Re: [Qemu-devel] [PATCH v2.5 00/10] block: incremental backup transactions

[Kashyap Chamarthy <kchamart@redhat.com>]

On Tue, Apr 21, 2015 at 03:53:11PM +0200, Kashyap Chamarthy wrote:

[. . .]

> And, invoking `drive-backup` *without* pre-creating the target image
> (i.e. 'incremental.o.img'):
> 
>     { 'execute': 'drive-backup',
>       'arguments': {
>         'device': 'ide0-0-0',
>         'bitmap': 'bitmap0',
>         'sync': 'dirty-bitmap',
>         'target': 'incremental.0.img',
>         'mode': 'existing',
>         'format': 'qcow2'
>       }
>     }
> 
> Results in:
> 
>     {"QMP": {"version": {"qemu": {"micro": 93, "minor": 2, "major": 2},
>     "package": ""}, "capabilities": []}} {"return": {}}
> 
> Instead of an "error". Is this a bug?

Answering myself, no, it's not.

I learnt from John Snow and Eric Blake that I needed additional 'reads'
to see event lines in my trivial QMP shell script[1]  -- "because 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" (detail
by Eric).

A minimal test below:

(1) This time, 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"}}
----------------

Thanks, John/Eric.


[1] Discussion thread here which contains the trivial script used:
http://lists.nongnu.org/archive/html/qemu-devel/2015-04/msg02432.html