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

[Wolfgang Richter]

On Mon, Sep 30, 2013 at 3:41 AM, Paolo Bonzini <address@hidden> wrote:
> Il 30/09/2013 00:46, Wolfgang Richter ha scritto:
> 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.

Yes, that makes sense and was what I thought as well: it should have been
canceled or ended in some bad state.  Instead my VM saw drive write errors
and remounted root read-only.  Not an issue for "real work" for me, just meant
my benchmark couldn't run.

>> 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.

Yeah, /dev/zero has the same issue.  I could make a null backend, or just make
my NBD server drop all the writes.  There will be extra overhead from TCP, but
it'll be good enough for me to measure (NBD is what I am using as a target
eventually anyways).

-- 
Wolf