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Container-kubernetes-linsto

Containerize LINSTOR

LINBIT and its Software-Defined Storage (SDS) solution LINSTOR has provided integration with Linux containers for quite some time. These range from a Docker volume plugin, to a Flexvolume plugin, and recently, a CSI plugin for Kubernetes. While we always provided excellent integration to the container world, most of our software itself was not available as a container/base image. Containerizing our services is a non-trivial task. As you probably know, the core of the DRBD software consists of a Linux kernel module and user space utilities that interact via netlink with this kernel module. Additionally, our software needs to create LVM devices and DRBD block devices within a container. These tasks are interesting and challenging to put into containers. For this article, we assume 3 nodes, one node that acts as a LINSTOR controller, and two that act as satellites. We tested this with recent Centos7 machines and with a current version of Docker.

Prerequisites

In this article, we assume access to our Docker registry hosted on drbd.io. On all hosts you should run the following commands:

docker login drbd.io
Username: YourUserName
Password: YourPassword
Login Succeeded

Installing the DRBD kernel modules

We need the DRBD kernel module and its dependencies on the LINSTOR satellites (the controller does not need access to DRBD). For that we provide a solution for the most common platforms, namely Centos7/RHEL7 and Ubuntu Bionic.

docker run --privileged -it --rm \
  -v /lib/modules:/lib/modules drbd.io/drbd9:rhel7
DRBD modul sucessfully loaded 

What this does is check which kernel is actually executed on the host, then found it the most appropriate package in the container and installed it. We ship the same, unmodified rpm/deb packages in the container as we provide in our customer repositories. If you are using Ubuntu Bionic, you should use the drbd.io/drbd9:bionic container.

Running a LINSTOR controller

docker run -d --name=linstor-controller \
  -p 3376:3376 -p 3377:3377 drbd.io/linstor-controller

The controller does not have any special requirements, it just needs to be accessible to the client via TCP/IP. Please note that in this configuration the controller’s database is not persisted. One possibility is to bind-mount the directory used for the controller’s database by adding
-v /some/dir:/var/lib/linstor .

Running a LINSTOR satellite

docker run -d --name=linstor-satellite --net=host \
 --privileged drbd.io/linstor-satellite 

The satellite is the component that creates actual block devices. On one hand the backing devices (usually LVM) and the actual DRBD block devices. Therefore this container needs access to/dev, and it needs to share the host networking. Host networking is required for the communication between drbdsetup and the actual kernel module.

Configuring the Cluster

We have to set up LINSTOR as usual, which fortunately, is an easy task and has to be done only once. In the spirit of this blog post, let’s use a containerized LINSTOR client as well. As the client obviously has to talk to the controller, we need to tell the client in the container where to find the controller. This is done by setting the environment variable LS_CONTROLLERS.

docker run -it --rm -e LS_CONTROLLERS=Controller \ 
  drbd.io/linstor-client interactive
  ...
- volume-definition (vd)
LINSTOR ==> node create Satellite1 172.42.42.10
LINSTOR ==> node create Satellite2 172.42.42.20
LINSTOR ==> storage-pool-definition create drbdpool
LINSTOR ==> storage-pool create lvm Satellite1 drbdpool drbdpool
LINSTOR ==> storage-pool create lvm Satellite2 drbdpool drbdpool 

Creating a replicated DRBD resource

So far we loaded the kernel module on the satellites, started the controller and satellite containers and configured the LINSTOR cluster. Now it is time to actually create resources.

docker run -it --rm -e LS_CONTROLLERS=Controller \ 
  drbd.io/linstor-client interactive
  ... 
- volume-definition (vd)
 LINSTOR ==> resource-definition create demo
 LINSTOR ==> volume-definition create demo 1G
 LINSTOR ==> resource create demo --storage-pool drbdpool --auto-place 2 

If you have drbd-utils installed on the host, you can now see the DRBD resource as usual viadrbdsetup status. But we can also use a container to do that. On one of the satellites you can run a throw-away linstor-satellite container which contains drbd-utils:

docker run -it --rm --net=host --privileged \
 --entrypoint=/bin/bash drbd.io/linstor-satellite
$ drbdsetup status
$ lvs

Note that by default you will not see the symbolic links for the backing devices created by LVM/udev in the LINSTOR satellite container. That is expected. In the container you will see something like /dev/drbdpool/demo_00000, while on the host you will only see/dev/dm-X, and  lvs will not show the LVs. If you really want to see the LVs on the host, you could execute  lvscan -a --cache, but there is no actual reason for that. One might also map the lvmetad socket to the container.

Summary

As you can see, LINBIT’s container story is now complete. It is now possible to deploy the whole stack via containers. This ranges from the lowest level of providing the kernel modules to the highest level of LINSTOR SDS including the client, the controller, and satellites.

Roland Kammerer
Software Engineer at Linbit
Roland Kammerer studied technical computer science at the Vienna University of Technology and graduated with distinction. Currently, he is a PhD candidate with a research focus on time-triggered realtime-systems and works for LINBIT in the DRBD development team.