Re: [Qemu-devel] [PATCH 0/2] block: allow commit active as top

[Fam Zheng <famz@redhat.com>]

On Mon, 07/22 14:43, Wenchao Xia wrote:
> 于 2013-7-22 11:46, Fam Zheng 写道:
> > Previously live commit of active block device is not supported, this series
> > implements it and updates corresponding qemu-iotests cases.
> > 
> > Please see commit messages for implementation details.
> > 
> > Fam Zheng (2):
> >    block: allow live commit of active image
> >    qemu-iotests: update test cases for commit active
> > 
> >   block.c                | 102 ++++++++++---------------------
> >   block/commit.c         | 160 +++++++++++++++++++++++++------------------------
> >   include/block/block.h  |   5 +-
> >   tests/qemu-iotests/040 |  68 ++++++++-------------
> >   4 files changed, 141 insertions(+), 194 deletions(-)
> > 
>   I have thought the commit with top issue before, a question comes to
> me: what is the relationship with block-stream?
> 
> for example:
> base->mid->top
> 
> you can block-stream mid to top(if I remember correctly), result is:
> base->(mid+top)
> 
> with this patch, you can block-commit top to mid, result is:
> base->(mod+top)
> 
> it seems the function is duplicated. Maybe you can raise another
> good user case. I must say it may help user to do every thing
> with block-commit, but if you find it is too much to make it work,
> maybe you can postpond it, since block-stream is available.
> 

Of course, you can either merge 'top' into 'mid', or merge 'mid' into
'top', you always get sum of them. However if 'mid' is very large while
you only allocated a few clusters in 'top', block-stream is not very nice
to use: you need to copy the whole 'mid' to 'top'. But committing 'top'
back to 'mid', if it has no other children, will be much faster.

-- 
Fam