deeper I get into APFS the more complex and frustrating it can be. The articles help to clarify a few things so I appreciate the links.

I remember seeing Carbon Copy Cloner years ago, but it fell off my radar at some point, so I'll definitely give it a test run.

Again, many thanks for the help.

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What is happening with System Data on MacOs 13.1 Ventura?

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