Re: [Qemu-block] [Qemu-devel] storing machine data in qcow images?

[Richard W.M. Jones]

On Mon, May 28, 2018 at 10:20:54PM +0100, Richard W.M. Jones wrote:
> On Mon, May 28, 2018 at 08:38:33PM +0200, Kevin Wolf wrote:
> > Just accessing the image file within a tar archive is possible and we
> > could write a block driver for that (I actually think we should do
> > this), but it restricts you because certain operations like resizing
> > aren't really possible in tar. Unfortunately, resizing is a really
> > common operation for non-raw image formats.
> 
> We do this already in virt-v2v (using file.offset and file.size
> parameters in the raw driver).
> 
> For virt-v2v we only need to read the source so resizing isn't an
> issue.  For most of the cases we're talking about the downloaded image
> would also be a template / base image, so I suppose only reading would
> be required too.
> 
> I also wrote an nbdkit tar file driver (supports writes, but not
> resizing).
> https://manpages.debian.org/testing/nbdkit-plugin-perl/nbdkit-tar-plugin.1.en.html

I should add the other thorny issue with OVA files is that the
metadata contains a checksum (SHA1 or SHA256) of the disk images.  If
you modify the disk images in-place in the tar file then you need to
recalculate those.

Rich.

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