Mobile Time Machine and its transformation in High Sierra

If you have a Mac laptop (MacBook Pro, MacBook Air, or MacBook), you will be aware of its ability to make ‘local snapshots’ or local backups, using Time Machine, when you are working away from your normal Time Machine backup drive(s).

Mobile Time Machine up to Sierra

Because these backups are made to your laptop’s internal drive, they are not as useful as regular Time Machine backups. If your internal drive gets corrupted or damaged, chances are the local backups have gone too. By default, they are also made infrequently: whereas normal backups occur every hour or so, ‘local snapshots’ are made once a day from when you last started your Mac up, and a weekly snapshot is also made.

Time Machine is sensitive to the amount of storage space available: these snapshots will only be made if there is sufficient free space. The way it assesses that is fairly complex:

when 20% or more of your startup drive is free, snapshots are made daily, and retained;
when free space falls below 20%, snapshots are removed until 20% is free, then kept normally again; snapshots are made daily;
when less than 10% remains free, snapshots are removed quickly in an effort to free space; when only one snapshot is left, Time Machine may stop making new snapshots until free space improves; it then replaces the remaining snapshot with a new one, until there is more than 10% free again.

Apple considers these snapshot backups to be akin to temporary files, so they are not included in the used space reported by the Finder (and in its Get Info dialog). The best way to tell how much space they occupy is to use the Storage tab in the About This Mac dialog, where they will be shown as Backups in the breakdown of space used. This can be confusing, as the free space reported in that dialog will then be less than that shown in the Finder or Get Info. They are also counted in the free space declared in Disk Utility, and third party tools vary in their treatment.

Although I cannot find any official documentation to confirm this, Apple engineers have warned that there are also limits to the size of files included in local snapshots, which could be as low as 20 MB. You may find it doesn’t back up files of several GB in size.

Local snapshots are removed if you turn Time Machine off, and of course no further snapshots are made until Time Machine is turned back on again.

Desktop Macs – that is, all models apart from MacBook Pro, MacBook Air, or MacBook – do not, by default, support local snapshots or ‘mobile Time Machine’, but it can be enabled at the command line. Enter
sudo tmutil enablelocal
and authenticate with your admin password to enable it, and
sudo tmutil disablelocal
and authenticate to disable it, which will also remove all existing snapshots.

You can also use the command
tmutil snapshot
to make a local snapshot at any time on any Mac running a reasonably recent version of OS X / macOS.

The ‘local snapshots’ made at present (up to and including Sierra) use their own virtual file system, mtmfs (Mobile Time Machine File System), and are mounted in /Volumes/MobileBackups/Backups.backupdb, although it may be that their data are actually stored in /.MobileBackups, as they used to be.

When you want to restore files from a snapshot, use the Time Machine app in the normal way. Backups shown there which you can restore from are indicated in red in the scale along the right-hand edge of the display. When you are away from your normal external backup drive, only local snapshots will be shown in red like that. If the backup is shown in dimmed red, that is one which is available when you are reconnected to your external backup drive. Older versions of OS X use different colour conventions, which are detailed here.

A good way of thinking about these local snapshots is that they are like versions of all your documents and other files: when you are working away from your backup drive, you can still restore old versions of those files backed up locally, even when the app which has written them does not support versioning. Indeed, apps which support versioning properly will not have those versions included in local snapshots, as old versions will already exist, and they will be made available as if they were local snapshots.

In versions of OS X and macOS up to and including Sierra, Time Machine local snapshots, or mobile Time Machine, are run by a service which is scheduled and dispatched by the DAS and CTS systems, which I believe is identified as backupd-auto.dryspell, and loaded soon after startup. This does the complicated task of deciding which files to back up; it does not create the huge number of hard links seen in normal Time Machine backups to make the illusion that each backup is a complete copy of the volume being backed up.

Mobile Time Machine in High Sierra

High Sierra changes ‘mobile Time Machine’ completely, using a new mechanism dependent on the APFS file system; when you install High Sierra, your startup volume will automatically be converted to APFS, and Time Machine will then start using APFS snapshots (which are very different to the old ‘local snapshots’) when your Mac is away from its normal Time Machine backup storage.

APFS snapshots are a feature of the file system itself. In effect, they are a snapshot of the directories and other metadata which track the physical location of all your files on disk. These are amazingly quick to make, even for volumes with a very large number of files, and don’t require any of the files to be copied. Because they are so quick and easy to make, in High Sierra, mobile Time Machine should make its snapshots every hour, just like normal Time Machine.

When you want to revert to an old version of a file, or restore a file which has since been deleted, you use the regular Time Machine interface. High Sierra will then look back at the snapshot, and reconstitute the file at the time it was made from the data which still exists on the disk.

This depends, of course, on the old data still being present, another feature of APFS called copy on write. This ensures that changed files do not overwrite the original file, but are written out to unused areas of disk. 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.

15Comments

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1

xz4gb8 on June 22, 2017 at 12:33 pm

You wrote, “when you install High Sierra, your startup volume will automatically be converted to APFS”.

Conversion to APFS will not be done if you uncheck the convert button (if displayed) or if installing onto an HFS+ volume which is on a multi-volume drive.

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- 2
  
  hoakley on June 22, 2017 at 12:41 pm
  
  Thanks: in the current pre-release, that is true. Apple has not announced whether the release version will allow those opt-outs.
  Currently, APFS on a conventional hard drive is not good news, and its performance is still being optimised.
  With almost all laptops, as they have internal SSDs, users are most unlikely to exercise any opt-out which might be offered anyway.
  Howard.
  
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3

Invicta on July 2, 2017 at 9:22 am

I’ve installed High Sierra with the APFS option on my iMac. In Time Machine Options my Mac HD is listed as a greyed out exclude item which I cannot delete. Is this because my two external backup drives are still in HFS+ format? If APFS on a conventional hard drive is not good news do you have any advice on how I should proceed? Is this an Apple ploy to encourage use of iCloud storage? Thanks for your advice.

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- 4
  
  hoakley on July 2, 2017 at 10:32 am
  
  As you are using a pre-release version on High Sierra, I recommend that you check with the documentation. If it is not made clear there, then you should report this as an issue to Apple using the beta reporting system.
  I suspect that, from what you say, that beta cannot make TM backups of APFS volumes yet. As I understand it, at least in the first release, TM will still expect (possibly require) to back up to an HFS+ volume, although that is of course subject to change.
  The current issues with APFS on hard disks are performance-related, I understand. As it gets closer to final release, you should find that successive betas perform better and better as APFS is tuned for hard drives. If you’re comfortable with its current performance, then there’s no need to worry about it.
  I don’t think that this has anything to do with iCloud – it is just a beta still.
  Howard.
  
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- 5
  
  krislord on July 3, 2017 at 10:02 pm
  
  Time Machine backups of your main drive is disabled in the public beta or the 2nd dev beta (update 1)
  
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  - 6
    
    hoakley on July 3, 2017 at 10:24 pm
    
    Thank you for that helpful information.
    Howard.
    
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  - 7
    
    Invicta on July 4, 2017 at 7:12 am
    
    Krislord, thanks for that info.
    
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8

fharvell on July 14, 2017 at 1:41 pm

It appears that the tmutil enablelocal and disablelocal commands are no longer available in High Sierra.

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  hoakley on July 14, 2017 at 1:45 pm
  
  Thank you.
  This is still a fairly early beta: Mobile Time Machine certainly is going to be fully functional in High Sierra release, and may well have a new tmutil command interface. It will be interesting to see that evolve through the betas.
  Howard.
  
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10

Thomas on November 14, 2017 at 9:29 am

I know that free space is not the only criterion for getting rid of snapshots, there’s also an “age” criterion. I’m not sure exactly what it is, I think it was very short, something like the last 24 hours but that might have changed since I looked at it.

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11

Alexander on December 29, 2017 at 5:37 am

Where does it store those snapshots in the file system?

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- 12
  
  hoakley on December 29, 2017 at 6:17 am
  
  As they aren’t discrete objects, they aren’t actually ‘stored’ anywhere. They do occupy space, though, in which the old data remains, so when they are purged from the file system, that space is freed up.
  They are only really exposed through Time Machine and the tmutil command tool. You cannot (until there are tools to work at a lower level in APFS) work with them any other way.
  The file system does give them some visibility in /Volumes/com.apple.TimeMachine.localsnapshots, but that is not a way in which you should access them in any useful way – use Time Machine or tmutil instead.
  I hope that answers your question, although I’m sure that’s a frustratingly evasive answer!
  Howard.
  
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  - 13
    
    Alexander on December 29, 2017 at 6:24 am
    
    Thank you so much !.
    
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14

Kos mOS on March 29, 2018 at 3:34 am

How can one check from High Sierra, the Time Machine’s disk for errors? I bought a 2 TB last year, and since a power outage when I had like 1.6 TB data on it, it got back online with Disk failure… I toke it to fix for Apple service, they gave it to me back after 4 weeks, and I still never managed to fully BACKUP/RESTORE from it. Last time I tried to recover from it, I got whole disk bad, and was missing headers to boot. I cannot use any other format other than the Zero bytes out, and even that one sometimes resets the whole Time Machine. In the end I am left with a rebooted TM and a warning that internal disk needs repairs, a few more restarts and format attempts in end will work. Still I had situations when I named the disk “Test”, and after a FORMAT, and a Rename to “Broken”, It got me back to previous name before the format. Thing that when it comes to formatting I did not understood how is possible. So basically now my Time Machine sees a Disk with name that was used before second rename and a format. Basically it ignored my initial rename of TM. Any suggestions what i could try or? Is very annoying I had to loose that 1.6 TB of data when in fact the data was there… Just weak support, and APPLE for having a backup solution they DO NOT OFFER ANY DAMAGE recovery for data. And yeah one disk is no redundancy/FO at all. Thank you! MC

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  hoakley on March 29, 2018 at 4:26 am
  
  I’m sorry to hear of your problems.
  Is this an Apple Time Capsule that you are writing about, or just a regular external hard disk?
  One important message is that you need to protect your Mac and any backup drive with a UPS. That will not only filter out any surges or fades in the mains supply, but should cover shorter periods of mains power outage. Unfortunately networked drives like Time Capsules don’t normally have a way of being shut down when they’re running on UPS – but a UPS with a USB connection to your Mac will allow your Mac to shut down external drives as it shuts down automatically.
  Howard.
  
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