Mac Backups and Cloning: Why APFS Snapshots Can be Problematic

My 2023 Mac Pro M2 Ultra has been one of the most stable Macs I have owned. But it still crashes about once a month which is too often and really quite unacceptable.

After a crash, is it all still good?

After a crash, it is best to check the file system on each and every attached drive. That is, a crash could cause damage to the file system. This damage won’t necessarily manifest in an immediate or obvious problem, or any problem of any kind. But sometime later something could go haywire, seemingly out of the blue.

Clone backups

Multiple clone backups are the mainstay of a solid backup strategy. I covered how to make clone backups in the article Backup Best Practices: How to Make Fast Clone Backups on a Mac. You should have at least two such backups for your critical data, and ideally have at least one offsite away from the computer and its associated risks.

An appealing feature of clone backups is the ability to take snapshots of the backup iteration. Apple defines APFS snapshots as “a read-only copy of its parent APFS volume, taken at a particular moment in time.” This makes it possible to get at something specific on your system 3 hours, 3 days, or 33 days ago—a similar idea as how Apple’s Time Machine backups work in practice.

APFS snapshots make file system verification glacially slow

But snapshots come with a catch—disk first aid with more than a few snapshots can take a very, very long time to check.

After a crash one day, I wanted to verify all my volumes. This is usually a quick process taking perhaps 30 seconds for most volumes, a few minutes for ones with millions of files. Very reasonable.

But on this occasion, I quickly ran into a problem: one of the backup drives had snapshots enabled—485 of them. When I started running Apple Disk First Aid, it was taking 20-30 seconds per snapshot. At that rate, it would take 3-4 hours in order to verify the file system on that drive. I find this unacceptable. In fact, I have 3 or 4 such volumes with a similar number of snapshots. What if I had to perform first aid on each of them?

Removing and disabling snapshots

Regrettably, my recommendation is to disable snapshots, as I know of no other workaround to making disk first aid usable.

Whatever theoretical benefit snapshots might offer, which I have never used, the cost of having to wait hours to verify the integrity of my volumes is simply unacceptable.

My backup program of choice is Carbon Copy Cloner. Luckily it makes it easy to remove the snapshots by turning them off. By turning off the option checkbox, CCC removes them on the spot very quickly. I was then able to verify the volume in short order. Problem solved!

Conclusions

Backup versioning has its place, which is why I run Apple Time Machine in addition to clone backups.  But clone backups are a mainstay, and so I will not be using snapshots with them given how long disk first aid takes to verify them.

Storing Important Data on the Boot Drive is Asking For Trouble — Always Make a Separate Volume
A Pro Photographer’s Guide to Fast Data Backup in the Field
The Best Drives and Smart Strategy for Safe, Secure and Consistent Mac Backups
Backup Best Practices: How to Make Fast Clone Backups on a Mac
What to Do Your When Mac’s Time Machine Says Its Backup Drive Is Full
Backup Best Practices: How to Use Time Machine Optimally With Other macOS Backups

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Need help working through your computing or photography system design? Consult with Lloyd before you potentially buy too much or too little, and see my Mac wishlist of current models, first pondering whether refurbished or used Macs might do the job for you.

View all posts by Lloyd Chambers… Lloyd’s photo web site is diglloyd.com, computers is MacPerformanceGuide.com, cycling and health are found at WindInMyFace.com, software tools including disk testing and data integrity validation at diglloydTools.com. Patreon page.