One of the more interesting features of Photos for Mac is its ability to not store my entire photo library on my Mac’s drive.¹ It does this by syncing the entire library to iCloud Photo Library² and then dynamically loading and unloading photos as you use it.

When you use Finder's List view to work with files on your Mac, a glance at the Size column tells you the size of each file, but when it comes to folders in the list, Finder just shows a couple of. To view the location of your Photos library in the Finder, click Show in Finder. To use the currently open library as your System Photo Library (which allows you to use it with iCloud Photos), click Use as System Photo Library. For information about how to switch libraries, see System Photo Library overview and Create additional photo libraries.

In true Apple fashion, Photos protects the user from thinking about managing storage — everything happens automatically, with absolutely no intervention from the user. That’s as it should be, but a few optional controls for the control freaks among us wouldn’t be the worst thing in the world.

This thought occurred to me when I was fishing a file out of my Pictures folder and noticed that my Photos library takes up 46GB of my precious iMac SSD storage space.

That’s a lot, especially when you’re supposed to render an HD video in Final Cut Pro, but you can’t because you’re out of disk space. There’s no button for me to press to put Photos in Austerity Mode, no interface to force it to slim down what it’s using. In fact, there’s no communication at all from the app about how it manages its own storage space.

Plumbing the depths of the Internet, I found this pretty great post on StackExchange that charts the size of the Photos library and a Mac’s disk usage. Photos is definitely optimizing the size of its library, though it’s still not entirely clear to me whether it only does this when it’s running, or if there’s some background process that might do it all the time. (My guess is that it’s the former.)

What that post does clarify is that Photos apparently has an optimization target: 10 percent of free disk space. So on my 467GB partition, it’s trying to free up roughly 47GB of free space. (At the moment that drive has 42GB free, so I guess it’s working?)

I’m also a little surprised at the 16GB of thumbnails in my Photos Library. That’s 240K in thumbnail data for every one of my 67,782 photos. It turns out that the Photos library actually generates two thumbnail files for each image: one “1024” image (roughly in the ballpark of 1024-by-768 pixels, though it varies based on aspect ratio) that’s 200K-300K, and a standard thumbnail that’s more like 480-by-360 and 50K-75K.

Those thumbnails are what make the Photos interface so pretty and responsive, even at Retina resolutions. At the same time… 135,000 thumbnail files on my SSD taking up 16GB of space. I guess that’s the trade-off of having a huge cloud photo library, but… wow.

I’m so happy that this feature exists, but in a future update I’d love to see a bit more transparency about how the storage is being optimized, and perhaps even a user option to blow out the cache or reduce the library size by some amount. Or, failing that, it needs to be much more aggressive in pruning its library in low-disk situations.

After all, my photo library is larger than my Mac’s drive, so it just won’t fit! I had to break my old iPhoto Library into pieces and store it on a server. ↩
You can’t use this feature if you aren’t using iCloud Photo Library, because Photos needs a data source for the files it’s deleting. ↩

Mac Photos Maximum Library Size

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