Containers are stateless by nature and likely to be short-lived. They are quite ephemeral than VMs. What it actually means? Say, you have any data or logs generated inside the container and you don’t really care about loosing it no matter how many times you spin it up and down, like HTTP requests, then the ideal stateless feature should be good enough. BUT in case you are looking out for a solution which should record “stateful” applications like storing databases, storing logs etc. you definitely need persistent storage to be implemented. This is achieved by leveraging Docker’s volume mounts to create either a data volume or a data volume container that can be used and shared by other containers.
In case you’re new to Docker Storage, Docker Volumes are logical building blocks for shared storage when combined with plugins. It helps us to store state from the application to locations outside the docker image layer. Whenever you run a Docker command with -v, and provide it with a name or path, this gets managed within /var/lib/docker or in case you’re using a host mount, it’s something that exists on the host. The problem with this implementation is that the data is pretty inflexible, which means anything you write to that specific location, yes, it’ll stay there after the container’s life cycle, but only on that host. If you lose that host, the data will get erased. This clearly means that the situation is very prone to data loss. Within Docker, it looks very similar to what shown in the above picture, /var/lib/docker directory structure. Let’s talk about how to implement the management of data with an external storage. This could be anything from NFS to distributed file systems to block storage.
In my previous blog, we discussed about Persistent Storage implementation with DellEMC RexRay for Docker 1.12 Swarm Mode. Under this blog, we will look at how NFS works with Swarm Mode.I assume that you have an existing NFS server running in your infrastructure. If not, you can quickly set it up in just few minutes. I have Docker 1.12 Swarm Mode initialized with 1 master node and 4 worker nodes. Just for an example, I am going to leverage a Ubuntu 16.04 node(outside the swarm mode cluster) as NFS server and rest of the nodes( 1 master & 4 workers) as NFS client systems.
Setting up NFS environment:
There are two ways to setup NFS server – either using available Docker image or manually setting up NFS server on the Docker host machine. As I already have NFS server working on one of Docker host running Ubuntu 16.04 system, I will just verify if the configuration looks good.
Let us ensure that NFS server packages are properly installed as shown below:
raina_ajeet@master1:~$ sudo dpkg -l | grep nfs-kernel-server
1:1.2.8-9ubuntu12 amd64 support for NFS kernel server
I created the following NFS directory which I want to share across the containers running the swarm cluster.
$sudo mkdir /var/nfs
$sudo chown nobody:nogroup /var/nfs
It’s time to configure NFS shares. For this, let’s edit the export file to look like as show below:
raina_ajeet@master1:~$ cat /etc/exports
# /etc/exports: the access control list for filesystems which may be exported
# to NFS clients. See exports(5).#
# Example for NFSv2 and NFSv3:
# /srv/homes hostname1(rw,sync,no_subtree_check) hostname2
# Example for NFSv4:
# /srv/nfs4 gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)
# /srv/nfs4/homes gss/krb5i(rw,sync,no_subtree_check)
As shown above, we will be sharing /var/nfs directory among all the worker nodes in the Swarm cluster.
Let’s not forget to run the below commands to provide the proper permission:
$sudo chown nobody:nogroup /var/nfs
$sudo exportfs -a
$sudo service nfs-kernel-server start
Great ! Let us cross-check if the configuration holds good.
raina_ajeet@master:~$ sudo df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.9G 0 1.9G 0% /dev
tmpfs 370M 43M 328M 12% /run
/dev/sda1 20G 6.6G 13G 35% /
tmpfs 1.9G 0 1.9G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
tmpfs 100K 0 100K 0% /run/lxcfs/controllers
tmpfs 370M 0 370M 0% /run/user/1001
10.140.0.7:/var/nfs 20G 19G 1.3G 94% /mnt/nfs/var/nfs
As shown above, we have NFS server with IP: 10.140.0.7 and ready to share volume to all the worker nodes.
Running NFS service on Swarm Mode
In case you are new to –mount option introduced under Docker 1.12 Swarm Mode, here is an easy explanation:-
$sudo docker service create –mount type=volume,volume-opt=o=addr=<source-host which is master node>, volume-opt=device=:<NFS directory>,volume-opt=type=nfs,source=<volume name>, target=/<insideContainer> –replicas 3 –name <service-name> dockerimage <command>
Let’s verify if NFS service is up and running across the swarm cluster –
We can verify the NFS volume driver through ‘docker volume’ utility as shown below –
Inspecting the NFS Volume on the nodes:
docker volume inspect <vol_name> rightly displays the Mountpoint, Labels and Scope of the NFS volume driver.
Verifying if worker nodes are able to see the Volumes
Let’s verify by logging into one of worker node and trying to check if volume is shared across the swarm cluster:
Let us create a file under /var/nfs directory so as to see if storage persistent is actually working and data is shared across the swarm cluster:
Let’s verify if one of worker node can see the created file.
Hence, we implemented persistent storage for Docker 1.12 Swarm Mode using NFS. In the future post, we will talk further on the available storage plugin and its implementations.