What is the maximum/optimized size of the OSX photos.app library?

So after some time (learning python on how to monitor and plot library size and disk usage) this is what I have observed regarding my first question. As written by @will-mallard and @bmike the optimizer targes for percentage free disk space. As shown below: the size of the picture library increases and decreases together with my disk usage. During those three days I did not use my mac book air for work. I only added pictures two times.

Over a longer time scale, one can see the reduction of picture library disc space over time while I was working on my macbook.

My followup question is now: What triggers the optimization? At first I thought it is some kind of free space threshold. But as it is easily seen in the last plot, something at the 24th May triggered a reduction of library space. I am not sure what that was.

Create a partition to the photo library dedicated, if you create a new photo library in there, and the maximum size is limited to the partition size. Of the 500GB of system drive, are using divided into partitions for the photo library of 50GB (10%).

I have a picture of 200GB on icloud, are used without any problems with the above settings.

I practice, the optimize space has zero effect on your photo library on OS X initially.

When you enable a system Photos library and opt in to the iCloud Photo storage, it tells the Photos Agent background process to start uploading photos to the cloud. When that initial pass is done, then deduplication in the cloud and download of new photos starts.

Assuming your Mac has room for the original photos - nothing gets optimized. Also, nothing gets optimized until the original is stored in iCloud.

Once your Mac runs into a storage issue on the volume where the Photos files are stored, it will start to optimize the local copies and reduce the amount of space needed so that the drive can have free space for other files.

I haven't seen Apple document these low storage space limits and I also haven't determined it empirically. Similarly, I don't have documented or empirical data on how far "optimization" will go and if at some point an error will be raised if photos cannot be compressed further.

I'm sure the code handles all of the above, but I don't have details on that.

As to your two specific items, it seems the software is smart enough to handle even your case where the library is large enough to have optimization happen before upload is finished.

I would say that I've seen it take 24 hours post upload to start reconciliation and download, so my guess is you might have to wait a while for the space savings to kick in as well as have Photos on a volume with low free space as an additional factor.

After about 2 weeks of working with the Photos app on a Macbook Air 128GB, with a much larger photos library, I've noticed the following things.

There appears to be no fixed target size for the actual Photos library - instead its a target "free" space on my Macbook Air.

Currently, the Mac Optimisation appears to target roughly 10% free space on my Macbook (about 12GB)

Obviously you can only upload up to 12GB, but as soon as the import process begins, the Mac starts to re-allocate storage as it imports the photos to the Mac, and then begins uploading to iCloud.

It's a lengthy process importing 100's GBs of photos, but it should be possible.

My Photos library totals around 40GB on my MacBook Air

Hope that sheds some light... at least a little.