The Photos app on the Mac has two options for storing your photos. You can tell it to keep the full-size originals of everything, or you can have it self-manage, keeping your master library in iCloud and storing a mixture of full-resolution and low-res versions locally to save space.
The trouble is, even when you choose the “Optimize Mac Storage” option, the Photos app’s storage can metastasize and take over your whole drive. Today we’ll see how to cap this storage, giving Photos a hard limit on how much space it can use. For instance, if you have a MacBook with a 128GB SSD, you could choose to only use 30GB for Photos — and it will never, ever use more.