Is there a block storage system that can create volumes span across multiple physical servers?
Solution 1:
They're also called network file systems. NFS for UNIX and SMB for Windows are the primary and most popular protocols used for accessing files over the network. I know Starwind can do both. Check out their vSAN that involves multiple nodes and can be easily scalable. With the free version you'll be able to create at least 2-node scale-out file server and setup either NFS or SMB file share on top of it: https://www.starwindsoftware.com/starwind-virtual-san-free
Solution 2:
Ceph and/or CephFS may be what you looking for.