Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

[Stefan Hajnoczi]

On Fri, May 17, 2013 at 02:58:57PM +0800, Wenchao Xia wrote:
> 于 2013-5-16 15:47, Stefan Hajnoczi 写道:
> >On Thu, May 16, 2013 at 02:16:20PM +0800, Wenchao Xia wrote:
> >>   After checking the code, I found it possible to add delta data backup
> >>support also, If an additional dirty bitmap was added.
> >
> >I've been thinking about this.  Incremental backups need to know which
> >blocks have changed, but keeping a persistent dirty bitmap is expensive
> >and unnecessary.
> >
>   Yes, it would be likely another block layer, so hope not do that.

Not at all, persistent dirty bitmaps need to be part of the block layer
since they need to support any image type - qcow2, Gluster, raw LVM,
etc.

> >I don't consider block jobs to be "qemu device" layer.  It sounds like
> >you think the code should be in bdrv_co_do_writev()?
> >
>   I feel a trend of becoming fragility from different solutions,
> and COW is a key feature that block layer provide, so I wonder if it
> can be adjusted under block layer later

The generic block layer includes more than just block.c.  It also
includes block jobs and the qcow2 metadata cache that Dong Xu has
extracted recently, for example.  Therefore you need to be more specific
about "what" and "why".

This copy-on-write backup approach is available as a block job which
runs on top of any BlockDriverState.  What concrete change are you
proposing?