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Re: [Qemu-devel] drive-backup locks VM if target has issues?
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] drive-backup locks VM if target has issues? |
Date: |
Mon, 30 Sep 2013 09:41:30 +0200 |
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Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130923 Thunderbird/17.0.9 |
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