Re: [Qemu-devel] vmdk stream-optimised format

[Paolo Bonzini <pbonzini@redhat.com>]

Il 20/08/2013 09:08, Fam Zheng ha scritto:
> On Tue, 08/20 07:51, Alex Bligh wrote:
>>
>> On 20 Aug 2013, at 02:42, Fam Zheng wrote:
>>
>>> OK, thanks for explaination. That sounds a valid use case for
>>> streamOptimized. However I am afraid QEMU and its users benefit not much
>>> from this feature anyway, because it's moving a VM away to VMware, :)
>>> that might be the reason it's not there yet, and I don't know about any
>>> plan to do it in the near future.
>>
>> Well, given it is an open source project, the more interoperability
>> the better. Even if it just means users need not worry about lock
>> in to faster hypervisors ... Being more serious, qemu-img is
>> part of the project too.
>>
>>> But if someone sends patches for this, I think it is possible to get in.
>>
>> I guessed "send code" might be the answer :-)
>>
>> What I'm not sure of is whether the streaming format has to be written
>> sequentially from as opposed to random writes. I believe the way
>> qemu-img convert works, one can't guarantee the writes are
>> sequential.
>>
> The order of sectors doesn't matter, but granularity should be aligned
> to, as the data is compressed cluster by cluster. And no overwrite, of
> course. The challenge may be that header comes at the end of file
> (well, called footer), which is not decided at create time.

It doesn't look too different from what "qemu-img convert -c" does,
except that you need to use the right "-o" incantation to specify the
format type.

Paolo