MacOS – How does the new “local snapshots” feature of Time Machine in Lion work

I have seen this on several occasions and I have done this to clear up the corruption or glitch that prevents browsing local snapshots without an actual full time machine destination mounted:

1. Connect to one of my destinations and force a good backup

   tmutil startbackup --block --rotation
   

2. Disable local snapshots

   sudo tmutil disablelocal
   

3. Enable local snapshots - being sure the cleanup is done

   sudo tmutil enablelocal
   

I do watch the console app (or tail -f /var/log/system.log) during the backup and commands above to make sure that the local filesystem gets cleaned up and give the computer several minutes between the disable and the re-enable since it does have to unmount the network filesystem, delete old copies of files and clean up things before trying again with a local snapshot.

The downside of this is you do lose any copies of files that only existed on the local snapshot intervals and didn't get saved to an actual remote location. It also doesn't really dig into why the store became corrupt so you might have the same thing happen again at a later date. I've also had good luck with reproducible time machine problems reporting it to Apple for assistance. They'll want you to run tmdiagnose and probably not reset things like I mentioned above if you want to get at the root cause of what really happened as opposed to getting it working again.

The simplest way is for you to note the date and time of your last backup in Time Machine on OS X Mountain Lion. You can get back to it at any point in time in the future just by restoring to the backup with that date/time (this will remain on the disk as long as you have adequate space so that Time Machine does not delete these older versions).

• You can easily find this date and time by navigating into the "Backups.backupdb" folder on the Time Machine disk and looking at the timestamp for the "Latest" folder (it is actually a link - so you can even open it to see the latest backup folder name with the date and time).

If you do not wish to note down the date and time of your last backup with OS X Mountain Lion, the next simplest way is to change the computer name either on the Mac on OS X Mavericks before you do your first Time Machine backup on it or on the Time Machine backup disk for the older backup.

In either case, this will make Time Machine create a new folder with the new machine name on the disk and it will do a full backup the first time (so you need adequate disk space to hold almost two copies of your Mac). This way, you can restore from the old or new backup in the future (if you rename the computer name on the Time Machine disk, then you would have to revert that to do a full restore of the old system).

• To change your computer name, go to System Preferences > Sharing and edit the Computer Name: text.
  OR

• To rename the computer name on the Time Machine disk instead, use the following command in Terminal.app:

  sudo /System/Library/Extensions/TMSafetyNet.kext/Contents/Helpers/bypass mv /Volumes/<TMDiskName>/Backups.backupdb/<ComputerName> /Volumes/<TMDiskName>/Backups.backupdb/<OSXMountainLionBackupComputerName>
  

  Substitute the following in the above command before running it:

  • <TMDiskName> is the name of the Time Machine volume name
  • <ComputerName> is the current computer name (which you will continue using for OS X Mavericks)
  • <OSXMountainLionBackupComputerName> is the name of the snapshot for you to do a full restore from in the future (you would have to rename it back using a similar command as above before the full restore)