iCloud Photo Library & Faces

i have my iCloud Photo Library downloading all originals to a Mac Mini but on my MBP its set to Optimize.

how does Faces deal with this? do they sync across the library if tagged on one machine to the other? both directions? or is it a local setting?

Faces do only sync partly to iCloud Photo Library, independent of the setting for "optimize Mac Storage".

If you tag faces, the searchable keywords will sync to iCloud, but not the faces thumbnails, and the faces will not appear in the Faces album.

Have you seen this support document? iMore: How to use Photos for OS X with multiple Macs | iMore

and: Tag faces in Photos to find people on all your devices - Apple Support

Cloud doesn't automatically update changes to the names associated with Faces or the contents of the Faces photo album in the Photos app. You can tag people's faces with names on multiple Macs, but the face tags and Faces photo albums will remain unique to each computer.

Learn more about using Photos on multiple Macs.

i now understand the Faces is a local setting but what about my Smart Search? i clearly have lots of one Face with 2015 when i go to that persons Face in 'Faces', but when i smart search that face for 2015 dates, there are hardly any showing when i know they exist???

The Faces Smart search seems to be buggy.

When I want to search for faces with a smart album I search "Text contains" instead of "Face is". That is working much better.