[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:lp-2028213 into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

Michael Hudson-Doyle has proposed merging ~mwhudson/livecd-rootfs/+git/livecd-rootfs:lp-2028213 into livecd-rootfs:ubuntu/master.

Commit message:
lb_chroot_layered: use rsync to make more minimal overlay layers. (LP: #2028213)



Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)
Related bugs:
  Bug #2028213 in livecd-rootfs (Ubuntu): "ISOs with the new installer are 34% bigger than legacy ISOs"
  https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/2028213

For more details, see:
https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/447749

I have validated this approach by testing a previous version of this by running two livefs builds, one with the change and one without:

with: https://launchpad.net/~mwhudson/+livefs/ubuntu/mantic/test/+build/472518
without: https://launchpad.net/~mwhudson/+livefs/ubuntu/mantic/test/+build/472519

I mounted the stack of squashfses that make up the live session from both builds and ran

$ sudo diff --recursive --no-dereference --brief vanilla/live changed/live

To compare them. The only differences were in files like var/lib/snapd/state.json and var/log/apt/history.log and var/cache/ldconfig/aux-cache (full output here: https://paste.ubuntu.com/p/cnZ32jnPGz/) so I am happy with that. And it certainly helps!

$  sudo du --one-file-system --human-readable --summarize --apparent-size changed/ vanilla/
3.0G	changed/
4.2G	vanilla/

-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~mwhudson/livecd-rootfs/+git/livecd-rootfs:lp-2028213 into livecd-rootfs:ubuntu/master.
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