Ceph incremental backup

This is based on http://ceph.com/dev-notes/incremental-snapshots-with-rbd/ which is incomplete (missing the initial send step).

    make sure there is an rbd to mount (-l also lists snapshots/clones):
        rbd ls -l

    put some files there (this assumes it is already mounted)::
        cd /mnt/ceph/foo/
        echo snapshot test file1 - before snapshots > snaptest1

    snapshot it as "snap1":
        rbd snap create rbd/foo@snap1

    clone it to another new rbd:
        # unlike "import", "import-diff" does not create an image automatically, so we create one first
        sudo rbd create foo-bak --size 4096
        rbd export-diff rbd/foo@snap1 - | rbd import-diff - rbd/foo-bak

    put more files there:
        echo snapshot test file2, with first snap already done > snaptest2

    snapshot it snap2:
        rbd snap create rbd/foo@snap2

    do the export and import the changes to the other rbd

        rbd export-diff --from-snap snap1 rbd/foo@snap2 - | rbd import-diff - rbd/foo-bak