Re: [Qemu-devel] [PATCH 3/3] block: mirror - zero unallocated target sectors when zero init not present

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/09/2015 18:02, Jeff Cody wrote:
> As a general rule for blockjobs, I disagree.
> 
> Right away, there is a key difference: we don't know that the image is
> (or should be) empty.

Not necessarily empty.  sync='top' && mode='existing' &&
!target->backing_file, for example, makes sense if the target is a copy
of source->bs.

In fact, commit of the active layer is almost exactly a mode='existing'
drive-mirror operation.

But if you use mode == 'existing', and don't provide an image that
follows the rules, it's garbage-in garbage-out.  The sequence of
operation makes sense, but the resulting image does not.

Paolo

> With mode != "existing", we know the image
> should be empty, since we just created it (although for a host device,
> it may have extraneous data in it).  So I think it is not so much what
> we can assume about an existing image, as it is what we cannot assume.
> And that could potentially influence some block jobs.