Re: [Qemu-devel] vpc size reporting problem

[Peter Lieven <pl@kamp.de>]

Am 07.07.2015 um 03:50 schrieb Chun Yan Liu:
>
>>>> On 7/6/2015 at 06:42 PM, in message <559A5B79.4010707@kamp.de>, Peter Lieven
> <pl@kamp.de> wrote:
>> Am 06.07.2015 um 11:44 schrieb Chun Yan Liu:
>>> While testing with a 1GB VHD file created on win7, found that the VHD file
>>> size reported on Windows is different from that is reported by qemu-img
>>> info or within a Linux KVM guest.
>>>
>>> Created a dynamic VHD file on win7, on Windows, it is reported 1024MB
>>> (2097152 sectors). But with qemu-img info or within a Linux KVM guest,
>>> it is reported 1023MB (2096640 sectors).
>>>
>>> The values in the footer_buf are as follows:
>>> creator_app: "win "
>>> cylinders: 0x820 (2080)
>>> heads: 0x10 (16)
>>> cyl/sec: 0x3f (63)
>>> current_size: 0x40000000 (1G)
>>>
>>> So, if using current_size, it's correct; but using CHS will get a smaller
>> size.
>>> Should we add a check in this case and use "current_size" instead of
>>> CHS?
>>   
>> As far as I remember the issue was and still is that there is no official
>> spec that says
>> use current_size in case A and CHS in case B.
> Understand.
>
>>   
>> If currrent_size is greater than CHS and Windows would use CHS (we don't
>> know that) we might run into issues if Qemu uses current_size. In this
>> cas we would write data beyond the end of the container (from Windows
>> perspective).
> That's right. The fact is in our testing we found Windows does not use CHS
> but current_size (from testing result), we create and get the VHD parted on
> Windows, then take the VHD file into Linux KVM guest, it fails to show partition
> table (since the reported disk size is shrinking, some of the partitions extend
> beyond the end of the disk).

Which version of Windows are you referring to?

I personally think that it might be ok to use current_size if its greater
than the size derived from CHS. Our current implementation when
creating an image assumes we have to choose CHS to be equal
or greater to current_size. This can cause the same issue in the
other direction.

Peter