LXD polish for xenial

[John Meinel]

...


> So the plan as I understand it is that we're planning on updating Bundles
>> to use the term "lxd" as the container they are requesting. And then
>> updating the deployer and other tools to understand that they need to
>> translate that back to LXC for Juju-1.X. The rationale is that we don't
>> want to be stuck using old terminology forever, and the change is easy to
>> do for bundles.
>>
>
> My understanding was different. My understanding was that Juju 2.0 was to
> understand both lxc and lxd so old bundles work just fine with Juju 2.0. If
> you have a bundle with lxd in it, it was clearly written for 2.0 so it's
> fine that it doesn't deploy with Juju 1.x.
>
> Having Juju 2.0 not understand lxc seems silly given that in fact a lxd
> container is just an lxc. We seem to be splitting hairs at the cost of
> users.
>
> -Dean
>

So I'd like to clarify a few points here. The first *very* key point is
that the old "lxc" containers are *not* the same as "lxd" containers. It is
a bit unfortunate, but I'll go through some reasons:

   1. Both of them do use cgroups, etc to create isolation between
   containers, but so does docker, and I don't think people feel docker
   containers are interchangable with lxc or lxd containers.
   2. There is a package called "lxc" that you can install, which provides
   the old "lxc-foo" commands (lxc-start, lxc-stop, lxc-launch, etc)
   3. There is also a package called "lxdclient" which installs a local
   binary named "/usr/bin/lxc". That, however, does *not* interact with the
   former package.
   4. Very concretely, if you do "lxc-launch -t ubuntu-cloud" then that
   container will *not* show up in "lxc list". "lxc" is the lxdclient and
   talks to the lxd daemon to get work done. "lxc-*" commands do all of the
   container creation, etc, themselves.
   5. Going forward I'll call the old thing 'lxc1' because that is the new
   package for it (AIUI). And I'll enumerate some more of the differences
      1. lxc1 containers are priviledged by default and require you to be
      root to create them. lxd containers are unpriviledged by default
and can be
      requested by any user. The daemon itself runs as root to provide the
      functionality, but the container you get will not have a root-elevation
      escape mechanism.
      2. lxc1 containers download from cloud-images to /var/cache/lxc and
      populate /var/lxc/* with the rootfs and where the container files
      themselves are. lxd caches images differently (/var/lib/lxd/images, IIRC)
      and supports the use of things like ZFS filesystem mounts to provide fast
      cloning to launch a new image.


Juju itself *could* continue to support its existing logic to create and
manage 'lxc' containers as a separate bunch of containers from 'lxd'
containers. They would end up on different bridges, have different code
paths for creating them (lxd we talk directly to the HTTP REST api of the
daemon, 'lxc' we have to exec a command and parse the string output.)
We have been directed that we really don't want to be supporting 2 very
similar-but-not-the-same container mechanism for the next 5 years, and
going to 2.0 is the one time we're going to get to break support for the
old mechanism.

John
=:->
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