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FAQ

Please open an issue if you want to add a question here.

How do updates work?

LinuxKit does not require being installed on a disk, it is often run from an ISO, PXE or other such means, so it does not require an on disk upgrade method such as the ChromeOS code that is often used. It would definitely be possible to use that type of upgrade method if the system is installed, and it would be useful to support this for that use case, and an updater container to control this for people who want to use this.

We generally use external tooling such as Infrakit or CloudFormation templates to manage the update process externally from LinuxKit, including doing rolling cluster upgrades to make sure distributed applications stay up and responsive.

Updates may preserve the state disk used by applications if needed, either on the same physical node, or by reattaching a virtual cloud volume to a new node.

What do I need to build LinuxKit?

We have tried to make this as simple as possible, by using containers for the build process, so you should be able to build LinuxKit on any OSX or Linux laptop; we should have Windows build support soon.

Why not use systemd?

In order to keep the system minimal, systemd did not seem appropriate, as it brings in a lot of dependencies and functionality that we do not need. At present we are using the busybox init process, and a small set of minimal scripts, but we expect to replace that with a small standalone init process and a small piece of code to bring up the system containers where the real work takes place.

Console not displaying init or containerd output at boot

If you're not seeing containerd logs in the console during boot, make sure that your kernel cmdline configuration doesn't list multiple consoles.

init and other processes like containerd will use the last defined console in the kernel cmdline. When using qemu, to see the console you need to list ttyS0 as the last console to properly see the output.

Enabling and controlling containerd logs

On startup, linuxkit looks for and parses a file /etc/containerd/runtime-config.toml. If it exists, the content is used to configure containerd runtime.

Sample config is below:

cliopts="--log-level debug"
stderr="/var/log/containerd.out.log"
stdout="stdout"

The options are as follows:

  • cliopts: options to pass to the containerd command-line as is.
  • stderr: where to send stderr from containerd. If blank, it sends it to the default stderr, which is the console.
  • stdout: where to send stdout from containerd. If blank, it sends it to the default stdout, which is the console. containerd normally does not have any stdout.

The stderr and stdout options can take exactly one of the following options:

  • stderr - send to stderr
  • stdout - send to stdout
  • any absolute path (beginning with /) - send to that file. If the file exists, append to it; if not, create it and append to it.

Thus, to enable a higher log level, for example debug, create a file whose contents are --log-level debug and place it on the image:

files:
  - path: /etc/containerd/runtime-config.toml
    source: "/path/to/runtime-config.toml"
    mode: "0644"

Note that the package that parses the cliopts splits on all whitespace. It does not, as of this writing, support shell-like parsing, so the following will work:

--log-level debug --arg abcd

while the following will not:

--log-level debug --arg 'abcd def'

Troubleshooting containers

Linuxkit runs all services in a specific containerd namespace called services.linuxkit. To list all the defined containers:

(ns: getty) linuxkit-befde23bc535:~# ctr -n services.linuxkit container ls
CONTAINER               IMAGE    RUNTIME
getty                   -        io.containerd.runtime.v1.linux

To list all running containers and their status:

(ns: getty) linuxkit-befde23bc535:~# ctr -n services.linuxkit task ls
TASK                    PID    STATUS
getty                   661    RUNNING

To list all processes running in a container:

(ns: getty) linuxkit-befde23bc535:/containers/services/getty# ctr -n services.linuxkit task ps getty
PID     INFO
661     &ProcessDetails{ExecID:getty,}
677     -
685     -
686     -
687     -
1237    -

To attach a shell to a running container:

(ns: getty) linuxkit-befde23bc535:/containers/services/getty# ctr -n services.linuxkit tasks exec --tty --exec-id sh sshd /bin/ash -l
(ns: sshd) linuxkit-befde23bc535:/#

Containers are defined as OCI bundles in /containers.