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

[Kevin Wolf]

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.

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".

Kevin