Time Machine backups don’t add up! Or, rather, if you’re viewing the contents of a Time Machine backup volume in the Finder and choose File > Get Info and enable Calculate All Sizes, the totals of all the snapshots typically far exceed the capacity of the volume. That can’t be—and it perturbed one reader whose snapshots were 1.7 terabytes—he thought he was running out of space on a 12TB drive.
Apple’s Time Machine is an idiosyncratic method of backing up volumes and providing snapshots compared to how most cloning and archiving software works and ever worked. Similar software typically makes an initial snapshot and then stores the changed portions of files along with a log of deletions and folder changes. When you restore a file, you’re offered timestamped versions to choose among; restore a snapshot and the software figures out which combination of files from the original snapshot it has to juggle with later changes to reproduce the results.
Time Machine offers a different approach. Instead of making a single comprehensive volume snapshot or ones at long intervals and only storing differences, Time Machine creates the equivalent of timestamped virtual volumes that appear when browsing a Time Machine volume to store every file from the backed-up volume.
The backup algorithm does create an initial snapshot to ensure that every file and folder on a volume has a corresponding entry on the Time Machine volume. After that, however, any file that hasn’t changed since the previous Time Machine backup isn’t copied. Instead, the next backup creates a link to the original copy of the file. When you browse a snapshot on a Time Machine volume by opening a timestamped folder, it appears that every file is in the snapshot, not just the modified or new ones. And the Finder relies on those links to perform a calculation, making each snapshot folder seem to contain the full set of files.
(A technical aside: With HFS+ volumes, Apple’s long-running filesystem, these were hard links, a special kind of one-to-many filesystem linking format. In Time Machine backups to an APFS-formatted drive, Apple uses a kind of central data store and creates a link in Time Machine snapshots from that store. The effect is the same.)
Behind the scenes, Time Machine prunes older snapshots both over time and as the storage in an allotted volume fills up. You should always have the most recent 24 hourly snapshots and the last rolling month’s worth of daily snapshots. Time Machine retains weekly snapshots older than a month until it has to delete them.
If you want to know the actual amount of storage remaining on a Time Machine volume or occupied by a snapshot on the volume:
For a volume, select it in the Finder and choose File > Get Info. The number following Available is the true remaining capacity.
For a snapshot, select its folder on the Time Machine volume and choose File > Get Info. The Size field indicates its occupied storage on disk.
This Mac 911 article is in response to a question submitted by Macworld reader Jim.
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