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

From: Nir Soffer
Subject: Re: [Qemu-discuss] [Qemu-devel] Estimation of qcow2 image size converted from raw image
Date: Wed, 15 Feb 2017 17:49:58 +0200

On Wed, Feb 15, 2017 at 5:14 PM, Stefan Hajnoczi <address@hidden> 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.


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