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

["Mars.cao" <mars@linux.vnet.ibm.com>]

On 10/07/2011 11:49 PM, Stefan Hajnoczi wrote:
> 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(-)
>
I tested the patch by qemu-iotest 029 test case and also by manually, it 
worked as expected.

Tested-by: Cao,Bing Bu <mars@linux.vnet.ibm.com>