Fortunately, Time Machine is built on top of a command-line utility called Terminal that can tell us everything we need to know to measure drift. Here’s a look at how to use Terminal and the tmutil command to measure the changes that occur between one backup and the next.
Why Is Drift Information Helpful?
There are many reasons users want to know the drift rate. If you measure drift and discover you’re adding large chunks of data each time you run a backup, it may be time to buy a larger backup drive. On the flip side, you may discover that your current backup drive is much larger than you need, now or in the foreseeable future. Time Machine’s user interface doesn’t include a method for measuring drift. While it’s possible to measure the amount of data stored on your backup drive before and after Time Machine runs, you’ll see only the total amount of change, not specifically how much data was added and how much data was removed. This is where Terminal and the tmutil (Time Machine Utility) command come into play.
Use Terminal and Tmutil to Measure Drift
Terminal’s tmutil command allows you to set up, control, and interact with Time Machine. Anything you can do in Time Machine’s user interface, you can also do with tmutil, but tmutil allows much more functionality. Here’s how to use Terminal and tmutil to measure drift.
Reading and Understanding Drift Calculations
It can take some time to run the drift calculations, depending on the size of your backup drive, so be patient. When the calculations are finished, Terminal will display drift data for each Time Machine backup slice in the following format: You will see multiple groups of the above output. This will continue until the final average is displayed. Here’s an example: Don’t use only the average drift to make decisions about storage upgrades. It’s important to look at the drift data for each time slice. Your drift measurement will likely show that more data gets added to a backup than gets deleted, so a larger backup drive may eventually be necessary.
