Re: [Qemu-devel] Estimation of qcow2 image size converted from raw image

[Nir Soffer <nsoffer@redhat.com>]

On Wed, Feb 15, 2017 at 5:20 PM, Daniel P. Berrange <berrange@redhat.com> wrote:
> On Wed, Feb 15, 2017 at 03:14:19PM +0000, Stefan Hajnoczi wrote:
>> On Mon, Feb 13, 2017 at 05:46:19PM +0200, Maor Lipchuk wrote:
>> > I was wondering if that is possible to provide a new API that
>> > estimates the size of
>> > qcow2 image converted from a raw image. We could use this new API to
>> > allocate the
>> > size more precisely before the convert operation.
>> >
>> [...]
>> > We think that the best way to solve this issue is to return this info
>> > from qemu-img, maybe as a flag to qemu-img convert that will
>> > calculate the size of the converted image without doing any writes.
>>
>> Sounds reasonable.  qcow2 actually already does some of this calculation
>> internally for image preallocation in qcow2_create2().
>>
>> Let's try this syntax:
>>
>>   $ qemu-img query-max-size -f raw -O qcow2 input.raw
>>   1234678000
>>
>> As John explained, it is only an estimate.  But it will be a
>> conservative maximum.
>
> This forces you to have an input file. It would be nice to be able to
> get the same information by merely giving the desired capacity e.g
>
>   $ qemu-img query-max-size -O qcow2 20G

Without a file, this will have to assume that all clusters will be allocated.

Do you have a use case for not using existing file?

For ovirt we need this when converting a file from one storage to another,
the capabilities of the storage matter in both cases.

(Adding all)

Nir