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I'm thinking about setting up a private cloud using Ubuntu with 2 nodes, but I don't quite understand how disk space from each node is handled in the cloud.

Can somebody explain how (and if) the disk space is accessible from within the cloud? For instance when I have 2 nodes with varying amount of disk space, for example 320GB and 1TB, can I set up a "shared storage" from both devices?

edit for clarification:

Currently I have 3 Windows Servers, consisting of a Domain Controller, a Database-/File-Server and a Web-/Mail-Server. I want to switch to an Ubuntu Server-based private cloud with VMs serving these purposes instead of dedicating each physical server to a certain job. As my database- and web-server differ largely in their available disk space, I wondered whether I can manage the entire disk space from all nodes in the cloud and make it accessible similar to using DFS in Windows AD to provide a network share that underlyingly uses two multiple servers.

More generally what are common practices to share disk space within a cloud based infrastructure? Are there only Key-/Value-storages, or is something like the before mentioned scenario using network shares feasible?

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There are actually a few options. In writing this answer I came across this Xen page which covers what I've said

  • Virtual block-level-device sharing over SAN Amazon has banks of disks that are networked out to their EC2 VMs and exposed as physical disks. One drive can really only be [safely] used by one VM at a time, with the exception of making snapshot backups.

    You'd be looking at technologies like iSCSI and nbd for this.

  • SAN-based network file systems Then there are the more traditional storage over IP methods like NFS. As they're essentially just another filesystem they're considerable more transparent for maintaining large repositories but they're less cross-platform (Windows and NFS don't mix well).

    The clear advantage over block-level SANs is you can have more than one computer connected to the same data at the same time. NFS supports file locking and permissions so you can do all sorts of nifty sharing that wouldn't otherwise be possible.

  • Physical, local RAID And even more traditional than that, just sticking a RAID array in the host box and portioning that out to local VMs. A lot of VM providers do this still because it means you don't have a monolith disk array waiting to blow up.

    And that does happen. I had a lot of downtime from a VPS provider because they couldn't reliably keep their SAN online.

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