[libvirt] Notes: Non-shared storage live migration w/ active blockcommit

[Kashyap Chamarthy]

This notes is based on an IRC conversation with Eric Blake, to have
efficient non-shared storage live migration. Thought I'd post my notes
here before I forget. Please review and spot if there are any
inaccuracies.

Procedure
---------

(1) Starting from disk A, create a snapshot A <- A':
        
    $ virsh snapshot-create-as \
        --domain f20vm snap1 snap1-desc \
        --diskspec hda,file=/export/vmimages/A'.qcow2 \
        --disk-only --atomic

(2) Background copy of A to B:

    $ virsh blockcopy \
        --domain vm1 vda /export/vmimages/B.qcow2 \
        --wait --verbose --shallow \
        --finish

(3) Create an empty B' with backing file B:

    $ qemu-img create -f qcow2 -b B.qcow2 \
        -o backing_fmt=qcow2 B'.qcow2

    [or]

    $ virsh vol-create-as default B'.qcow2 1G \
        --format qcow2 \
        --backing-vol B.qcow2 --backing-vol-format qcow2 

(4) Do a shallow blockcopy of A' to B':

    $ virsh blockcopy \
        --domain vm1 vda /export/vmimages/B'.qcow2 \
        --wait --verbose --shallow \
        --finish

(5) Then live shallow commit of B:

    $ virsh blockcommit \
        --domain f20vm vda \
        --wait --verbose --shallow \
        --pivot --active --finish
    Block Commit: [100 %]
    Successfully pivoted

--
/kashyap