Snapshots Aren’t Backups
What is different is that restoring a whole volume from a snapshot is a one-way trip, and there is no undo. This is because snapshots subsequent to that used to restore from will be removed, and you won’t then be able to ‘roll forward’ to a later snapshot. That contrasts with a normal backup, where items remain available from any other backup that is retained in the backup store.
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Because snapshots share the same container as the current volume, and share many file extents with them, they are prone to common errors. In particular, common file extents make it more likely that faults occurring in extents and data storage will affect them both. This is particularly important as one of the most common file system errors that corrupts data in files occurs when extents for two separate files overlap. A snapshot is thus more vulnerable than a backup on a different disk, or even one in a different container on the same physical store.
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Snapshots do have one specific advantage over backups when it comes to their coverage. As they include the whole file system metadata for the volume, no items present in that volume are excluded from its snapshots. If you want to restore an item that has been excluded from backups made of any volume, you can therefore do that from its latest snapshot, if that item was present in the volume at the time that was made.
The only disadvantage to this is that snapshots can be disproportionately large compared to volume backups.
Snapshots are a great tool, but they don’t replace backups. The combination can be powerful. All my clone backups are now to APFS drives that make a new snapshot for each backup. I would like to be able to restore previous versions of files or folders from a year ago or more. Every once in a while I archive a clone drive and stop updating it. But I don’t have enough drives in rotation to keep a version for each week or month. Snapshots make that possible, albeit with less redundancy. Unfortunately, Mac backup software has kind of regressed in that it no longer provides great tools for browsing and searching old versions, but at least with snapshots we can easily and efficiently store them.
Previously:
- Carbon Copy Cloner 7
- Clearing Space on Your Mac
- Why You Can No Longer Roll Back a macOS Update
- Time Machine Evolution and APFS
- The Role of Bootable Duplicates in a Modern Backup Strategy
- Carbon Copy Cloner and APFS Snapshots
- SuperDuper 3.1 Supports APFS Snapshots for Both Source and Destination
- Local Time Machine Uses APFS Snapshots
- Testing Out Snapshots in APFS
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@Michael - What software are you cloning with these days? Where are we currently with SuperDuper, CCC, and ChronoSync for this? And I’m assuming this is also aimed to obtain bootable clones otherwise you’d use TM?
@Sean I’m using a mix of Carbon Copy Cloner and SuperDuper. I don’t think clones are really bootable these days. I use Time Machine, too, but it has lots of limitations. For example, all the source drives have to fit on a single backup drive, which is impossible for me.
@Michael Thanks a lot for surfacing this article!
To my knowledge, as soon as an external drive is formatted as APFS, Time Machine makes snapshots rather than traditional delta backups.
Besides, HFS+ drive performance tend to continually degrade with the number of backups being made on spinning HDDs, which means APFS is strongly recommended for these types of external drives.
Have you looked into that and if so, what strategy do you recommend?
Backup is fully broken on Mac. Nobody pays attention but not only it is super slow - and plus it backups tons of data you probably don’t need like OS. Would be great to have an option. And yes I forgot last time Time Machine actually worked. And backing up over network is broken as well. Bad times.
@Nathan Roughly 16 TB for the main Mac. Actually, I see that they now have 24 TB drives, so right now Time Machine could actually fit with enough room for versions. But this has not always been the case, and it’s slower and more expensive to put everything on the same destination drive. I really do not recommend Time Machine for big backups.
Has anyone used (is using) both Carbon Copy Cloner and SuperDuper?
Are there particular pros and cons to each/either?
@David They are both good. A big advantage is that CCC can do multiple backups simultaneously. I use SD, too, to spread around my risk in the event that there’s a bug.
@Michael Thanks a lot for the links!
So, if I understand correctly, spinning drive should ideally be formatted in HFS+ while using APFS for SSDs.
@David
I much prefer the user interface of CCC, plus it’s very easy to organize backup tasks in groups plus it allows to easily see how much space TM snapshots take on each drive, plus you can configure the equivalent of a trash when files get deleted from the destination.
@Damien My backups are almost all to spinning drives, and for me the benefits of snapshots outweigh the performance hit of APFS.
