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Re: [Xen-devel] organizing virtual machines

Eric S. Johansson wrote:

Every virtual machine must have effectively two partitions. The first being a root partition containing all of the system executables and configuration files as well as the usual /var, /tmp, etc. The second being storage for your application/user.

This is one approach.  Not necessarily a perfect solution.

Obviously, this seems like a terrible waste of space but given the current dogs breakfast known as /etc, I'm not sure that is another solution. I have a few ideas on how to fix this that may or may not pan out but not the hands (rsi).

There are really two other solutions that I know of. Either some sort of content-addressable storage based file system (like Plan9's Venti) which would provide an optimum storage scenario (although at a performance/complexity cost) or some sort of Copy-On-Write filesystem or block device.

LVM snapshots has been suggested a COW mechanism. The most appealing to me is something like UnionFS (http://www.fsl.cs.sunysb.edu/project-unionfs.html) however it's rather unstable and I don't think FiST is going in the kernel anytime soon. UnionFS does COW on a file-access level. You have one read-only mount that's your root and if a file is changed, the read-only version is copied over to a read/write partition and that's used as the working copy.

A fantastic project for someone would be a from-scratch simplistic union-fs clone that could actually be integrated into the kernel. Linux used to have such a filesystem (IFS) but it became unmaintained and eventually removed from the kernel.


Anthony Liguori

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