Re: [Qemu-devel] [PATCH v2 3/5] vmdk: Add option to create zeroed-grain image

[Fam Zheng <famz@redhat.com>]

On Mon, 04/22 16:06, Stefan Hajnoczi wrote:
> On Mon, Apr 22, 2013 at 10:07:57AM +0800, Fam Zheng wrote:
> > Add image create option "zeroed-grain" to enable zeroed-grain GTE
> > feature of vmdk sparse extents. When this option is on, header version
> > of newly created extent will be 2 and VMDK4_FLAG_ZG flag bit will be
> > set.
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/vmdk.c | 22 +++++++++++++++++-----
> >  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> Is there a way to upgrade an image file to use zeroed-grain GTEs once
> the file has been created?
> 

Yes. We have discussed this on IRC.

Here's how we do it:
 - If file version is 2 and zeroed-grain GTE bit set in header, we can
   support bdrv_co_write_zeroes (this patch series implemented).
 - If the file version is 2, we can enable zeroed-grain flag in header
   on bdrv_co_write_zeroes, and use zeroed-grain GTE. (need another
   patch to update the header).
 - Otherwize, -ENOTSUP

For background, here's how vmware uses this feature:
 - On shrinking a *child* disk (e.g.: vmware-vdiskmanager -k
   child.vmdk), if the child cluster is filled with zero, then the image
   version is upgraded to 2 and zeroed-grian flag is set, and
   zeroed-grain GTE is used to clear out the cluster, no need to take
   care of parent cluster allocation and actual data at all. That's
   where the efficiency comes.

-- 
Fam