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Re: [Qemu-devel] vpc size reporting problem


From: Peter Lieven
Subject: Re: [Qemu-devel] vpc size reporting problem
Date: Tue, 07 Jul 2015 08:03:42 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

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 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




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