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Re: Exporting qcow2 images as raw data from ova file with qemu-nbd

From: Richard W.M. Jones
Subject: Re: Exporting qcow2 images as raw data from ova file with qemu-nbd
Date: Mon, 29 Jun 2020 11:10:53 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Jun 26, 2020 at 09:47:24PM +0300, Nir Soffer wrote:
> Whats's missing in the current solution is supporting compressed raw
> disk. I don't think oVirt creates such ova files, so this should not
> be an issue. However if nbdkit tar plugin can support this (current
> implementation does not), this will be a good reason to integrate
> with it.

As far as I know the OVA format requires that the tar file is
uncompressed.  Also the disk images inside the tar file are supposed
to be uncompressed (except maybe for qcow2 compression, but that's

> I wonder if it is possible to  add gzip driver in qemu block layer?

Kevin already answered this, and as he said no it's not possible.
There's already a gzip plugin in nbdkit but it has to uncompress the
whole file so it's not very practical except for small images.

XZ (lzma) is much better here -- nbdkit has an xz filter already --
but you have to prepare the XZ file with special flags.  Eventually
zstd may support a seekable format

I'm looking into making it easier to put nbdkit in front of qemu-nbd.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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