I am migrating from Aperture to the new Photos. In Aperture, I had my originals on an external hard drive (i.e. the library was local, but the originals were on the external drive). Now in iPhoto, I can still access those externally saved photos, but they don't upload to iCloud (instead, it always asks me for permissions whenever I open any external image, which is also annoying).
I found out that I can select an album and go on File->Consolidate to have it copy the originals over into the Photos library. When I do this, the images are being uploaded to iCloud. So far so good.
But this is when I run into space issues: I want all the originals to live in the cloud and have a thin library on my Mac. With storing the originals on the external hard drive, this was working (still ~50 GB on my Mac). But now it seems to store all the originals on my Mac anyway. I have selected Preferences->Optimize Mac Storage, yet it claims there is not enough storage on my mac to keep all the optimized version:
Your Mac Storage is full This Mac is no longer updating because there is not enough space to store optimized versions of your photos and videos.
When I right-click an album, I can select "Download Originals to this Mac" - what I want to do is the opposite: delete the originals locally, keep only low-res versions and have the originals in full resolution available in iCloud. How can I achieve that?