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

From: Daniel P. Berrange
Subject: Re: [Qemu-discuss] [Qemu-devel] Estimation of qcow2 image size converted from raw image
Date: Wed, 15 Feb 2017 15:20:13 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

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

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