Please provide feedback about a proposal to add 'tar' as a new Glance 
disk_format.[0]

The Ironic team is adding support for "OS tarball images" in Mitaka.  This is a 
compressed tar archive of a / (root filesystem). These tarballs are created by 
first installing the OS packages in a chroot and then compressing the chroot as 
tar.*.  The proposal is to store such images as disk_format == tar and 
container_format == bare.

Intuitively, 'tar' seems more like a container_format.  The Glance developer 
documentation, however, says that "The container format refers to whether the 
virtual machine image is in a file format that also contains metadata about the 
actual virtual machine."[1]  Under this proposal, there is no such metadata 
included.

The Glance docs say this about disk_format: "The disk format of a virtual 
machine image is the format of the underlying disk image. Virtual appliance 
vendors have different formats for laying out the information contained in a 
virtual machine disk image."[1]  Under this definition, 'tar' as used in this 
proposal [0] does in fact seem to be a disk_format.

There is not currently a 'tar' container format defined for Glance.  The 
closest we have now is 'ova' (an OVA tar archive file) and 'docker' (a Docker 
tar archive of the container filesystem).  And, in fact, 'tar' as a container 
format wouldn't be very helpful, as it doesn't indicate where in the tarball 
the metadata should be found.

The goal here is to come up with an identifier for an "OS tarball image" that's 
acceptable across projects and isn't confusing for people who are creating 
images.

Thanks in advance for your feedback,
brian

[0] https://bugs.launchpad.net/glance/+bug/1535900
[1] https://github.com/openstack/glance/blob/master/doc/source/formats.rst

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