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rbd-scheme.md

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Scheme: RBD

The RBD scheme is similar to the Scheme: iSCSI in structure, as both are network block device attachment schemes. In our testing, RBD + SquashFS + Overlay provided the most scalable results. Moreover, the added features of Ceph/RBD like replication and failover, make this a promising solution for large-scale, demanding clusters.

Note: in this example we use the admin account to attach the RBD for simplicity. You almost certainly want a more restricted account for this use.

Creating the image

We will demostrate using SquashFS here. Other filesystem types would follow by running mkfs.* against the rbd device, then copying files over (or writing them in place).

  1. Create image chroot

    Follow some procedure that provides a subdirectory containing the image files, e.g. a dnf --installroot=... ..., or perhaps a container build and mount.

  2. Create the RBD object

    rbd create --image-feature layering --image-shared -s 100G image1

  3. Map the RBD object to the image creation machine

    # rbd map --monitor 192.168.3.253 --pool rbd --image iamge1 --id admin --secret XXXXXXXXXXXX
/dev/rbd0

  4. Write the image

    From within the chroot directory, build the squashfs directly to the rbd device:

    mksquashfs . /dev/rbd0 -noI -noX -noappend

  5. Cleanup attachment

    rbd unmap -d 0

    The image is now ready to use.

Container definition

{
  "name": "test",
  "command": "/usr/lib/systemd/systemd",
  "mount": {
    "kind": "overlay",
    "overlay": {
      "lower": [
        {
          "kind": "attach",
          "attach": {
            "kind": "rbd",
            "fs_type": "squashfs",
            "mount_options": [
              "ro"
            ],
            "attach": {
              "kind": "rbd",
              "rbd": {
                "image": "image1",
                "monitors": [
                  "192.168.3.253"
                ],
                "options": {
                  "name": "admin",
                  "ro": true,
                  "secret": "XXXXXXXXXXXX"
                },
                "pool": "rbd"
              }
            }
          }
        }
      ]
    }
  },
  "state": "running",
  "systemd": true
}