Re: [Qemu-devel] [PATCH 0/3] block: zero write detection

[Stefan Hajnoczi]

On Tue, Oct 11, 2011 at 03:46:28PM +0200, Kevin Wolf wrote:
> Am 07.10.2011 17:49, schrieb Stefan Hajnoczi:
> > Image streaming copies data from the backing file into the image file.  It 
> > is
> > important to represent zero regions from the backing file efficiently during
> > streaming, otherwise the image file grows to the full virtual disk size and
> > loses sparseness.
> > 
> > There are two ways to implement zero write detection, they are subtly 
> > different:
> > 
> > 1. Allow image formats to provide efficient representations for zero 
> > regions.
> >    QED does this with "zero clusters" and it has been discussed for qcow2v3.
> > 
> > 2. During streaming, check for zeroes and skip writing to the image file 
> > when
> >    zeroes are detected.
> > 
> > However, there are some disadvantages to #2 because it leaves unallocated 
> > holes
> > in the image file.  If image streaming is aborted before it completes then 
> > it
> > will be necessary to reread all unallocated clusters from the backing file 
> > upon
> > resuming image streaming.  Potentionally worse is that a backing file over a
> > slow remote connection will have the zero regions fetched again and again if
> > the guest accesses them.  #1 avoids these problems because the image file
> > contains information on which regions are zeroes and do not need to be
> > refetched.
> > 
> > This patch series implements #1 with the existing QED zero cluster feature. 
> >  In
> > the future we can add qcow2v3 zero clusters too.  We can also implement #2
> > directly in the image streaming code as a fallback when the BlockDriver does
> > not support zero detection #1 itself.  That way we get the best possible 
> > zero
> > write detection, depending on the image format.
> > 
> > Here is a qemu-iotest to verify that zero write detection is working:
> > http://repo.or.cz/w/qemu-iotests/stefanha.git/commitdiff/226949695eef51bdcdea3e6ce3d7e5a863427f37
> > 
> > Stefan Hajnoczi (3):
> >   block: add zero write detection interface
> >   qed: add zero write detection support
> >   qemu-io: add zero write detection option
> > 
> >  block.c     |   16 +++++++++++
> >  block.h     |    2 +
> >  block/qed.c |   81 
> > +++++++++++++++++++++++++++++++++++++++++++++++++++++------
> >  block_int.h |   13 +++++++++
> >  qemu-io.c   |   35 ++++++++++++++++++++-----
> >  5 files changed, 132 insertions(+), 15 deletions(-)
> 
> It's good to have an option to detect zero writes and turn them into
> zero clusters, but it's something that introduces some overhead and
> probably won't be suitable as a default.

Yes, this series simply has a bdrv_set_zero_detection() API to toggle it
at runtime.  By default it is off to save CPU cycles.

> I think what we really want to have for image streaming is an API that
> explicitly writes zeros and doesn't have to look at the whole buffer (or
> actually doesn't even get a buffer).

I didn't take this approach to avoid having block drivers handle the
zero buffers that need to be allocated when the region does not cover
entire clusters.  It can be done for sure but I'm not sure how to do it
nicely yet.

Stefan