Efficient live disk backup with active blockcommit
==================================================

Purpose: Take efficient live disk backup using virsh blockcommit

Procedure
---------

(1) Start with a single disk image:

    $ virsh domblklist vm1
    Target     Source
    ------------------------------------------------
    vda        /export/images/base.qcow2 


(2) Create an external disk snapshot:

    $ virsh snapshot-create-as --domain vm1 sn1 \
        --diskspec vda,file=/export/images/sn1.qcow2 \
        --disk-only --atomic --no-metadata

    Now, the chain is: [base] <-- [sn1] (live QEMU)

    NOTE (thanks Eric Blake): Above, if you have QEMU guest agent
    installed in your virtual machine, try --quiesce option with
    'snapshot-create-as' to ensure you have a consistent disk
    state.


(3) Take a backup of the original disk in backround:

    $ rsync -avh --progress /export/images/base.img \
            /export/images/copy.img


(4) Now that the backup is finished, perform active blockcommit by
    mergning contents of sn1 into base:

    $ virsh domblklist vm1
    Target     Source
    ------------------------------------------------
    vda        /export/images/sn1.qcow2

    $ virsh blockcommit vm1 vda --active --verbose --pivot

    $ virsh domblklist vm1
    Target     Source
    ------------------------------------------------
    vda        /export/images/base.qcow2


The final resulting disk image chain (a single consolidated disk image):

    [base] (live QEMU)