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

[Peter Lieven <pl@kamp.de>]

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
>> 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?

Peter