Re: [Qemu-devel] vpc size reporting problem

[Peter Lieven <pl@kamp.de>]

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).

>
> 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).

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