Monday, November 13, 2017

Local Time Machine Uses APFS Snapshots

Lex Friedman, writing in 2012:

With Lion, Apple introduced local Time Machine snapshots. This mostly-silent feature lets your Mac use free space on your main drive to create iterative backups of your files when you’re away from your external Time Machine disk.

By default, Apple disables local snapshots on desktop Macs; the assumption is that you only need them when you’re using a laptop, and that your trusty desktop machine is always connected to a Time Machine drive.

This was called Mobile Time Machine and managed by the mtmd process.

Apple, describing macOS 10.13 (via Accidental Tech Podcast):

Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups to your built-in startup drive and other local drives. These backups are called local snapshots.


  • A bright red tick mark is a backup that can be restored now, either from a local snapshot or your backup drive. When your backup drive isn’t available, only the local snapshots are bright red.

  • A dimmed red tick mark is a backup that can be restored from your backup drive after that drive becomes available. Until then, the stack of windows on the screen shows a blank window for that backup.


Time Machine in macOS High Sierra stores snapshots on every APFS-formatted, all-flash storage device in your Mac or directly connected to your Mac.

Howard Oakley:

Inevitably, depending on how full your disk is and how often new data has to be written to it, old versions of files will be lost over time, as they have to be re-used to make free space. But if you can keep a reasonable amount of space free on your internal disk, mobile Time Machine should give you a valuable means of going back to any version of a document over the last several days, maybe even weeks.

What in Sierra is of relatively limited value will, therefore, become very useful indeed in High Sierra – and come with no performance or storage penalties. For laptop users, this will be an important feature to consider when deciding how quickly to upgrade to High Sierra.

Previously: SuperDuper and APFS, Finder 10.9 Disk Space Embellishment.

Update (2017-11-27): Rich Trouton:

As part of macOS High Sierra, Apple has added a new feature to Apple software updates which require a restart. When these updates are installed onto a boot drive which is using Apple File System (APFS), an APFS snapshot is automatically created on the boot drive prior to installing the software update. An APFS snapshot is a read-only copy of the state that the boot drive was in at a certain point in time, so it can be used as a backup in case something goes wrong with the update.

4 Comments RSS · Twitter

[…] Local Time Machine Uses APFS Snapshots, APFS […]

My Time Machine backup always worked perfectly until I upgraded to High Sierra 10.13.1. Now from time to time I get the message saying backup disc not available. Nothing has changed--all the connections are secure. Sometimes a restart helps but why is this happening.

[…] example, Time Machine’s restoration interface no longer has a star field. Other than some additional colors (more complexity) it works pretty much the same as before, so I don’t see how you can make […]

I am getting occasional Time Machine failures trying to back up an APFS volume, running 10.14.3.
The error message "-128" is described on Ask Different as indicating that a local snapshot could not be unmounted, apparently because of the spotlight indexer.

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