From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:39641)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <kchamart@redhat.com>) id 1aVHDG-0004nV-RX
	for qemu-devel@nongnu.org; Mon, 15 Feb 2016 06:21:07 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <kchamart@redhat.com>) id 1aVHDC-0002Nt-Sx
	for qemu-devel@nongnu.org; Mon, 15 Feb 2016 06:21:06 -0500
Date: Mon, 15 Feb 2016 12:20:54 +0100
From: Kashyap Chamarthy <kchamart@redhat.com>
Message-ID: <20160215112054.GC15761@tesla.redhat.com>
References: <56C0A2B2.8070202@redhat.com> <56C0A75E.4020807@redhat.com>
	<20160215091259.GB5244@noname.str.redhat.com>
	<56C199B9.5000400@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <56C199B9.5000400@redhat.com>
Subject: Re: [Qemu-devel] crash with -incoming and -snapshot
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: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, qemu block <qemu-block@nongnu.org>

On Mon, Feb 15, 2016 at 10:26:17AM +0100, Paolo Bonzini wrote:
> 
> 
> On 15/02/2016 10:12, Kevin Wolf wrote:
> > Am 14.02.2016 um 17:12 hat Paolo Bonzini geschrieben:
> >> On 14/02/2016 16:52, Paolo Bonzini wrote:
> >>> Reproducer:
> >>>
> >>> x86_64-softmmu/qemu-system-x86_64 \
> >>>    -incoming tcp:localhost:12345 -snapshot \
> >>>    /vm/virt_test/images/jeos-21-64-base.qcow2
> >>>
> >>>
> >>> Weird as it may seem, this actually makes some sense for testing
> >>> migration with non-shared storage...
> >>
> >> In fact, migration with non-shared storage (migrate -b) is broken in
> >> general, even without -snapshot.
> > 
> > Oh cool, does this mean that we can finally remove it? :-)
> 
> I wouldn't object, but people are using it. :)

One case I know for sure: Upstream libvirt (consequently, OpenStack
Nova) falls back to 'migrate -b'/("inc:" true) approach when there's no
NBD support on the destination during live block migration:


    # From qemuMigrationRun() function in libvirt source
    $ less libvirt/src/qemu/qemu_migration.c
    [...]
    if (migrate_flags & (QEMU_MONITOR_MIGRATE_NON_SHARED_DISK |
                         QEMU_MONITOR_MIGRATE_NON_SHARED_INC)) {
        if (mig->nbd) {
            /* This will update migrate_flags on success */
            if (qemuMigrationDriveMirror(driver, vm, mig,
                                         spec->dest.host.name,
                                         migrate_speed,
                                         &migrate_flags,
                                         nmigrate_disks,
                                         migrate_disks,
                                         dconn) < 0) {
                goto cleanup;
            }
        } else {
            /* Destination doesn't support NBD server.
             * Fall back to previous implementation. */
            VIR_DEBUG("Destination doesn't support NBD server "
                      "Falling back to previous implementation.");
        }
    }
    [...]


This fallback also kicks in when people enable TUNNELLED migration (as
NBD does not have support for it in libvirt, due to valid technical
reasons documented else where).

-- 
/kashyap