[Lxc-users] Dependencies Use Cases

[Serge Hallyn]

Quoting Alex Eagar (alexeagar at gmail.com):
> Can LXC use cgroups without libcgroup? For that matter, just to be
> clear, can LXC use cgroups without cgroup-bin?

LXC doesn't need anything from cgroup-bin, and, if it did, cgroup-bin
could not deliver.  (see below)

> In what use case would
> using LXC without cgroups make sense? Aren't cgroups fundamental to

I think you misunderstand cgroup-bin.  The point of cgroup-bin is to
try and catch applications/daemons as they start and classify them
into cgroups according to a configuration.  However because tasks are
classified by placing their pids one at a time into a file, there are
cases where it misses tasks, and it's not entirely reliable.

LXC controls cgroups (the kernel feature) itself through the cgroup
filesystem.  cgroup-bin is not needed for this.  The cgroups just need
to be composed in a (set of) cgroup mount(s) somewhere.

> happening, but based on my presumptions, which presumptions I am
> actively asking you to correct, that is what appears to have occurred
> in Ubuntu.

Hopefully the above explained why that's not what happened.

> serge, as a fellow member of the Ubuntu community, please do not refer
> to others' efforts as 'papering over' even if it perhaps is in

I wrote lxcguest.  'Papering over' is not meant as a put-down.  The
point is that there are things in a stock Ubuntu install which stop
a container from booting.  For each of those, the right thing to do
is to update the packages involved so that they can work just as well
in a container as on hardware/kvm.  But for a first step, I chose
to create a package to hide the problems.  In part, that gave us a
better chance to figure out what the real problems were.  Currently
there are (if I'm thinking right), at core, two:  1. the need for the
lxc-monitor to watch /run/utmp in the container to detect reboot/shutdown.
That means the guest can't mount tmpfs on /run, which suddenly creates
a whole set of issues.  Daniel is hoping to resend a kernel patchset
this week or next which well let us not do that.  2. mountall needs
to not mount certain things in a container at boot.  Here is where
I almost literally paper over :)  : lxcguest just bind-mounts a different
file over /lib/init/fstab to make mountall do what we want.  This
can break upgrades, when they want to overwrite /lib/init/fstab.  So
I intend to fix mountall so we don't need that.

-serge