Re: [Qemu-devel] [PATCH] qcow2: Bring synchronous read/write back to life

[Mark McLoughlin <markmc@redhat.com>]

On Thu, 2009-10-08 at 10:01 -0500, Anthony Liguori wrote:
> Kevin Wolf wrote:
> > Am 08.10.2009 16:30, schrieb Anthony Liguori:
> >   
> >> Kevin Wolf wrote:
> >>     
> >>> When the synchronous read and write functions were dropped, they were replaced
> >>> by generic emulation functions. Unfortunately, these emulation functions don't
> >>> provide the same semantics as the original functions did.
> >>>
> >>> The original bdrv_read would mean that we read some data synchronously and that
> >>> we won't be interrupted during this read. The latter assumption is no longer
> >>> true with the emulation function which needs to use qemu_aio_poll and therefore
> >>> allows the callback of any other concurrent AIO request to be run during the
> >>> read.
> >>>       
> >> Perhaps you could create a mechanism to freeze the qcow2 image by 
> >> queuing all completions within qcow2 until the image was unfrozen.  This 
> >> would have the same effect switching to synchronous read/write.
> >>
> >> You may also have to queue new read/write requests...
> >>
> >> Introducing sync read/write seems like a major step backwards to me.
> >>     
> >
> > Right, I was expecting your reaction. ;-) I do even agree that it's not
> > nice to have the synchronous functions back. But removing them caused a
> > regression, so the removal should be reverted until it is done right.
> >
> > I just want to make clear that we're talking about data corruption here.
> > This is not just something that we can care about when we are bored some
> > time in the future.
> >   
> 
> Yeah, okay.  Can we do a more direct revert though so that it's clearer 
> in the commit log?

FWIW, here's the Fedora 12 (qemu-kvm-0.11.0) report on this:

  https://bugzilla.redhat.com/524734

Cheers,
Mark.