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[Qemu-devel] drive-backup locks VM if target has issues?


From: Wolfgang Richter
Subject: [Qemu-devel] drive-backup locks VM if target has issues?
Date: Sun, 29 Sep 2013 18:46:46 -0400

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).

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.

-- 
Wolf



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