Re: [Qemu-devel] vpc size reporting problem

[Chun Yan Liu]

>>> On 7/7/2015 at 02:19 PM, in message <559B6F79.237 : 102 : 21807>, Chun Yan 
>>> Liu
wrote: 
> 
>  
>>>> On 7/7/2015 at 02:03 PM, in message <address@hidden>, Peter Lieven 
> <address@hidden> wrote:  
> > Am 07.07.2015 um 07:59 schrieb Chun Yan Liu:  
>> >  
>> >>>> On 7/7/2015 at 01:50 PM, in message <address@hidden>, Peter Lieven  
>> > <address@hidden> wrote:  
>> >> Am 07.07.2015 um 03:50 schrieb Chun Yan Liu:  
>> >>>>>> On 7/6/2015 at 06:42 PM, in message <address@hidden>, Peter Lieven  
>> >>> <address@hidden> 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?  

And imported to a Win7 guest on KVM as IDE device, it's also reported as
1024MB (not CHS value, CHS is 1023MB).

>  
> I'm not sure. See from "device management" -> "disk drive", it's named as  
> "Msft 
> virtual disk SCSI disk device", and from "storage controller", it has a  
> separate 
> controller named as "Microsoft VHD HBA". Anyway, seems not controlled by 
> ATA/ATAPI. 
>  
>>   
>> 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  
>>   
>>   
>>   
>