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

From: Zhang Haoyu
Subject: Re: [Qemu-devel] [RFC] introduce bitmap to bdrv_commit to trackdirty sector
Date: Tue, 10 Mar 2015 09:30:53 +0800

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.

> 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?
If yes, it will impact on IO throughput in two sides, I think,
1) extro IO  2) randomize the IO,
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.

> For the same reason we have drive-backup (write intercept) instead of a
> "snapshot + drive-mirror" analogue.
drive-backup is not incremental backup, full copy is needed in every time 
so it dosen't meet our requirements.

Zhang Haoyu
> Fam

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