On Tue, Feb 10, 2015 at 02:34:14PM +0100, Kevin Wolf wrote:
Am 10.02.2015 um 12:41 hat Peter Lieven geschrieben:
Am 09.02.2015 um 17:09 schrieb Kevin Wolf:
The CHS calculation as done per the VHD spec imposes a maximum
image size of ~127 GB. Real VHD images exist that are larger than
that.
Apparently there are two separate non-standard ways to achieve
this: You could use more heads than the spec does - this is the
option that qemu-img create chooses.
However, other images exist where the geometry is set to the
maximum (65536/16/255), but the actual image size is larger.
Until now, such images are truncated at 127 GB when opening them
with qemu.
This patch changes the vpc driver to ignore geometry in this case
and only trust the size field in the header.
Signed-off-by: Kevin Wolf <address@hidden> ---
Peter, I'm replacing some of your code in the hope that the new
approach is more generally valid. Of course, I haven't tested if
your case with disk2vhd is still covered. Could you check this,
please?
I checked this and found that disk2vhd always sets CHS to 65535ULL
* 16 * 255 independed of the real size.
But, as the conversion to CHS may have an error its maybe the best
solution to ignore CHS completely and always derive total_sectors
from footer->size unconditionally.
I had a look at what virtualbox does and they only rely on
footer->size. If they alter the size or create an image the write
the new size into the footer and recalculate CHS by the formula
found in the appendix of the original spec.
Check vhdCreateImage, vhdOpen in
http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Storage/VHD.cpp
The original spec also says that CHS values purpose is the use in
an ATA controller only.
The problem with just using footer->size back then when I
implemented this was that from the perspective of a VirtualPC guest
run in qemu, the size of its hard disk would change, which you don't
want either. Going from VPC to qemu would be ugly, but mostly
harmless as the disk only grows. But if you use an image in qemu
where the disk looks larger and then go back to VPC which respects
geometry, your data may be truncated.
I believe the vpc "creator" field is different if the image was
created by Virtual PC, versus created by Hyper-V ("vpc" and "win",
respectively, I think). Perhaps we could use that to infer a guest
image came from VirtualPC, and thus not use footer->size in that
scenario?