Treating the whole thing as one FS is the current architecture GitLab uses, so is more of an existing constraint than a proposed architecture. To get distribute

Treating the whole thing as one FS is the current architecture GitLab uses, so is more of an existing constraint than a proposed architecture. To get distributed storage you either need to rewrite GitLab to deal with distributed storage, or run it on another layer that presents an illusion of one big FS (whether that's CephFS or a storage appliance).