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Re: [Qemu-discuss] Converting qcow2 image on the fly to raw format

From: Kevin Wolf
Subject: Re: [Qemu-discuss] Converting qcow2 image on the fly to raw format
Date: Mon, 9 Jul 2018 18:56:07 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 09.07.2018 um 18:38 hat Nir Soffer geschrieben:
> We are discussing importing VM images to KubVirt. The goal is to be
> able to import an existing qcow2 disk, probably some appliance stored
> on http server, and and convert it to raw format for writing to storage.
> This can be also useful for for oVirt for importing OVA, since we like to
> pack
> disks in qcow2 format inside OVA, but the user may like to use raw disks, or
> for uploading existing disks.
> Of course converting the image using qemu-img is easy, but requires
> downloading the image to temporary disk. We would like to avoid temporary
> disks, or telling users to convert the image.
> Base on the discussion we had here:
> https://lists.ovirt.org/archives/list/address@hidden/thread/GNAVJ253FP65QUSOONES5XZGRIDX5ABC/#YMLSEGU7PN3MX5MUORGEGGAQLLSL4KKJ
> I think this is impossible since qcow2 is not built for streaming. But both
> Richard and Eric suggested some solutions.
> The flow is:
>     qcow2 image -- http --> importer -> raw file
> Is it possible to implement the importer using qemu-img and qemu-nbd,
> or maybe nbdkit?

This direction is actually easy when you use the curl block driver in
QEMU instead of downloading the image externally:

qemu-img convert -f qcow2 http://.../image.qcow2 local.raw

This works because HTTP allows to make requests for arbitrary parts of
the image, so we actually have random access. We only get problems when
we need to process the image file sequentially.


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