add secondary disk instructions

        echo "$KEY_FROM_KVM_HOST" >> /etc/ssh/userkeys/root

 4. copy the `.qcow` file over, from the KVM host to the SPARE NODE:

 4. copy the `.qcow` file(s) over, from the KVM host to the SPARE NODE:

        rsync -P $KVM_HOST:/srv/vmstore/$INSTANCE/$INSTANCE-root /srv/

        rsync -P $KVM_HOST:/srv/vmstore/$INSTANCE/$INSTANCE-lvm /srv/ || true

 5. on the SPARE NODE, create and initialize a logical volume with the predetermined size:

        lvcreate -L 20GiB -n $INSTANCE-root vg_ganeti

        qemu-img convert /srv/$INSTANCE-root  -O raw /dev/vg_ganeti/$INSTANCE-root

        lvcreate -L 4GiB -n $INSTANCE-swap vg_ganeti

        mkswap --uuid $SWAP_UUID /dev/vg_ganeti/$INSTANCE-swap

        lvcreate -L 20GiB -n $INSTANCE-root vg_ganeti

        qemu-img convert /srv/$INSTANCE-root  -O raw /dev/vg_ganeti/$INSTANCE-root

        lvcreate -L 40GiB -n $INSTANCE-lvm vg_ganeti

        qemu-img convert /srv/$INSTANCE-lvm  -O raw /dev/vg_ganeti_hdd/$INSTANCE-lvm

    TODO: the above assumes only two disk devices, which is usually

    incorrect.

    Note how we assume two disks above, but the instance might have a

    different configuration that would require changing the above. The

    above, common, configuration is to have an LVM disk separate from

    the "root" disk, the former being on a HDD, but the HDD is

    sometimes completely omitted and sizes can differ.

    Sometimes it might be worth using pv to get progress on long

    transfers:

        qemu-img convert /srv/$INSTANCE-lvm -O raw /srv/$INSTANCE-lvm.raw

        pv /srv/$INSTANCE-lvm.raw | dd of=/dev/vg_ganeti_hdd/$INSTANCE-lvm bs=4k

    TODO: ideally, the above procedure (and many steps below as well)

    would be automatically deduced from the disk listing established

    in the first step.

 6. on the MASTER NODE, create the instance, adopting the LV:

        gnt-instance add -t plain \

            -n fsn-node-03 \

            --disk 0:adopt=$INSTANCE-root \

            --disk 1:adopt=$INSTANCE-swap \

            --disk 2:adopt=$INSTANCE-lvm,vg=vg_ganeti_hdd \

            --backend-parameters memory=2g,vcpus=2 \

            --net 0:ip=pool,network=gnt-fsn \

            --no-name-check \

        gnt-instance stop $INSTANCE

     on the KVM node:

        rsync -P /srv/vmstore/$INSTANCE/$INSTANCE-root $SPARE_NODE:/srv/

     on the Ganeti node:

        qemu-img convert /srv/$INSTANCE-root  -O raw /dev/vg_ganeti/$INSTANCE-root

        rsync -P $KVM_HOST:/srv/vmstore/$INSTANCE/$INSTANCE-root /srv/ &&

        qemu-img convert /srv/$INSTANCE-root  -O raw /dev/vg_ganeti/$INSTANCE-root &&

        rsync -P $KVM_HOST:/srv/vmstore/$INSTANCE/$INSTANCE-lvm /srv/ && 

        qemu-img convert /srv/$INSTANCE-lvm  -O raw /dev/vg_ganeti_hdd/$INSTANCE-lvm

 13. switch to DRBD, still on the Ganeti MASTER NODE: