Re: [Qemu-devel] [Qemu-trivial] [PATCH] vmdk: Fix cylinders number during convert

[Arthur Gautier <baloo@gandi.net>]

On Thu, Oct 30, 2014 at 06:27:20PM +0800, Fam Zheng wrote:
> On Thu, 10/30 10:09, Arthur Gautier wrote:
> > On Wed, Oct 29, 2014 at 09:28:52AM +0800, Fam Zheng wrote:
> > > On Tue, 10/28 16:00, Stefan Hajnoczi wrote:
> > > > On Thu, Oct 23, 2014 at 10:03:25AM +0200, Markus Armbruster wrote:
> > > > > Michael Tokarev <mjt@tls.msk.ru> writes:
> > > > > 
> > > > > > On 10/22/2014 05:25 PM, Arthur Gautier wrote:
> > > > > >> We can not rely on int cast to get a correct number of cylinders. The
> > > > > >> cylinders information was wrong in 49.9999% of cases.
> > > > > >> 
> > > > > >> This ensures the cylinders always gets the ceiling value.
> > > > > >
> > > > > > Good thing, especially the good probability :), and also a good patch
> > > > > > which comes with a test.
> > > > > >
> > > > > > But I wonder if anything can break this way?  Migration, windows guest
> > > > > > being unable to find its partitions, something else?
> > > 
> > > I'd like to hear an answer to this question too, so we know why it's right and
> > > worth to have.
> > > 
> > Sincerely, I didn't tried myself.
> > 
> > > > > >
> > > > > > And more.  What-if our drive size in cylinders will be larger than
> > > > > > the size in bytes?  The proposed div_round_up() will increase number
> > > > > > of cylinders, so size in CHS will be larger than size in bytes.  Maybe
> > > > > > there was a reason why originally the size in cylinders was calculated
> > > > > > by truncating extra fractional part?  What-if guest will try to access
> > > > > > the very last CHS which is incomplete?
> > > 
> > > I don't remember a reason why truncating (I doubt there is any), OTOH I'm not
> > > sure what is the right thing to do if the guest tries to write to the last
> > > incomplete CHS either.
> > > 
> > > Fam
> > 
> > Maybe we can expand the disk to the size matching cylinders * sectors *
> > sector_size and issue a warning to the user? This way it will always be
> > safe.
> > 
> 
> Possible, but I don't know if it's worth it. Could you explain what doesn't
> work now, in order to show *why* you need this change?
> 
> Thanks,
> Fam

Please take a look at the test in the patch. I have been able to
generate an image with a number of cylinders of zero which I believe is
wrong.



-- 
\o/ Arthur
 G  Gandi.net