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

[Fam Zheng]

On Tue, 03/10 09:30, Zhang Haoyu wrote:
> 
> On 2015-03-10 08:29:19, Fam Zheng wrote:
> > 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". 
> I think we can backup the incremental image(e.g., A, B, C) to destination in 
> top mode,
> although I haven't read the code in detail, it can work theoretically, I 
> think.

No, we don't have any command do that.

> 
> > Even if we do that, there will be severe IO amplification
> > compared to the dirty bitmap way.
> > 
> Yes, block-commit will produce extra IO.
> But regarding incremental backup, when guest IO is performed, 
> will the corresponding dirty bit be synchronized to qcow2 image 
> simultaneously?

No, that would have a huge performance penalty. It will only be synced
at shutdown and or periodically, therefore it has the same implications with
other cache, such as page cache or block driver metadata cache.

> if no, if source VM is shut-down in non-normal way, like killed by force or 
> by mistake or server poweroff suddenly,
> some dirty bits maybe lost, then full copy is needed.

Yes, it is a reasonable rule.

> > 
> drive-backup is not incremental backup, full copy is needed in every time 
> backup,
> so it dosen't meet our requirements.

I didn't mean drive-backup already provides incremental backup, but we do need
it to implement it (see the patch series posted by John Snow).

Thanks,
Fam