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


From: Fam Zheng
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] vmdk: Fix cylinders number during convert
Date: Thu, 30 Oct 2014 18:27:20 +0800
User-agent: Mutt/1.5.23 (2014-03-12)

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 <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.
> 

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



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