On Wed, Feb 15, 2017 at 5:14 PM, Stefan Hajnoczi <stefa...@gmail.com> 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
This is little bit verbose compared to other commands
(e.g. info, check, convert)
Since this is needed only during convert, maybe this can be
a convert flag?
qemu-img convert -f xxx -O yyy src dst --estimate-size --output json
{
"estimated size": 1234678000
}
> As John explained, it is only an estimate. But it will be a
> conservative maximum.
>
> Internally BlockDriver needs a new interface:
>
> struct BlockDriver {
> /*
> * Return a conservative estimate of the maximum host file size
> * required by a new image given an existing BlockDriverState (not
> * necessarily opened with this BlockDriver).
> */
> uint64_t (*bdrv_query_max_size)(BlockDriverState *other_bs,
> Error **errp);
> };
>
> This interface allows individual block drivers to probe other_bs in
> whatever way necessary (e.g. querying block allocation status).
>
> Since this is a conservative max estimate there's no need to read all
> data to check for zero regions. We should give the best estimate that
> can be generated quickly.
I think we need to check allocation (e.g. with SEEK_DATA), I hope this
is what you mean by not read all data.
Nir
