Re: [Qemu-devel] [RFC 0/7] qcow2: implement lazy refcounts optimization

[Zhi Yong Wu]

On Fri, Jun 22, 2012 at 11:08 PM, Stefan Hajnoczi
<address@hidden> wrote:
> This series aims to improve qcow2 performance with cache=writethrough and
> cache=directsync.  In particular it reduces the impact of metadata updates for
What does cache=directsync mean? what is the difference between them?
> allocating writes.
>
> Allocating writes are expensive because they involve updating L2 tables and
> refcount blocks.  In addition they can also cause L2 table allocation and
> refcount block allocation but these remain unaffected by this optimization.
>
> The key insight is that refcounts are not required to access data in the 
> image.
Can you elaborate this? why?
> This means that we can postpone refcount updates without violating the data
> integrity guarantee that cache=writethrough and cache=directsync give.
>
> The trade-off for postponing refcount updates is that the image may not be
> completely consistent in case of power failure or crash.  If the image is 
> dirty
> then it must be repaired before performing further modifications, in other
> words we need an fsck-like scan on startup.
>
> I don't have performance results to share yet but I wanted to get the code out
> there.  The bigger picture is that this optimization should help make qcow2 a
> good choice even for cache=writethrough and cache=directsync where QED has
> traditionally had an advantage due to less metadata - this allows us to
> converge image format development in QEMU around the qcow2v3 format.
Why do we design one new image format? e.g. use btree to manage
metadata and disk data layout? Since btree has a lot of advantages
than bitmap or tables.
>
> Stefan Hajnoczi (7):
>  docs: add dirty bit to qcow2 specification
>  qcow2: introduce dirty bit
>  docs: add lazy refcounts bit to qcow2 specification
>  qemu-iotests: ignore qemu-img create lazy_refcounts output
>  qcow2: implement lazy refcounts
>  qemu-io: add "abort" command to simulate program crash
>  qemu-iotests: add 039 qcow2 lazy refcounts test
>
>  block/qcow2-cluster.c        |    5 +-
>  block/qcow2.c                |  111 
> +++++++++++++++++++++++++++++++++++++++---
>  block/qcow2.h                |   11 +++++
>  block_int.h                  |   26 +++++-----
>  docs/specs/qcow2.txt         |   12 ++++-
>  qemu-io.c                    |   12 +++++
>  tests/qemu-iotests/039       |   99 +++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/039.out   |   34 +++++++++++++
>  tests/qemu-iotests/common.rc |    3 +-
>  tests/qemu-iotests/group     |    1 +
>  10 files changed, 292 insertions(+), 22 deletions(-)
>  create mode 100755 tests/qemu-iotests/039
>  create mode 100644 tests/qemu-iotests/039.out
>
> --
> 1.7.10
>
>



-- 
Regards,

Zhi Yong Wu