I want to use the Photos.app option "Optimize Mac Storage" to free up space on my Mac, rather than downloading my entire iCloud Photos Library to my small laptop SSD.
Is there any way to strictly limit the space given to the Photos library?
I want to give, say, 40 GB to Photos. The app can "optimize" that space as it sees fit, but I never want it to take more than 40 GB.
iCloud Photos > Optimize Mac Storage
If your Mac is low on space, full-resolution photos and videos are automatically replaced with smaller, device-sized versions. Full-resolution versions can be downloaded from iCloud anytime.
Best Answer
Limiting Photos iCloud Photos Library sizes was discussed on Accidental Tech Podcast:
https://atp.fm/334 (1:36:45)
https://atp.fm/335 (0:04:20)
https://atp.fm/410 (2:19:05)
They originally suggested using a Sparse Disk Image with a size quota.
Follow-up suggested instead using an APFS Space-Sharing volume.
APFS Space-Sharing Volume
In Disk Utility, select
Add APFS Volume..
Set a Quota Size to strictly limit the size of the new volume:
When you move your Photos library, your iCloud photos will only be permitted to fill this Quota Size set for the volume.
Move Photos Library
Finally, move your Photos library file to the new size-limited volume.
Quit Photos.app > Move the library file > Hold Option and relaunch Photos > Pick the library’s new location > Photos Preferences > Use as System Photo Library
How to put Photos.app main library on an external disk?
Or, start a new Photos library on the volume and allow iCloud to re-download your photos.