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

[Alexander Graf]

On 01.06.2011, at 11:11, Kevin Wolf wrote:

> Am 01.06.2011 10:49, schrieb 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.
> 
> Hm, wondering if that's something to consider for qcow2v3, too... Do you
> think it's still useful when doing this on a cluster granularity? It
> would only work for well-aligned partitions then, but I think that
> shouldn't be a problem for current OSes.

Well, we could always just hack around for bits where it overlaps. When passing 
in a differently aligned partition for example, we could just declare the odd 
sector as COW sector and copy the contents over :). Though that might not be 
what the user really wants. Hrm.

> Basically, additionally to the three cluster types "read from this
> image", "COW from backing file" and "zero cluster" we could introduce a
> fourth one "read/write to backing file".

Yup, sounds very much straight forward! Then all we need is some tool to create 
such a qcow file :)


Alex