secluded/content/posts/lxd-containers-for-human-beings.md

---
title: "LXD: Containers for Human Beings"
subtitle: "Docker's great and all, but I prefer the workflow of interacting with VMs"
date: 2023-09-19T14:26:00-04:00
categories:
  - Technology
tags:
  - Sysadmin
  - Containers
  - VMs
  - Docker
  - LXD
draft: false
toc: true
rss_only: false
cover: ./cover.png
---

This is a blog post version of a talk I presented at both Ubuntu Summit 2022 and
SouthEast LinuxFest 2023. The first was not recorded, but the second was and is
on [SELF's PeerTube instance.][selfpeertube] I apologise for the terrible audio,
but there's unfortunately nothing I can do about that. If you're already
intimately familiar with the core concepts of VMs or containers, I would suggest
skipping those respective sections. If you're vaguely familiar with either, I
would recommend reading them because I do go a little bit in-depth.

[selfpeertube]: https://peertube.linuxrocks.online/w/hjiTPHVwGz4hy9n3cUL1mq?start=1m

{{< adm type="warn" >}}

**Note:** Canonical has decided to [pull LXD out][lxd] from under the Linux
Containers entity and instead continue development under the Canonical brand.
The majority of the LXD creators and developers have congregated around a fork
called [Incus.][inc] I'll be keeping a close eye on the project and intend to
migrate as soon as there's an installable release.

[lxd]: https://linuxcontainers.org/lxd/
[inc]: https://linuxcontainers.org/incus/

{{< /adm >}}

Questions, comments, and corrections are welcome! Feel free to use the
self-hosted comment system at the bottom, send me an email, an IM, reply to the
fediverse post, etc. Edits and corrections, if there are any, will be noted just
below this paragraph.

## The benefits of VMs and containers

- **Isolation:** you don't want to allow an attacker to infiltrate your email
  server through your web application; the two should be completely separate
  from each other and VMs/containers provide strong isolation guarantees.
- **Flexibility:** <abbr title="Virtual Machines">VMs</abbr> and containers only
  use the resources they've been given. If you tell the VM it has 200 MBs of
  RAM, it's going to make do with 200 MBs of RAM and the kernel's <abbr
  title="Out Of Memory">OOM</abbr> killer is going to have a fun time 🤠
- **Portability:** once set up and configured, VMs and containers can mostly be
  treated as closed boxes; as long as the surrounding environment of the new
  host is similar to the previous in terms of communication (proxies, web
  servers, etc.), they can just be picked up and dropped between various hosts
  as necessary.
- **Density:** applications are usually much lighter than the systems they're
  running on, so it makes sense to run many applications on one system. VMs and
  containers facilitate that without sacrificing security.
- **Cleanliness:** VMs and containers are applications in black boxes. When
  you're done with the box, you can just throw it away and most everything
  related to the application is gone.

## Virtual machines

As the name suggests, Virtual Machines are all virtual; a hypervisor creates
virtual disks for storage, virtual <abbr title="Central Processing
Units">CPUs</abbr>, virtual <abbr title="Network Interface Cards">NICs</abbr>,
virtual <abbr title="Random Access Memory">RAM</abbr>, etc. On top of the
virtualised hardware, you have your kernel. This is what facilitates
communication between the operating system and the (virtual) hardware. Above
that is the operating system and all your applications.

At this point, the stack is quite large; VMs aren't exactly lightweight, and
this impacts how densely you can pack the host.

I mentioned a "hypervisor" a minute ago. I've explained what hypervisors in
general do, but there are actually two different kinds of hypervisor. They're
creatively named **Type 1** and **Type 2**.

### Type 1 hypervisors

These run directly in the host kernel without an intermediary OS. A good example
would be [KVM,][kvm] a **VM** hypervisor than runs in the **K**ernel. Type 1
hypervisors can communicate directly with the host's hardware to allocate RAM,
issue instructions to the CPU, etc.

[debian]: https://debian.org
[kvm]: https://www.linux-kvm.org
[vb]: https://www.virtualbox.org/

```kroki {type=d2,d2theme=flagship-terrastruct,d2sketch=true}
hk: Host kernel
hk.h: Type 1 hypervisor
hk.h.k1: Guest kernel
hk.h.k2: Guest kernel
hk.h.k3: Guest kernel
hk.h.k1.os1: Guest OS
hk.h.k2.os2: Guest OS
hk.h.k3.os3: Guest OS
hk.h.k1.os1.app1: Many apps
hk.h.k2.os2.app2: Many apps
hk.h.k3.os3.app3: Many apps
```

### Type 2 hypervisors

These run in userspace as an application, like [VirtualBox.][vb] Type 2
hypervisors have to first go through the operating system, adding an additional
layer to the stack.

```kroki {type=d2,d2theme=flagship-terrastruct,d2sketch=true}
hk: Host kernel
hk.os: Host OS
hk.os.h: Type 2 hypervisor
hk.os.h.k1: Guest kernel
hk.os.h.k2: Guest kernel
hk.os.h.k3: Guest kernel
hk.os.h.k1.os1: Guest OS
hk.os.h.k2.os2: Guest OS
hk.os.h.k3.os3: Guest OS
hk.os.h.k1.os1.app1: Many apps
hk.os.h.k2.os2.app2: Many apps
hk.os.h.k3.os3.app3: Many apps
```

## Containers

VMs use virtualisation to achieve isolation. Containers use **namespaces** and
**cgroups**, technologies pioneered in the Linux kernel. By now, though, there
are [equivalents for Windows] and possibly other platforms.

[equivalents for Windows]: https://learn.microsoft.com/en-us/virtualization/community/team-blog/2017/20170127-introducing-the-host-compute-service-hcs

**[Linux namespaces]** partition kernel resources like process IDs, hostnames,
user IDs, directory hierarchies, network access, etc. This prevents one
collection of processes from seeing or gaining access to data regarding another
collection of processes.

**[Cgroups]** limit, track, and isolate the hardware resource use of a
collection of processes. If you tell a cgroup that it's only allowed to spawn
500 child processes and someone executes a fork bomb, the fork bomb will expand
until it hits that limit. The kernel will prevent it from spawning further
children and you'll have to resolve the issue the same way you would with VMs:
delete and re-create it, restore from a good backup, etc. You can also limit CPU
use, the number of CPU cores it can access, RAM, disk use, and so on.

[Linux namespaces]: https://en.wikipedia.org/wiki/Linux_namespaces
[Cgroups]: https://en.wikipedia.org/wiki/Cgroups

### Application containers

The most well-known example of application container tech is probably
[Docker.][docker] The goal here is to run a single application as minimally as
possible inside each container. In the case of a single, statically-linked Go
binary, a minimal Docker container might contain nothing more than the binary.
If it's a Python application, you're more likely to use an [Alpine Linux image]
and add your Python dependencies on top of that. If a database is required, that
goes in a separate container. If you've got a web server to handle TLS
termination and proxy your application, that's a third container. One cohesive
system might require many Docker containers to function as intended.

[docker]: https://docker.com/
[Alpine Linux image]: https://hub.docker.com/_/alpine

```kroki {type=d2,d2theme=flagship-terrastruct,d2sketch=true}
Host kernel.Container runtime.c1: Container
Host kernel.Container runtime.c2: Container
Host kernel.Container runtime.c3: Container

Host kernel.Container runtime.c1.One app
Host kernel.Container runtime.c2.Few apps
Host kernel.Container runtime.c3.Full OS.Many apps
```

### System containers

One of the most well-known examples of system container tech is the subject of
this post: LXD! Rather than containing a single application or a very small set
of them, system containers are designed to house entire operating systems, like
[Debian] or [Rocky Linux,][rocky] along with everything required for your
application. Using our examples from above, a single statically-linked Go binary
might run in a full Debian container, just like the Python application might.
The database and webserver might go in _that same_ container.

[Debian]: https://www.debian.org/
[rocky]: https://rockylinux.org/

You treat each container more like you would a VM, but you get the performance
benefit of _not_ virtualising everything. Containers tend to be _much_ lighter
than most VMs.[^1]

```kroki {type=d2,d2theme=flagship-terrastruct,d2sketch=true}
hk: Host kernel
hk.c1: Container
hk.c2: Container
hk.c3: Container
hk.c1.os1: Full OS
hk.c2.os2: Full OS
hk.c3.os3: Full OS
hk.c1.os1.app1: Many apps
hk.c2.os2.app2: Many apps
hk.c3.os3.app3: Many apps
```

## When to use which

These are personal opinions. Please evaluate each technology and determine for
yourself whether it's a suitable fit for your environment.

### VMs

As far as I'm aware, VMs are your only option when you want to work with
esoteric hardware or hardware you don't physically have on-hand. You can tell
your VM that it's running with RAM that's 20 years old, a still-in-development
RISC-V CPU, and a 420p monitor. That's not possible with containers. VMs are
also your only option when you want to work with foreign operating systems:
running Linux on Windows, Windows on Linux, or OpenBSD on a Mac all require
virtualisation. Another reason to stick with VMs is for compliance purposes.
Containers are still very new and some regulatory bodies require virtualisation
because it's a decades-old and battle-tested isolation technique.

{{< adm type="note" >}}
See Drew DeVault's blog post [_In praise of qemu_][qemu] for a great use of VMs

[qemu]: https://drewdevault.com/2022/09/02/2022-09-02-In-praise-of-qemu.html

{{< /adm >}}

### Application containers

Application containers are particularly popular for [microservices] and
[reproducible builds,][repb] though I personally think [NixOS] is a better fit
for the latter. App containers are also your only option if you want to use
cloud platforms with extreme scaling capabilities like Google Cloud's App Engine
standard environment or AWS's Fargate.

[microservices]: https://en.wikipedia.org/wiki/Microservices
[repb]: https://en.wikipedia.org/wiki/Reproducible_builds
[NixOS]: https://nixos.org/

Application containers also tend to be necessary when the application you want
to self-host is _only_ distributed as a Docker image and the maintainers
adamantly refuse to support any other deployment method. This is a _massive_ pet
peeve of mine; yes, Docker can make running self-hosted applications easier for
inexperienced individuals,[^2] but an application orchestration system _does
not_ fit in every single environment. By refusing to provide proper "manual"
deployment instructions, maintainers of these projects alienate an entire class
of potential users and it pisses me off.

Just document your shit.

### System containers

Personally, I prefer the workflow of system containers and use them for
everything else. Because they contain entire operating systems, you're able to
interact with it in a similar way to VMs or even your PC; you shell into it,
`apt install` whatever you need, set up the application, expose it over the
network (for example, on `0.0.0.0:8080`), proxy it on the container host, and
that's it! This process can be trivially automated with shell scripts, Ansible
roles, Chef, Puppet, whatever you like. Back the system up using [tarsnap] or
[rsync.net] or [Backblaze,][bb] Google Drive, and [restic.][restic] If you use
ZFS for your LXD storage pool, maybe go with [syncoid and sanoid.][ss]

[tarsnap]: https://www.tarsnap.com/
[rsync.net]: https://rsync.net/
[bb]: https://www.backblaze.com/
[restic]: https://restic.net/
[ss]: https://github.com/jimsalterjrs/sanoid

My point is that using system containers doesn't mean throwing out the last few
decades of systems knowledge and wisdom.

I wrote [a follow-up post] with a crash course to actually using LXD in the real
world along with a few configuration tips.

[a follow-up post]: {{< ref "crash-course-to-lxd" >}}

[^1]:
    There's a [technical
    publication](https://dl.acm.org/doi/10.1145/3132747.3132763) indicating that
    specialised VMs with unikernels can be far lighter and more secure than
    containers.

[^2]:
    Until they need to do _anything_ more complex than pull a newer image. Then
    it's twice as painful as the "manual" method might have been.