We provide an in-memory writable filesystem (e.g. you want to do file transformations) but it's not persistent across all instances. We're looking into Filestore integration for a mountable NFS product, though as mentioned in another comment about that, reading is easy but writing is hard.

Why is writing hard? We already use Filestore across multiple VMs in read-write mode. We ensure that writes are unique i.e no two machines will use the same filename. There are also no file modifications.