Re: [Qemu-devel] [PATCH v5 0/6] block: zero writes

[Kevin Wolf <kwolf@redhat.com>]

Am 07.02.2012 14:27, schrieb Stefan Hajnoczi:
> This series adds an interface for efficient writes when data contains all
> zeros.  It also takes advantage of this new interface by extending the
> copy-on-read feature to perform zero-detection.
> 
> The details of efficient zero representations depend on the image format.  This
> series includes a patch for the QED image format to write special "zero
> clusters" that keep the image file compact.  In the future qcow2v3 could also
> support an efficient zero representation.
> 
> The new BlockDriver interface is called .bdrv_co_write_zeroes() and is
> optional.  If the interface is not implemented by a BlockDriver then a regular
> .bdrv_co_writev() operation will be performed.  The public interface is called
> bdrv_co_write_zeroes() and can be tested via the new qemu-io write -z option.
> 
> Copy-on-read is extended to detect zeroes and invoke the
> .bdrv_co_write_zeroes() interface when possible.  As a result we avoid bloating
> the image file if the backing file contains zeroes.
> 
> My motivation for this feature is efficient image streaming.  The destination
> file must stay compact even when copying zeroes from the source file.
> 
> We now only do zero detection for copy-on-read requests, whereas previous
> revisions of this patch series scanned all write requests for zeroes.  The old
> behavior wasted CPU cycles in most cases but we could add a feature to
> explicitly scan guest writes for zeroes in the future, if desired.
> 
> v5:
>  * assert length constraint in buffer_is_zero() [Kevin]
>  * explain why bdrv_co_write_zeroes() is not suitable for huge requests [Kevin]
> 
> v3:
>  * Rebased on image streaming series
> 
> v2:
>  * Introduce .bdrv_co_write_zeroes() [Kevin]
>  * Perform zero detection only on copy-on-read requests [Kevin]
> 
> Stefan Hajnoczi (6):
>   cutils: extract buffer_is_zero() from qemu-img.c
>   block: add .bdrv_co_write_zeroes() interface
>   block: perform zero-detection during copy-on-read
>   qed: replace is_write with flags field
>   qed: add .bdrv_co_write_zeroes() support
>   qemu-io: add write -z option for bdrv_co_write_zeroes
> 
>  block.c       |   67 ++++++++++++++++++++++++++----
>  block.h       |    8 ++++
>  block/qed.c   |  125 ++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  block/qed.h   |    7 +++-
>  block_int.h   |    8 ++++
>  cutils.c      |   35 ++++++++++++++++
>  qemu-common.h |    2 +
>  qemu-img.c    |   46 +++------------------
>  qemu-io.c     |   77 +++++++++++++++++++++++++++++++----
>  trace-events  |    3 +-
>  10 files changed, 305 insertions(+), 73 deletions(-)

Thanks, applied all to the block branch.

Kevin