For minimalist Linux Operating systems, you’ll need a little extra configuration to use Longhorn with RKE (Rancher Kubernetes Engine). This document outlines the requirements for using RKE and CoreOS.

Background

CSI doesn’t work with CoreOS + RKE before Longhorn v0.4.1. The reason is that in the case of CoreOS, RKE sets the argument root-dir=/opt/rke/var/lib/kubelet for the kubelet , which is different from the default value /var/lib/kubelet.

For k8s v1.12+, the kubelet will detect the csi.sock according to argument <--kubelet-registration-path> passed in by Kubernetes CSI driver-registrar, and <drivername>-reg.sock (for Longhorn, it’s io.rancher.longhorn-reg.sock) on kubelet path <root-dir>/plugins.

For k8s v1.11, the kubelet will find both sockets on kubelet path /var/lib/kubelet/plugins.

By default, Longhorn CSI driver creates and expose these two sock files on the host path /var/lib/kubelet/plugins. Then the kubelet cannot find <drivername>-reg.sock, so CSI driver doesn’t work.

Furthermore, the kubelet will instruct the CSI plugin to mount the Longhorn volume on <root-dir>/pods/<pod-name>/volumes/kubernetes.io~csi/<volume-name>/mount. But this path inside the CSI plugin container won’t be bind mounted on the host path. And the mount operation for the Longhorn volume is meaningless.

Therefore, in this case, Kubernetes cannot connect to Longhorn using the CSI driver without additional configuration.

Requirements

• Kubernetes v1.11 or higher.
• Longhorn v0.4.1 or higher.

1. Add extra binds for the kubelet

This step is only required for For CoreOS + and Kubernetes v1.11. It is not needed for Kubernetes v1.12+.

Add extra_binds for kubelet in RKE cluster.yml:

services:
  kubelet:
    extra_binds:
    - "/opt/rke/var/lib/kubelet/plugins:/var/lib/kubelet/plugins" 

This makes sure the kubelet plugins directory is exposed for CSI driver installation.

2. Start the iSCSI Daemon

If you want to enable iSCSI daemon automatically at boot, you need to enable the systemd service:

sudo su
systemctl enable iscsid
reboot

Or just start the iSCSI daemon for the current session:

sudo su
systemctl start iscsid

Troubleshooting

Failed to get arg root-dir: Cannot get kubelet root dir, no related proc for root-dir detection …

This error happens because Longhorn cannot detect the root dir setup for the kubelet, so the CSI plugin installation failed.

You can override the root-dir detection by setting environment variable KUBELET_ROOT_DIR in https://github.com/longhorn/longhorn/blob/v1.8.0/deploy/longhorn.yaml.

How to find root-dir?

Run ps aux | grep kubelet and get the argument --root-dir on host node.

For example,

$ ps aux | grep kubelet
root      3755  4.4  2.9 744404 120020 ?       Ssl  00:45   0:02 kubelet --root-dir=/opt/rke/var/lib/kubelet --volume-plugin-dir=/var/lib/kubelet/volumeplugins

You will find root-dir in the cmdline of proc kubelet. If it’s not set, the default value /var/lib/kubelet would be used. In the case of CoreOS, the root-dir would be /opt/rke/var/lib/kubelet as shown above.

If the kubelet is using a configuration file, you need to check the configuration file to locate the root-dir parameter.

References

https://github.com/kubernetes-csi/driver-registrar

https://coreos.com/os/docs/latest/iscsi.html