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

From: Stefan Hajnoczi
Subject: Re: [Qemu-discuss] [Qemu-devel] Estimation of qcow2 image size converted from raw image
Date: Wed, 15 Feb 2017 15:14:19 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

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

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.


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