Re: [Qemu-devel] [RFC PATCH 0/2] Migration: support working on file:url

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jul 21, 2016 at 06:41:23PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Thu, Jul 21, 2016 at 01:05:58PM +0800, zhanghailiang wrote:
> > > It is more simple to use file:url to migrate VM into file.
> > > Besides, it will be used in memory snapshot.
> > 
> > NB, you can already migrate into a file
> > 
> >    "exec:/bin/cat > /path/to/file"
> > 
> > and likewise
> > 
> >   qemu -incoming "exec:/bin/cat /path/to/file"
> > 
> > This avoids the problem with POSIX I/O on plain files not actually
> > supporting O_NONBLOCK in any sensible manner, which your file:
> > suggestion suffers from.
> 
> Hmm that's a shame; I liked this idea as a nice simplification of the
> exec: stuff.

The only way to achieve that would be to spawn a thread in QEMU that
does I/O to the actual file, and then have the migration code do I/O
to/from that thread via a pipe. IOW, the thread would take the role
of cat.  What that's worth doing or not I don't know, given that it
is already possible to use exec+cat.

Regards,
Daniel
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