Question about icloud photos across devices and storage

[brollikk]

So lets say I have an iphone with 256gb storage and im using about 150gb of space for storage. If i have an ipad with 128 gb of storage, and I turn on icloud photos for that device, will it fail to sync/and/or fill the entire storage up with photos until it can't? Anyone with experience here able to chime in?

 

[contacos]

With optimized storage, it only takes little space on the device itself.

I have a 80 GB phot library but it takes up only 4 GB on my device