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

[Stefan Hajnoczi]

On Tue, Jun 23, 2015 at 01:57:55PM -0400, John Snow wrote:
> On 06/15/2015 10:42 AM, Stefan Hajnoczi wrote:
> > 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.
> > 
> > Stefan
> > 
> 
> I still don't follow you, actually.
> 
> The dirty bitmap only tracks changed virtual disk sectors, not actual
> file sectors, right? Writing a bitmap that describes foo.qcow2 to
> foo.qcow2 won't dirty bitmaps, it's an out-of-channel write as far as
> the bitmap is concerned.
> 
> Right? Am I fatally misunderstanding the situation?

There is no out-of-channel for bs->file.  The bs->file raw-posix image
file includes the bs qcow2 sectors that hold the dirty bitmap.

This is a corner case and I don't think any valid configuration would
hit it.

Stefan

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