[Merge] livecd-rootfs:default-compat into livecd-rootfs:ubuntu/master

[Brian Murray]

Brian Murray has proposed merging livecd-rootfs:default-compat into livecd-rootfs:ubuntu/master.

Requested reviews:
  John Chittum (jchittum)
  Christian Ehrhardt  (paelzer)
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/427296

The other day I noticed that autopkgtest images are using qemu compat format 0.10 and took some time to dig into it and discovered that this is a very old format and should be updated.

 $ qemu-img info https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img
image: https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img
file format: qcow2
virtual size: 2.2 GiB (2361393152 bytes)
disk size: unavailable
cluster_size: 65536
Format specific information:
    compat: 0.10
    compression type: zlib
    refcount bits: 16

Looking at livecd-rootfs the code to create the qcow2 images was created (in 2015) and sets the compat format to 0.10. However, there is a compat from of 1.1 which is the current default and provides numerous advantages including "zero clusters, which allow efficient copy-on-read for sparse images". Additional information about the benefits and changes can be found at https://wiki.qemu.org/Features/Qcow3.

Every supported release of Ubuntu has a version of qemu greater than 1.1 so this should be entirely safe.
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of livecd-rootfs:default-compat into livecd-rootfs:ubuntu/master.
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