Re: [Qemu-devel] [PATCH v7 11/19] block: vhdx write support

[Stefan Hajnoczi]

On Fri, Oct 18, 2013 at 02:01:03PM +0200, Stefan Hajnoczi wrote:
> On Fri, Oct 11, 2013 at 02:17:31PM -0400, Jeff Cody wrote:
> > +            /* check the payload block state */
> > +            switch (s->bat[sinfo.bat_idx] & VHDX_BAT_STATE_BIT_MASK) {
> > +            case PAYLOAD_BLOCK_ZERO:
> > +                /* in this case, we need to preserve zero writes for
> > +                 * data that is not part of this write, so we must pad
> > +                 * the rest of the buffer to zeroes */
> > +
> > +                /* if we are on a posix system with ftruncate() that 
> > extends
> > +                 * a file, then it is zero-filled for us.  On Win32, the 
> > raw
> > +                 * layer uses SetFilePointer and SetFileEnd, which does not
> > +                 * zero fill AFAIK */
> > +
> > +                /* Queue another write of zero buffers if the underlying 
> > file
> > +                 * does not zero-fill on file extension */
> > +
> > +                if (bdrv_has_zero_init(bs->file) == 0) {
> 
> Maybe this should be done differently once Peter Lieven's thin
> provisioning patches are merged.  I think bdrv_has_zero_init() only
> tells you whether the file is zeroed after creation.  It doesn't
> guarantee zeroes for other operations like growing the file.
> 
> I'm not 100% sure right now, will follow-up on this point later when
> I've reviewed Peter's series and thought more about it.

With Peter's thin provisioning patches this call would be changed to
bdrv_has_discard_zeroes().  We're discussing changing the name of that
function though.

Stefan