Sharing disk space between two Ubuntu hosts [closed]

Rather than bumping Alice up to a more expensive image, is there an
effective way of sharing some of Bob’s disk to Alice?

What you are talking about comes down to being so intricate—NFS mounts, etc…—that ultimately you might be better off growing Alice’s space to a more expensive image.

But part of the problem with the question is what exactly is eating up space? Actual application usage on the server? Files that need to be accessed immediately or archival items? Because you say:

I’m looking to transparently extending the filesystem rather than
creating an additional partition if possible.

But honestly doing that transparently is not going to be easy. You need to better define—for us and for you—what’s eating up the space.

Perhaps something like Amazon S3 can help you offload your storage for the application running on the server itself.

But you need to provide more details since simply saying “more storage” does not address the issue.

EDIT: Based on the original poster’s comments this is about Bitcoin related files stored in .bitcoin/. The key thing you need to keep in mind is these are simply files store on a oath. So figure out where your .bitcoin/ directory is located, then see if you can adjust a config to change that path where they are stored. If you can change that path—which I am pretty confident yo can—then perhaps an NFS mount would work. Just setup an NFS mount, move the old block chain stuff to the new space, change the config to point to that new space & restart the process. But honestly you might be doing more work than it’s worth.