[21-APR-2015]

Purpose: Test dirty-bitmap with `drive-backup` in QEMU


Prequisites
===========

I tested with this branch:

    https://github.com/jnsnow/qemu/commits/incremental-transactions

    $ git checkout -b v2.5-and-v6-inc-backup jsnow/incremental-transactions
    $ git describe
    v2.3.0-rc3-37-g6932a32


Test
====

(1) Invoke QEMU w/ QMP server over Unix socket:

---------------
$ ./invoke-qemu-with-qmp.sh
. . .
-qmp unix:./qmp-sock,server
char device redirected to /dev/pts/49 (label charserial0)
QEMU waiting for connection on: disconnected:unix:./qmp-sock,server
---------------


(2) Pre-create the target destination:

    $ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2


(3) And, use `rlwrap` in combination with `socat` (previously learnt
this trick from Markus Armbruster, it conveniently retains the command
history) to connect to the QMP server and issue 'block-dirty-bitmap-add'
and 'drive-mirror' commands with 'dirty-bitmap':

----------------
$ rlwrap -H ~/.qmp_history socat UNIX-CONNECT:./qmp-sock STDIO
{"QMP": {"version": {"qemu": {"micro": 93, "minor": 2, "major": 2}, "package": ""}, "capabilities": []}}
{"execute":"qmp_capabilities"}
{"return": {}}

{ 'execute': 'block-dirty-bitmap-add', 'arguments': { 'node': 'drive-ide0-0-0', 'name': 'bitmap0' } }
{"return": {}}

{ 'execute': 'drive-backup', 'arguments': { 'device': 'drive-ide0-0-0', 'bitmap': 'bitmap0', 'sync': 'dirty-bitmap', 'target':
+'/home/kashyapc/work/virt/qemu/incremental-backup-test-qemu/tests/incremental.0.img', 'mode': 'existing', 'format': 'qcow2' } }
{"return": {}}
{"timestamp": {"seconds": 1429647518, "microseconds": 663755}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "drive-ide0-0-0", "len": 41126400, "offset": 41156608, "speed": 0, "type":
+"backup"}}
----------------


Additional notes
================

Comments from eblake on running QMP:

 - It will require additional reads to see the event lines (qemu sends
   events as the job makes progress)
 - when you first connect, the qemu sends a line BEFORE you send your
   'qmp_capabilities' response so you're off by one if you don't read
   the initial server greeting
 - I'm not sure how modal QMP is - that is, I don't know if it refuses
   to read new commands from your input until after you have drained the
   output it was trying to send you , or whether you can send commands
   as fast as you want and then read replies
 - There's a limit as to how much you can send before you must read,
   based on buffer sizes, but I don't know if there are additional
   limits on per-line reads


Trivial QMP scripts
===================

#!/bin/bash
set -x
# Create bitmap

exec 3<>/dev/tcp/localhost/4444
echo -e "{ 'execute': 'qmp_capabilities' }" >&3
read response <&3
read response <&3
echo "$response"
echo -e "{ 'execute': 'block-dirty-bitmap-add', 'arguments': { 'node': 'drive-ide0-0-0', 'name': 'bitmap0' } }" >&3
read response <&3
echo "$response"


#!/bin/bash
set -x
# Test incremental backup with `drive-backup`

exec 3<>/dev/tcp/localhost/4444
echo -e "{ 'execute': 'qmp_capabilities' }" >&3
read response <&3
read response <&3
echo "$response"
echo -e "{ 'execute': 'drive-backup', 'arguments':
      { 'device': 'drive-ide0-0-0', 'bitmap': 'bitmap0', 'sync': 'dirty-bitmap', 'target':
      '/home/kashyapc/work/virt/qemu/incremental-backup-test-qemu/tests/incremental.0.img', 'mode': 'existing', 'format': 'qcow2' } }" >&3
read response <&3
echo "$response"