Docker in LXC

A hack or a haven?

Docker is a great containerization technology for running applications. It keeps multiple applications completely isolated from each other, only allowing connections exactly when you tell them to. But what if you’re on a hypervisor? You want your host OS to be as lean as possible (else it defeats the point), but you don’t want the overhead and additional complexities which come from full on VMs?

LXC is the technology for you!

LXC (Linux Containers) is a technology which sits somewhere in between VMs and docker containers. It’s still a container technology, so shared kernel and all that, but it’s designed to run a full persistent OS rather than a throw-away environment for an application. LXC is a great technology for a lot of things, but the differences between it, docker and VMs is a topic for another day.

By running docker inside LXC, you get all the gains of running docker in its own isolated environment away from the host, but without the complexities and overhead that would come with running docker in a full VM.

In a similar vein to this, DinD (docker in docker) exists, which lets you run docker from inside docker, so long as the outer container is configured correctly. Although it exists, DinD is definitely not something you’d want to depend on in production. Docker in LXC can have the same issues, but because the technologies are different, and the isolation is different, it actually works far better. In contrast, trying to run LXC inside docker may just open a pathway to hell.


For once, I’m writing a post where the “how-to” section is surprisingly simple:

  1. Create an LXC container
    1. Ensure “nesting” is enabled (it is by default in Proxmox)
    2. If using unprivileged, ensure the “keyctl” option is also enabled
  2. Install Docker (other OCI technologies also available) in the LXC container however you wish
  3. Profit?

Yes, it’s really that simple! Now, running docker run hello-world should start a container and work absolutely flawlessly! At least excluding some minor storage issues.

For reasons I don’t quite understand, I wasn’t able to get Alpine linux working as the base OS for the LXC container at the time. I really like Alpine as a minimal base OS, and given everything would be running in docker, the musl complexities didn’t bother me. Instead, I just run Debian, and it worked absolutely perfectly first time. Arch also works perfectly for me, as too I suspect would almost any other distro. Revisiting it whilst writing this post, and it seems to work fine. In cases like this, there is no “best distro”, the distro doesn’t matter - what does is that you’re comfortable maintaining it for its lifespan.

#But why?

As mentioned before, one of the great things about LXC is that it’s not a VM. Not being a VM gives a number of major advantages due to the fact it’s a shared kernel.

I personally like to keep as much storage as possible outside the relevant VMs and LXC containers, as it makes backup much easier, and means the machines can be nuked at will should something go wrong (remember cattle, not pets). With a VM, this would require something like NFS or CIFS to mount the storage in, which adds additional network overhead, not to mention not being suitable for some applications (most databases for example really won’t like it). With LXC, you can use good ol’ bind mounts to just mount in directories or files as you need, with no overhead, meaning it works absolutely fine for databases. If you want the user ids to match up inside and outside the LXC you’ll need to enable “privileged” mode, but after that it truly does “just work”.

Speaking of passing through mountpoint, passing through GPUs is also much simpler. Because it’s a shared kernel, and LXC is just isolating processes, the GPU doesn’t know it’s being controlled by a separate LXC, nor does it care. Meaning you can split up a GPU between multiple LXCs with no issues at all, all whilst allowing the host to keep using it as a graphics output. Passing a GPU through to a VM is a well known process by now, but the host loses access to it. And splitting a single GPU up between multiple VMs is a very complicated process.

And of course, that’s to say nothing of the reduced overhead. Virtualizing a whole OS adds a lot of extra layers between the containers running your application and the CPU executing their instructions. These layers exist for good reason, they add isolation and allow the VM to be completely different to the host OS. But for a docker container, it’s probably going to be a Linux-based guest OS anyway, so why not cut out the abstraction layers and benefit from the extra performance.

I’ve been running Docker inside LXC for a little under a year now, and it’s been absolutely fantastic. I have a few separate LXCs running separate Docker instances for things like monitoring (to keep it separate), but my primary “throw-all” LXC has a number of different docker containers with a number of mount points. I wrote up a post a few months ago about how exactly my Proxmox server is setup, which may inspire your next server project.

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