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

[Kevin Wolf]

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.

First of all, thanks a lot for doing all the testing!

> 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.

Okay. I had hoped that Win 7 would be fine as well and we might just get
away with applying the patch, but this we can't possibly ignore.

> 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.

That they fill in different creator strings is good news. This isn't a
nice solution, but then it wasn't nice to change the semantics between
the two versions, so it's better than nothing. I'd vote for comparing
the creator field.

One open question is what to do with images that are neither "vpc " (or
"qemu") nor "win ". I think the newer behaviour is saner, but I don't
know if there are other tools for which the old semantics should be used.

The other question is what to do with newly created images. Maybe we
should add a creation option. At the same time we should probably switch
from the "qemu" creator string to something different if Hyper-V
semantics is used, in order to allow distinguishing the version. I'm
also not sure what the right default would be for now.

Kevin