Re: [Qemu-devel] [RFC] introduce bitmap to bdrv_commit to track dirty sector

[Fam Zheng]

On Mon, 03/09 16:14, Zhang Haoyu wrote:
> Hi John, Vladimir 
> We can using active block commit to implement incremental backup without 
> guest disruption,
> e.g., 
> origin <= A <= B <= C <= current BDS,
> a new external snapshot will be produced before every time backup,
> we can migrate A, B, C, ... to destination, 
> then commit_active_start() the unneeded snapshot in source or destination end.
> 
> So, comparing with above mechanism,
> what's the advantages of the incremental backup implemented by John and 
> Vladimir?

We can't migrate A, B, C because they are buried in the backing chain under
"current BDS". Even if we do that, there will be severe IO amplification
compared to the dirty bitmap way.

For the same reason we have drive-backup (write intercept) instead of a
"snapshot + drive-mirror" analogue.

Fam