Re: [Qemu-devel] VMDK development plan for Summer of Code 2011

[Alexander Graf]

On 01.06.2011, at 06:29, Stefan Hajnoczi wrote:

> On Sun, May 29, 2011 at 2:19 PM, Fam Zheng <address@hidden> wrote:
>> As a project of Google Summer of Code 2011, I'm now working on
>> improving VMDK image support. There are many subformats of VMDK
>> virtual disk, some of which have separate descriptor file and others
>> don't, some allocate space at once and some others grow dynamically,
>> some have optional data compression. The current support of VMDK
>> format is very limited, i.e. qemu now supports single file images, but
>> couldn't recognize the widely used multi-file types. We have planned
>> to add such support to VMDK block driver and enable more image types,
>> and the working timeline is set in weeks (#1 to #7) as:
>> 
>> [#1] Monolithic flat layout support
>> [#2] Implement compression and Stream-Optimized Compressed Sparse
>> Extents support.
>> [#3] Improve ESX Server Sparse Extents support.
>> [#4] Debug and test. Collect virtual disks with various versions and
>> options, test qemu-img with them. By now some patches may be ready to
>> deliver.
>> [#5, 6] Add multi-file support (2GB extent formats)
>> [#7] Clean up and midterm evaluation.
> 
> Thanks to Fam's work, we'll hopefully support the latest real-world
> VMDK files in qemu-img convert within the next few months.
> 
> If anyone has had particular VMDK "problem files" which qemu-img
> cannot handle, please reply, they would make interesting test cases.

There is one very useful use-case of VMDK files that we currently don't 
support: remapping.

A vmdk file can specify that it really is backed by a raw block device, but 
only for certain chunks, while other chunks of it can be mapped read-only or 
zero. That is very useful when passing in a host disk to the guest and you want 
to be sure that you don't break other partitions than the one you're playing 
with.

It can also shadow map those chunks. For example on the case above, the MBR is 
COW (IIRC) for the image, so you can install a bootloader in there.


Alex