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Re: [Qemu-devel] vpc size reporting problem

From: Chun Yan Liu
Subject: Re: [Qemu-devel] vpc size reporting problem
Date: Mon, 06 Jul 2015 19:50:13 -0600

>>> 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. 


> 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).

> > 
> > BTW, before commit 0444dceee, there is a similar check for 'd2v', 
> > if creator_app is 'd2v', using "size" instead of CHS. But in commit 
> > 0444dceee, this check is removed. To me, the new check and 'd2v' 
> > check seem to be two different cases, why removing 'd2v' check? 
> d2v always writes a magic combination of 65535x16x255 for CHS. So commit 
> 0444dceee just changed the behaviour to always use current_size in 
> case CHS is 65535x16x255 (including d2v). 

Got it. Thanks!

- Chunyan

> I personally wouldn't mind to always use current_size that is what e.g.  
> VirtualBox does. 
> Or use current_size if it is greater than the size derived from CHS. But  
> this might 
> break things. 
> Peter 

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