Along with the launch of macOS High Sierra, due in the fall of 2017, Apple has tweaked a nice, little-known feature by the name of mobile Time Machine. This shouldn’t be confused with Time Machine, though, which is an essential tool for backing up your data at a given frequency. While most Mac users are aware of this, the same can’t be said of mobile Time Machine. Despite being available since MacQuest and Lion, this feature – known as local snapshots – wasn’t a feature that Apple highlighted much, although Time Machine does inform users that it kept these snapshots whenever space permitted.
This important update to mobile Time Machine was possible due to the new Apple File System (APFS), as it takes advantage of the new speeds offered by the file system.
What Is Mobile Time Machine and How Does It Work?
When you use Time Machine on a MacBook, the feature saves some of its backups on the startup disk. Referred to as local snapshots in Apple support documents, this feature is as its name suggests: it takes a snapshot of your entire system and stores it locally on your hard drive. Local snapshots are automatically enabled when you activate Time Machine and are disabled when it is turned off. By default the system takes one daily snapshot at the beginning of the day when you start (or restart) your computer, while one weekly snapshot is saved every week.
You can activate mobile Time Machine using the terminal command “sudo tmutil enablelocal”. Like regular Time Machine backups, local snapshots are taken hourly and are stored for a length of time determined by the available storage on your Mac. The mobile Time Machine will store snapshots in the Mobilebackups folder. Find the folder by launching Finder and select Go> Go to Folder and type /Volumes.
Since local snapshots save information about the actual state of your system and are stored locally they each take up space on your storage drive, which macOS highlights if you are interested in what is using up space on your Mac. If there is less than 20% storage space available on your startup disk, Time Machine starts clearing older snapshots until the machine is back over 20%. If it falls below 10% or less than 5GB, Time Machine removes local snapshots more quickly.
Since Time Machine removes local snapshots if space is needed, neither Finder nor Get Info displays size information. The only way to access information about their size is by selecting the About This Mac from the Apple menu, clicking Storage and finding the Backups label.
Mobile Time Machine is particularly useful when the backup drive you use to store data is unavailable and a file (or more) is accidentally deleted. However, if the startup disk is damaged, this local backup can’t save your data.
APFS Enables New Features in Mobile Time Machine
The version of mobile Time Machine currently available with macOS Sierra has file size limits for backup: it cannot back up changes to files larger than 20MB. That limit has been removed with macOS High Sierra thanks to the new APFS, explained the manager of Finder and Time Machine teams, Pavel Cisler, during a developer session at WWDC 2017.
If you have accidentally deleted a file, you can restore it using the Time Machine just as you would with an external drive. The backups available are indicated in red in the scale along the right-hand side of the display.
The key to this feature is that it uses snapshotting, a copy and write technology that means it grabs references to your data and that the actual copying only happens if there is an actual change, Cisler said.
How to Remove Local Backups From the Startup Disk
A quick check of your storage (Apple button > About this Mac > Storage) will give you information about how much disk space backups are taking up on your startup drive. Although the feature is designed to not fill up your drive, it still occupies precious space that can be used to store other, maybe more important files.
To remove local snapshots, you can either wait for the system to delete them as the disk fills up, or manually remove them completely. For the latter, however, you’ll need to disable mobile Time Machine using the following command in Terminal: “sudo tmutil disablelocal”. This command will disable the feature and automatically remove all previous snapshots stored on the startup drive.