Most of current block storage systems allow you to create volumes that span across multiple disks, but those disk must be on the same server. Is there any system that can create a volume that spans across multiple disk on multiple servers, and that volume can grow by adding more servers to the cluster?
Edit: for example, I need to write log continuously to a remote file. The file is never closed, so a caching model like those in object storage systems is not feasible. I'm looking for a solution that is similar to a SAN system, but 1 volume can be scaled beyond a single server.
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
Ceph and/or CephFS may be what you looking for.