Re: [PATCH 0/4] (Refactored) VHD Support Addition

[Steve Ofsthun <sofsthun@virtualiron.com>]

Anthony Liguori wrote:
> Steve Ofsthun wrote:
>> Daniel P. Berrange wrote:
>>> On Thu, Jun 21, 2007 at 01:27:32PM -0400, Ben Guthro wrote:
>>
>>>> ### vdisk_tool ###
>>>> The vdisk_tool provides a means of creating, and converting vhd files.
>>>> Like libvdisk, it is designed with future support of additional formats
>>>> in mind, but currently only supports vhd.
>>> Why not just add vhd support to qemu's existing qemu-img tool, then
>>> it will
>>> support all image formats, right from the start, rather than
>>> inventing yet
>>> another tool :-(
>>
>> Well, when we started this, it wasn't clear we were going to be able to
>> publicly release any code in support of the VHD format.  Microsoft later
>> relaxed the restrictions, and by that time the separate tool already
>> existed.  We can investigate integrating the work into qemu-img.
> 
> QEMU already supports VHD (it calls it VPC) and supported it long before
> Microsoft released any specs.  QEMU supported VMDK before VMware release
> the specs for that too.

This code was developed in the Xen 3.0.2 days when the integrated qemu had
no vpc support.  Our original code was targeted at PV on HVM drivers using
blocktap.  We needed to support the same format unaccelerated and so the
qemu integration was also done.

> Is there a particular feature of VHD/VPC that your code adds that QEMU
> didn't already support?

Not having any experience with VPC, I couldn't tell you.  I can say that we
tried to adhere to the Microsoft spec rather than reverse engineering the
format from examples.  Perusing the VPC code, there are many references to
unknown fields within the format.

Is there a complete and concise summary of the VPC feature coverage?

Thanks,
Steve
-- 
Steve Ofsthun - Virtual Iron Software, Inc.