Understanding Mac’s Time Machine
Time Machine has its own scripts for housekeeping, but sometimes (such as when you see the “Time Machine could not complete the backup” error message) you may need to intervene to clean up space on your Time Machine drive. Space can always be saved on your backup if you delete junk, cache and temporary files before you back up the computer. That’s particularly true if you haven’t set up any exceptions in Time Machine preferences, and everything is saved, even applications.
Since every external storage device has its own physical limits, if you are going to keep that drive in the long run then it’s good to optimize your backups so that they occupy less space. For that it is possible to compress Time Machine backups, but before you start deleting and compressing backups you need to understand how Time Machine works.
Time Machine uses File System Event Store, a hidden log macOS keeps of changes made to the data on each HFS+ formatted drive or partition. But this won’t list every file that has changed; instead, it will list the directories affected. Time Machine uses this log to find out what’s changed and what needs to be backed up.
When Time Machine does the first backup, it copies everything (except trash and system work files) and creates a dated backup folder (year-month-day-hhmmss) in the Backups.backupdb folder, placing hard links in it to all the backup copies it has just made. Another dated backup folder is then created with the second backup, and this repeats each time. There is a trick, however: the second backup folder contains hard links to the items that remained unchanged.
How to compress Time Machine backups
There are notable differences between the normal (or local) Backups.backupdb folder and a backup made over a network. Local backups are stored in a single Backups.backupdb folder at the top level of the external hard drive.
If you are backing up your Mac over a network (be it wired or wireless), you’ll find the Backups.backupdb folder inside a container called a “sparse bundle disk image”. While it works just as well as the local backup version, an extra step is needed to actually recoup the space gained by deleting earlier backups due to its format. Unlike a normal folder or disk image, when you delete a file in a sparse bundle, it won’t change the container size. To recoup the space, run the following command in Terminal (replacing the ‘Yourbackup’ entry with the necessary backup name):
% sudo su - root
% hdiutil compact /Volumes/TimeMachine/YourBackup.sparsebundle
What you’ll then see is:
Starting to compact…
Reclaiming free space…
……………………………………………
Finishing compaction…
Reclaimed X.X GB out of XXX.X GB possible.
Time Machine will prompt for the admin password and then delete the required file.
That’s a bit clumsy if you want to delete multiple backups, so here’s a Terminal command for this, which is very useful when deleting old backups:
tmutil delete /TimeMachine/Drive/Path/To/OldBackup/
Or, if you want to remove a backup from a specific date:
tmutil delete /Volumes/BackupDriveName/Backups.backupdb/MacComputerName/YYYY-MM-DD-HHMMSS/
How to delete old Time Machine backups automatically
If you don’t mind speaking with your Mac via Terminal commands, then it’s even possible to automate the removal of old Time Machine backups by running the following batch script. Note that this will find the oldest and newest backups and display a prompt to delete the oldest, which then requires you to enter “Y” and the admin password to finish.
COMPUTER_NAME=$(/usr/sbin/scutil --get ComputerName)
NBACKUPS=$(/usr/bin/tmutil listbackups |
/usr/bin/grep "$COMPUTER_NAME" |
/usr/bin/wc -l)
OLDEST_BACKUP=$(/usr/bin/tmutil listbackups |
/usr/bin/grep "$COMPUTER_NAME" |
/usr/bin/head -n1)
LATEST_BACKUP=$(/usr/bin/tmutil latestbackup)
echo Latest backup: $LATEST_BACKUP
if [[ -n "$LATEST_BACKUP" && "$LATEST_BACKUP" != "$OLDEST_BACKUP" ]] then
echo -n "$NBACKUPS backups. Delete oldest: ${OLDEST_BACKUP##*/} [y/N]? "
read answer
case $answer in
y*)
echo Running: /usr/bin/sudo /usr/bin/tmutil delete "$OLDEST_BACKUP"
/usr/bin/sudo time /usr/bin/tmutil delete "$OLDEST_BACKUP"
;;
*)
echo No change
;;
esac
else
echo "No backup available for deletion"
fi
István F.
Adam B.
