Re: [Qemu-devel] vpc size reporting problem

[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.

>  
> 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 
>  
>