Re: [Qemu-devel] [PATCH RFC for-2.6 0/3] block: Add meta dirty bitmap for migration/persistence

[John Snow <jsnow@redhat.com>]

On 12/07/2015 12:59 AM, Fam Zheng wrote:
> Vladimir,
> 
> This is what I propose to implement meta bitmap. It's implemented in the
> HBitmap level to be more efficient, and the interface slightly varies too.
> 

I missed it: What was wrong with Vladimir's approach / what are the
benefits of this approach?

> I'd like to use these operations to make dirty bitmap persistence more
> efficient too: unchanged dirty bits don't need to be flushed to disk. So I'm
> posting this as a separate series for a common base for both sides.
> 

This is a reasonable use of the meta-bitmap strategy in general.

Keep in mind Vladimir's approach to Meta bitmaps used a different
granularity such that 1 physical bit implied 1 sector needed to be
re-transmitted.

A meta-bitmap that keeps track of disk flushes may require a different
granularity than one used for migration.

> Posting as RFC as 2.6 dev phase is just starting, we can still tweak the
> interface and/or implementation to fit the need.
> 
> Fam Zheng (3):
>   HBitmap: Introduce "meta" bitmap to track bit changes
>   tests: Add test code for meta bitmap
>   block: Support meta dirty bitmap
> 
>  block.c                | 46 ++++++++++++++++++++++++++++++-
>  block/mirror.c         |  3 +-
>  blockdev.c             |  3 +-
>  include/block/block.h  | 11 ++++++++
>  include/qemu/hbitmap.h |  7 +++++
>  migration/block.c      |  2 +-
>  tests/test-hbitmap.c   | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  util/hbitmap.c         | 22 +++++++++++++++
>  8 files changed, 164 insertions(+), 4 deletions(-)
>