I have several virtual servers with the same provider. Typically I have around 50GB disk space.
On some I have largish databases, with consequently large database backups. The backups are for recovery from cock-ups rather than disaster recovery so retaining them on the same server is convenient, but they do tie up disk space, and the ones with the biggest databases lose the most to database backups.
So ideally I want to do something to share the spare capacity on other servers, but safely. for example in the event that one server got hacked I don't want that to provide a backdoor to another server, and nor should any data that's been shared onto it be easily accessible.
In an ideal world, it would be nice to ring fence (say) 15GB of capacity on server A and make it available over the network to server B such that A can't access B's files even though they're stored there (I accept that I can't stop someone with root on A deleting the files, but they should at least be encrypted so that they can't be read). And whilst B can use that allocated chunk of A's disk, it must not be able to access anything else on A.
Ideally the capacity that's shared should be pretty easy to change so I can easily adapt if A needs more space. (I wondered about trying to remotely mount an encrypted file on A as a filesystem on B, but adapting the size later would be difficult.)
As anything I do will be making changes to already functioning servers A & B the changes need to be relatively narrow to avoid the risk of breaking something.
Thoughts / suggestions / experience of doing something like this?
I can think of various paths to take, from simply pushing files over scp and working out how best to secure it (and manage the quota), to something far more complex like GlusterFS, with lots of options in between. So it's not really a lack of ideas as any idea which ones would be best!