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

[Kevin Wolf <kwolf@redhat.com>]

Am 28.05.2018 um 23:25 hat Richard W.M. Jones geschrieben:
> 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.

All of this means that OVA isn't really well suited to be used as a
native format for VM configuration + images. It's just for sharing
read-only images that are converted into another native format before
they are used.

Which is probably fair for the use case it was made for, but means that
we need something else to solve our problem.

Kevin