Re: [Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation

[Stefan Weil]

Am 11.02.2013 20:38, schrieb Jeff Cody:
> I've done some testing back and forth between VPC, Win Server 2012
> Hyper-V, and qemu/kvm with and without this patch applied.
>
> The short: this patch will break Virtual PC VHD compatibility (as it
> exists under Win7), but allow compatibility for VHD files created by
> Hyper-V (at least under Server 2012).
>
> VPC and Server 2012 Hyper-V are also incompatible with each other,
> when it comes to VHD files, if that is any consolation.
>
> I used sha1sums between the different combination of VHD files filled to
> capacity with random data under VPC, qemu/kvm (with & without the
> patch), and Hyper-V.  Without this patch we are an exact math with
> VPC, and with this patch we are an exact match with Hyper-V.
>
> However, perhaps we can support both, depending on who created the VHD
> file.  There is a "Creator Application" field in the VHD header - for
> the VPC-created VHD file, that field is ASCII "vpc ".  For the Hyper-V
> created VHD file, that field is ASCII "win ".  If we see the field as
> "vpc ", we could default to using the CHS geometry found in the
> header - otherwise, do as this patch does, and go by the virtual disk
> size.
>
> Jeff

Hi Jeff,

could you please give some more details of your VPC test scenario?

How did you create the VHD disk imagesfor VPC?
Which guest OS did you run?
Which disk size does the guest OS report?
Does VPC behave differently when it gets disk images with
a different signature (for example from Hyper V)?

I'd like to reproduce your test results in my environment.

Does the patch really break Virtual PC VHD compatibility?

Or does VPC simply pass the size calculated from CHSto the
guest - no matter how the disk image was created?In this case
I see no need that QEMU must show the same behaviour.
It's important to implement the VHD specification correctly,
but we should not duplicate bugs of other emulators just
to be compatible with them.

Thanks,

Stefan