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Re: [Qemu-devel] [PATCH 2/8] qcow2: add dirty-bitmaps feature

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 2/8] qcow2: add dirty-bitmaps feature
Date: Mon, 15 Jun 2015 15:42:37 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, Jun 12, 2015 at 03:02:33PM -0400, John Snow wrote:
> On 06/10/2015 10:30 AM, Stefan Hajnoczi wrote:
> > On Mon, Jun 08, 2015 at 06:21:20PM +0300, Vladimir Sementsov-Ogievskiy 
> > wrote:
> > 
> > I noticed a corner case, it's probably not a problem in practice:
> > 
> > Since the dirty bitmap is stored with the help of a BlockDriverState
> > (and its bs->file), it's possible that writing the bitmap will cause
> > bits in the bitmap to be dirtied!
> > 
> But since it's metadata and not stored within a disk sector, can this
> actually happen? Do you have an example of a scenario where this might
> come up?

The persistent dirty bitmap for bs->file is storeed in the qcow2 BDS.
This results in recursion.

This is a misconfiguration but I just want to understand what happens
when someone does this by mistake.


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