Re: [Qemu-devel] vpc size reporting problem

[Chun Yan Liu]

>>> 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? 

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 
>  
>  
>