Re: [Qemu-devel] vpc size reporting problem

[Peter Lieven <pl@kamp.de>]

Am 07.07.2015 um 07:59 schrieb Chun Yan Liu:
>
>>>> On 7/7/2015 at 01:50 PM, in message <559B68B2.5060402@kamp.de>, Peter Lieven
> <pl@kamp.de> wrote:
>> 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?
> Tested with WS2012R2 and Win7.

Which storage driver?

I had a look at the specs and in fact they more or less say: Use current_size
and if you have an ATA controller derive the disk size from CHS.

Peter