Re: [Qemu-devel] [PATCH 3/5] vpc: Ignore geometry for large images

[Max Reitz <mreitz@redhat.com>]

On 2015-02-24 at 01:45, Peter Lieven wrote:
> Am 23.02.2015 um 19:34 schrieb Max Reitz:
>> On 2015-02-23 at 09:27, Peter Lieven wrote:
>>> From: Kevin Wolf <kwolf@redhat.com>
>>>
>>> The CHS calculation as done per the VHD spec imposes a maximum image
>>> size of ~127 GB. Real VHD images exist that are larger than that.
>>>
>>> Apparently there are two separate non-standard ways to achieve this:
>>> You could use more heads than the spec does - this is the option that
>>> qemu-img create chooses.
>>>
>>> However, other images exist where the geometry is set to the maximum
>>> (65536/16/255), but the actual image size is larger. Until now, such

Notice just now: This should be 65535/16/255.

>>> images are truncated at 127 GB when opening them with qemu.
>>>
>>> This patch changes the vpc driver to ignore geometry in this case and
>>> only trust the size field in the header.
>>>
>>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>>> ---
>>>   block/vpc.c |   10 ++++------
>>>   1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> I'm trusting you this works for disk2vhd; at least it's in accordance 
>> with the VHD specification.
>
> When I wrote the hack for disk2vhd some time ago I just found that the 
> reported size was too big.
> I was not aware that it was exactly this special value.
>
> You say in the specs. Do you have a spec that is actually stating that 
> (65536/16/255) is special
> value that says ignore CHS and look at the footer?

No, but I do have a spec that says "if the number of sectors is greater 
than 65535 * 16 * 255, clamp it to 65535 * 16 * 255 for the geometry 
calculation". This implies to me that this value means that one cannot 
trust the geometry.

Max