From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:54511)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1aBgSB-0000Tw-Kp
	for qemu-devel@nongnu.org; Wed, 23 Dec 2015 05:15:32 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <famz@redhat.com>) id 1aBgSA-0007M4-Ig
	for qemu-devel@nongnu.org; Wed, 23 Dec 2015 05:15:31 -0500
Date: Wed, 23 Dec 2015 18:15:20 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20151223101520.GL14423@ad.usersys.redhat.com>
References: <87vb7vc2yt.fsf@blackfin.pond.sub.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87vb7vc2yt.fsf@blackfin.pond.sub.org>
Subject: Re: [Qemu-devel] Minutes from the "Stuttgart block Gipfele"
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Alberto Garcia <berto@igalia.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, John Snow <jsnow@redhat.com>

On Fri, 12/18 14:15, Markus Armbruster wrote:
> First, let's examine how such a chain could look like.  If we read the
> current code correctly, it behaves as if we had a chain
> 
>         BB
>          |
>       throttle
>          |
>      detect-zero
>          |
>     copy-on-read
>          |
>         BDS
> 
> Except for the backup job, which behaves as if we had
> 
>                backup job
>               /
>       notifier
>          |
>      detect-zero
>          |
>         BDS

Just to brainstorm block jobs in the dynamic reconfigured node graph: (not sure
if this is useful)

Nothing stops us from viewing backup as a self-contained filter,

        [backup]
           |
       detect-zero
           |
          BDS

where its .bdrv_co_writev copies out the old data, and at instantiation time
it also creates a long running coroutine (backup_run).

In that theory, all other block job types, mirror/stream/commit, fit into a
"pull" model, which follows a specified dirty bitmap and copies data from a
specified src BDS. In this pull model,

mirror (device=d0 target=d1) becomes a pull fileter:

        BB[d0]            BB[d1]
           |                 |
        throttle        [pull,src=d0]
           |                 |
       detect-zero       detect-zero
           |                 |
      copy-on-read      copy-on-read
           |                 |
          BDS               BDS

Note: the pull reuses most of the block/mirror.c code except the
s->dirty_bitmap will be initialized depending on the block job type. In the
case of mirror, it is trivially the same as now.

stream (device=d0 base=base0) becomes a pull filter:

          BB[d0]
           |
     [pull,src=base0]
           |
       detect-zero
           |
      copy-on-read
           |
          BDS
           |
          BDS[base0]

Note: s->dirty_bitmap will be initialized with the blocks which should be copied
by block-stream.

Similarly, active commit (device=d0 base=base0) becomes a pull filter:

          BB[d0]
           |
       detect-zero
           |
      copy-on-read
           |
          BDS
           |
      [pull,src=d0]
           |
          BDS[base0]

and commit (device=d0 top=base1 base=base0) becomes a pull filter:

          BB[d0]
           |
       detect-zero
           |
      copy-on-read
           |
          BDS
           |
          BDS[base1]
           |
      [pull,src=base1]
           |
          BDS[base0]


If this could work, I'm looking forward to a pretty looking diffstat if we can
unify the coroutine code of all four jobs. :)

Fam