A week ago I asked how much backup storage does your Mac need, and dropped a hint that I was looking at making this task easier. I think I might have done that in a free app DeltaFree, whose first release you can download below, for macOS Sonoma 14.6 and later, including Golden Gate.

Principles

If fixed-size storage has 700 GB free, and each week a net average of 14 GB is written to it, then it will take about 700/14 = 50 weeks for it to become full. This is simple arithmetic. All we need to know is how much free space there is at the start of a time period, which you can see in the Finder or Disk Utility if needed, but how do we measure the average rate that is disappearing?

What DeltaFree does is call the diskutil command tool and extract from its listing of APFS containers and volumes the required information. It then adds those to its database. You run the same check a day or two later, and DeltaFree gets a second set of points, and so on for a fortnight or so. You now know how much space was free in the container within which your backups are stored, and the rate at which free space is being used by those backups, making the arithmetic simple.

Not only that, but you can periodically keep an eye on all the APFS containers your Mac has access to, and watch for any change in the rate they are accumulating data.

First version

I know of several fine utilities that will analyse volumes, particularly the current Data volume, and tell you what’s taking the space on them. But those are static, and don’t look at how used and free space change over time. Maybe there is an app already out there that does this, but I’ve drawn a blank when looking. To that extent, DeltaFree is entering new territory, and is also a first version. For those reasons, it adopts a staged and gradual approach while we work out how this can be honed and improved – and that’s up to your thoughts and comments, please.

Get started

Once you’ve moved DeltaFree to an appropriate folder away from the contents of the archive it came in, perhaps into Applications, open the app.

On this occasion, click on the Check All tool first. This will get details of all accessible containers and list them in the upper part of the window. As the app only has a single set of points at this stage, there’s no point in trying to plot them yet. Click on the Save As JSON tool, and save the data to somewhere accessible before quitting the app.

The following day or a little later, open DeltaFree again, click on the Read JSON tool and select the data file you saved. Then click on Check All to get today’s values and add them to those you got a day or two ago. Click on the Save As JSON tool again, and either overwrite your original data file or save into a new file, before quitting the app again.

Repeat these another couple of times, using the same sequence of Read JSON ➤ Check All ➤ Save As JSON ➤ Quit.

Next time, once you’ve clicked on Check All, select one of the containers listed at the top of the window, such as your Mac’s backup storage, or the boot volume group. Then click Plot Selection.

Now it should all become worthwhile. For that container, the upper chart shows how little the amount used has changed, and the lower chart shows how the rate of increase in used space varies over time, in this case ranging between shrinking by about 25 GB/week when Time Machine must have thinned some old backups, up to a peak of just under 25 GB/week when there were a couple of chunky backups made. And the overall rate of growth of those backups is low, at around 4 GB/week. With over 900 GB free, that should be sufficient for a good 200 weeks, about 4 years.

This and much else is explained in its detailed Help book.

Download

DeltaFree version 1.0 is now available from here: deltafree01
and from Downloads above. I will allocate it a Product Page if you think it’s worthwhile.

Where next?

If we agree this is worth pursuing, there’s lots of choice as where to go next, including:

• use a fixed data file in Application Support and automatically open that and run Check All when the app is opened;
• automatically run Plot Selection whenever a new container is selected in the list;
• a background task to automatically gather data daily, with DeltaFree to access its database and chart it;
• more extensive time series analysis to improve accuracy of forecasting;
• additional reporting.

Please give DeltaFree a trial run over a few days and see what you think. I hope you find the idea worthwhile, and its information valuable.