Create a Docker Swarm with Volume Replication Using LINBIT SDS

With container orchestrators like Kubernetes, OpenShift and Nomad hogging the spotlight, Docker Swarm hasn’t been getting as much attention.That doesn’t mean we’ve forgotten about it! Docker Swarm is known for being easy to learn and use, making it an appealing container orchestration platform for smaller teams, or anyone looking to focus more on their applications than the platform they’re running on.

Since the inception of Docker Volume Plugins back in 2015, LINBIT has supported a plugin for managing DRBD replicated volumes through the Docker CLI. An older LINBIT blog post published with the release of LINBIT’s first Docker Volume Plugin highlighted its use by standing up a WordPress blog with a MariaDB database using Docker Compose. In the spirit of the original blog post, we’ll set up a similar WordPress blog and MariaDB database using Docker in Swarm Mode. Persistent data for both the blog and the database will be stored on, and replicated by, LINBIT SDS via LINBIT’s Docker Volume Plugin.

Using Docker in Swarm Mode will ensure that there is an instance of each container running somewhere in the Swarm cluster at all times. LINBIT SDS will ensure that each of those containers has access to a replica of the respective container’s Docker Volume. Combining Docker Swarm’s container orchestration with LINBIT SDS’ synchronously replicated Docker Volumes results in a Highly Available (HA) set of microservices.


Of course, we’re going to have to assume you’ve already done some things, otherwise this would be a very long blog post. You’ll need a LINSTOR cluster running, where each of your Docker Swarm nodes are added to the LINSTOR Cluster as a LINSTOR Satellite.

If you’re not sure about how to set that up, you can follow the subsections below for guidance. If you’ve already set that up, skip the subsections below and move on to installing the LINBIT SDS Docker Volume plugin.

Install and Initialize a LINSTOR Cluster

This is well covered in section-1 of the LINSTOR User’s Guide. Specifically, you’ll need to follow these short sections covering the following topics:

  1. Installation (of LINSTOR packages) OR running LINSTOR as Containers
  2. Initializing your cluster (if you’re using LINSTOR packages)
  3. Using the LINSTOR Client
  4. Adding nodes to your cluster
  5. and creating storage pools

Once you’ve initialized your LINSTOR cluster, you should see your cluster nodes and storage pool(s) in the following command’s outputs:

[root@swarm-0 ~]# linstor --no-utf8 node list
| Node    | NodeType  | Addresses                   | State  |
| swarm-0 | COMBINED  | (PLAIN) | Online |
| swarm-1 | SATELLITE | (PLAIN) | Online |
| swarm-2 | SATELLITE | (PLAIN) | Online |

[root@swarm-0 ~]# linstor --no-utf8 storage-pool list
| StoragePool          | Node    | Driver    | PoolName          8< | 
| DfltDisklessStorPool | swarm-0 | DISKLESS  |                   8< | 
| DfltDisklessStorPool | swarm-1 | DISKLESS  |                   8< | 
| DfltDisklessStorPool | swarm-2 | DISKLESS  |                   8< | 
| lvm-thick            | swarm-0 | LVM       | drbdpool          8< | 
| lvm-thick            | swarm-1 | LVM       | drbdpool          8< | 
| lvm-thick            | swarm-2 | LVM       | drbdpool          8< | 
| lvm-thin             | swarm-0 | LVM_THIN  | drbdpool/thinpool 8< | 
| lvm-thin             | swarm-1 | LVM_THIN  | drbdpool/thinpool 8< | 
| lvm-thin             | swarm-2 | LVM_THIN  | drbdpool/thinpool 8< | 

Install Docker CE and Initialize the Swarm Cluster

With the assumption that you’re using Linux, the best general purpose operating system ever created, you can follow the instructions from Docker documentation covering the following:

  1. Installing Docker CE from the Docker repositories
  2. Initialize the Swarm

Once you’ve initialized the Swarm cluster, you should see your nodes listed in the following command’s outputs:

[root@swarm-0 ~]# docker node ls
zc74...3b *   swarm-0    Ready     Active         Leader
a2ug...1p     swarm-1    Ready     Active
6tjt...ry     swarm-2    Ready     Active

Install and Configure the LINBIT SDS Docker Volume Plugin

The installation and configuration needs to be done on all nodes. Make sure you substitute your storagepool name (lvm-thin in my case) as well as the hostname or IP of your controller (swarm-0 in my case) in your plugin configuration. The other settings are fine as is and are used as defaults for volume creation.

Run the following commands to write the configuration file and install the LINBIT SDS Docker Volume Plugin:

# cat << EOF > /etc/linstor/docker-volume.conf
storagepool = lvm-thin
fs = xfs
size = 100MB
replicas = 2
controllers = linstor://swarm-0

# docker plugin install \
    linbit/linstor-docker-volume --grant-all-permissions

With the plugin installed and configured, you’re now able to create LINSTOR volumes using the Docker CLI.

[root@swarm-0 ~]# docker volume create \
                    -d linbit/linstor-docker-volume test-0

[root@swarm-0 ~]# linstor --no-utf8  resource list
| ResourceName | Node    | Port | Usage  | Conns |      State |
| test-0       | swarm-0 | 7000 | Unused | Ok    |   UpToDate |
| test-0       | swarm-1 | 7000 | Unused | Ok    |   UpToDate |
| test-0       | swarm-2 | 7000 | Unused | Ok    | TieBreaker |

[root@swarm-0 ~]# docker volume rm test-0
[root@swarm-0 ~]# linstor --no-utf8  resource list
| ResourceName | Node | Port | Usage | Conns | State |

Creating the Swarm Services

For our Swarm services to be able to communicate with one another, we have to create an overlay network that we can attach our services to. All commands in this section should only be run from the Swarm manager:

[root@swarm-0 ~]# docker network create -d overlay wp

Create the MariaDB root password and WordPress database secrets. You can echo and pipe any string you’d like to use as secrets to the docker secret create command, but I’ll just use random 20 character base64 encoded strings:

[root@swarm-0 ~]# openssl rand -base64 20 | docker secret create root_db_password -                
[root@swarm-0 ~]# openssl rand -base64 20 | docker secret create wp_db_password -

Create the MariaDB service’s LINSTOR volume, and then create the Swarm service:

[root@swarm-0 ~]# docker volume create \
            -d linbit/linstor-docker-volume --opt size=2G dbvol 
[root@swarm-0 ~]# docker service create \
  --name mariadb \
  --replicas 1 \
  --constraint=node.role==manager \
  --network wp \
  --publish 3306:3306 \
  --secret source=root_db_password,target=root_db_password \
  --secret source=wp_db_password,target=wp_db_password \
  -e MYSQL_ROOT_PASSWORD_FILE=/run/secrets/root_db_password \
  -e MYSQL_PASSWORD_FILE=/run/secrets/wp_db_password \
  -e MYSQL_USER=wp \
  --mount type=volume,src=dbvol,dst=/var/lib/mysql,volume-driver=linbit/linstor-docker-volume \

overall progress: 1 out of 1 tasks 
1/1: running   [==================================================>] 
verify: Service converged

Create the WordPress service’s LINSTOR volume, and then create the Swarm service:

[root@swarm-0 ~]# docker volume create \
                    -d linbit/linstor-docker-volume --opt size=1G wpvol                                                                                                             
[root@swarm-0 ~]# docker service create \
  --name wp \
  --constraint=node.role==worker \
  --replicas 1 \
  --network wp \
  --publish 80:80 \
  --secret source=wp_db_password,target=wp_db_password,mode=0444 \
  -e WORDPRESS_DB_PASSWORD_FILE=/run/secrets/wp_db_password \
  -e WORDPRESS_DB_HOST=mariadb \
  --mount type=volume,src=wpvol,dst=/var/www/html/wp-content,volume-driver=linbit/linstor-docker-volume \

overall progress: 1 out of 1 tasks 
1/1: running   [==================================================>] 
verify: Service converged

If everything was successful, you should now see that you have LINSTOR resources InUse and Swarm services running in your Docker Swarm cluster:

[root@swarm-0 ~]# linstor --no-utf8 resource list
| ResourceName | Node    | Port | Usage  | Conns |      State | CreatedOn           |
| dbvol        | swarm-0 | 7000 | InUse  | Ok    |   UpToDate | 2021-08-03 21:18:32 |
| dbvol        | swarm-1 | 7000 | Unused | Ok    |   UpToDate | 2021-08-03 21:18:32 |
| dbvol        | swarm-2 | 7000 | Unused | Ok    | TieBreaker | 2021-08-03 21:18:31 |
| wpvol        | swarm-0 | 7001 | Unused | Ok    |   UpToDate | 2021-08-03 21:19:03 |
| wpvol        | swarm-1 | 7001 | InUse  | Ok    |   Diskless | 2021-08-03 21:19:02 |
| wpvol        | swarm-2 | 7001 | Unused | Ok    |   UpToDate | 2021-08-03 21:19:03 |

[root@swarm-0 ~]# docker service ls
ID             NAME      MODE         REPLICAS   IMAGE              PORTS
kq7fqbu1d5lz   mariadb   replicated   1/1        mariadb:latest     *:3306->3306/tcp
i6dd73oq4zn8   wp        replicated   1/1        wordpress:latest   *:80->80/tcp

Finish Setting up WordPress

Point your browser at any Swarm node on port 80 and you should see you’ll be able to continue setting up your WordPress installation. Swarm will route requests to any running Swarm service through any Swarm node.

WordPress Installer

Once you click install, you can start creating your blogs! Go ahead and create one, then try halting the Docker Swarm Worker running the WordPress container. From there, you should see that Swarm will start another instance of the WordPress service, and LINSTOR will reattach the persistent storage to the instance’s new worker node.

There are a few exercises left for the reader to explore before a setup like this would be ready for a production setting. The following short list of to-dos will get you started in the right direction:

  • 3x LINSTOR controllers for an HA LINSTOR control plane
  • 3x Docker Swarm managers for an HA Docker Swarm control plane
  • SSL Certificates for WordPress

Also, if you’re looking to scale WordPress using Swarm, rather than simply utilizing Swarm’s HA container management as we did here, consider standing up an HA DRBD/Pacemaker cluster and storing your WordPress persistent volumes there. Alternatively, you can create a Swarm service that hosts NFS backed by LINSTOR SDS for Docker!

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