[24-FEB-2017]

Use case
~~~~~~~~

    Provide an NBD export that serves you a (read-only?) point-in-time
    (PIT) snapshot of a disk.  [Also sometimes referred to as "Image
    Fleecing"]

This test uses QEMU block primitives.

Tested with QEMU Git:

  $ git describe
  pull-block-2017-02-21-81-g612bb5d


High level procedure
~~~~~~~~~~~~~~~~~~~~

(1) Create the target image ('target.qcow2')

    $ qemu-img create -f qcow2 target.qcow2 1G

(2) Add the above via `blockdev-add` to the running QEMU instance, 
    with 'base.qcow2' as its backing file.

(3) Run `query-named-block-nodes` to find the node name (automatically assigned, 
    if it wasn't specified) of base.qcow2:

    https://kashyapc.fedorapeople.org/virt/qemu/query-named-block-nodes-QMP-return.txt

(4) Take the backup of device 'base.qcow2' into 'target.qcow2' using
    'blockdev-backup' (using node names "node-name":"#block197" for base.qcow2; 
    "node1" for target.qcow2)

(5) Then, export 'target.qcow2' over NBD with `nbd-server-start`, 
    and `nbd-server-add`.

(6) Discard the NBD export at anytime you wish (with 'nbd-server-stop')


Test with 'qmp-shell'
~~~~~~~~~~~~~~~~~~~~~

[$ qemu-img create -f qcow2 /export/target.qcow2 1G]

(QEMU) blockdev-add driver=qcow2 node-name=node2 file={"driver":"file","filename":"/export/target.qcow2"} backing={"driver":"qcow2","file":{"driver":"file","filename":"/export/base.qcow2"}}
(QEMU) query-named-block-nodes
(QEMU) blockdev-backup device=#block197 target=node1 sync=none
(QEMU) nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}
(QEMU) nbd-server-add device=node1
(QEMU) nbd-server-stop


With complete JSON
~~~~~~~~~~~~~~~~~~

Fire up QEMU on shell 1:

$ ~/build/qemu-upstream/x86_64-softmmu/qemu-system-x86_64 \
    -display none -nodefconfig -nodefaults -m 512 \
    -device virtio-scsi-pci,id=scsi -device virtio-serial-pci \
    -drive file=./base.qcow2,format=qcow2,if=virtio -monitor stdio \
    -qmp unix:./qmp-sock,server,nowait
QEMU 2.8.50 monitor - type 'help' for more information
(qemu) 


On shell 2:

-------
$ qemu-img create -f qcow2 /export/target.qcow2 1G
Formatting '/export/target.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
-------
$ ~/build/qemu-upstream/x86_64-softmmu/qemu-system-x86_64 --version
QEMU emulator version 2.8.50 (v2.8.0-1462-g612bb5d-dirty)
Copyright (c) 2003-2016 Fabrice Bellard and the QEMU Project developers
-------
$ socat UNIX:/export/qmp-sock READLINE,history=$HOME/.qmp_history,prompt='QMP> '
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 8, "major": 2}, "package": " (v2.8.0-1462-g612bb5d-dirty)"}, "capabilities": []}}
QMP> {"execute":"qmp_capabilities"}
{"return": {}}
-------
{
  "execute":"blockdev-add",
  "arguments":{
    "driver":"qcow2",
    "node-name":"node1",
    "file":{
      "driver":"file",
      "filename":"/export/target.qcow2"
    },                                                                                                                                                                                        
    "backing":{
      "driver":"qcow2",
      "file":{
        "driver":"file",
        "filename":"/export/base.qcow2"
      }                                                                                                                                                                                       
    }                                                                                                                                                                                         
  }                                                                                                                                                                                           
}
-------
[Run "query-named-block-nodes" and find the node name of 'base.qcow2',
which is "#block197" in this case, which was automatically assigned.]

https://kashyapc.fedorapeople.org/virt/qemu/query-named-block-nodes-QMP-return.txt
-------
{  
  "execute":"blockdev-backup",
  "arguments":{  
    "device":"#block197",
    "target":"node1",
    "sync":"none"
  }
}
-------
QMP> {  
QMP>   "execute":"blockdev-backup",
QMP>   "arguments":{  
QMP>     "device":"#block197",
QMP>     "target":"node1",
QMP>     "sync":"none"
QMP>   }
QMP> }
{"return": {}}
-------
{  
  "execute":"nbd-server-start",
  "arguments":{  
    "addr":{  
      "data":{  
        "path":"./nbd-sock"
      },
      "type":"unix"
    }
  }
}
-------
{  
  "execute":"nbd-server-add",
  "arguments":{  
    "device":"node1"
  }
}
-------
{  
  "execute":"nbd-server-stop"
}
-------

* * *