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Date: Mon, 30 Sep 2013 09:41:30 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] drive-backup locks VM if target has issues?
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To: Wolfgang Richter <wolf@cs.cmu.edu>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, imain <imain@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-devel <qemu-devel@nongnu.org>

Il 30/09/2013 00:46, Wolfgang Richter ha scritto:
> I wanted to explore overhead with the new drive-backup command and I
> noticed if I set the target to something like '/dev/null' the guest VM
> starts having IO errors and loses write access to its root file
> system.  Here is the qmp-shell command I'm using:
> 
>> drive-backup sync=none device=virtio0 target=/dev/null format=raw mode=existing
> 
> I have a guest running with a single virtio root disk (ext4, Ubuntu
> guest).  After that command, the guest sees write errors to its root
> block device (virtio0).

All writes to the drive-backup source have to first copy the pre-write
data to the target.  Thus, drive-backup usually works best if you are
using werror=stop on the source.  That said, I would have expected the
job to be cancelled instead.  Looks like there are bugs in the handling
of on_target_error.

> I didn't trace syscalls or dig deeper yet, but was wondering if you
> had an idea on why '/dev/null' as a target in a block job would cause
> the origin device to lockup/fail?
> 
> My overall goal is to drop the extra write traffic as early as
> possible to measure overhead of the drive-backup command in a few
> different scenarios, thus I was hoping /dev/null would help here.

I think you need a "null" backend instead that drops writes at the QEMU
level.  Perhaps /dev/zero helps too.

Paolo