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

[Arthur Gautier]

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 <address@hidden> 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.

WDYT?

-- 
\o/ Arthur
 G  Gandi.net