Raspberry Pi Cluster NFS

Well, it’s been a while since I’ve posted! It’s another summer and I am creating yet another cluster. One thing people may not know is back in the summer of 2014 as I anxiously waited for the release of the Parallella I was originally going to use Raspberry Pis in my parallel computing course. Of course the Parallella came out, and so I put my Pis back on the shelf and concentrated on the Parallella for the course.

While my love affair with the Parallella was intense and short, I have been using the Raspberry Pi for quite a few research projects involving students, to great success. I may eventually go back to the Parallella, but it occurred to me that some of my old Raspberry Pi materials could also use an update, which was originally written in August 2014.

Unlike what I did with the Parallella tutorial I posted a couple years back, I’m NOT going to transcribe the entire tutorial from the PDF. The instructions largely still work. A couple things I should note:

• my webpage lists images for the Raspberry Pi 2 and the Raspberry Pi 3 You can use the Raspberry Pi 3 image to create a cluster of Raspberry Pi 3s.
• you really should log into your router (192.168.1.1) to get the IP addresses to each of the nodes in your cluster.

Setting up a NFS shared hard drive

One of the things that I always struggle with when I put together these clusters is the creation of the Network File System (NFS). The instructions are always a little different between image to image. So, the instructions below are for a cluster of Raspberry Pi 3s, though it is very likely they will apply to Raspberry Pi 2s as well.

As a reminder, pre-requisites for a NFS setup include:

• a hard drive that can be read/written to on Linux
• a working Raspberry Pi cluster (see tutorial)

Step 1: Install requisite software on master node

For the Raspberry Pi 3, the nfs-common and rpcbind packages are installed by default. This makes it a lot easier to set up NFS when you already have a Raspberry Pi cluster. Thus, the only thing you need to install is the nfs-kernel-server on the master node:

sudo apt-get install nfs-kernel-server

Step 2: Mount the hard drive on the master node

After you attach your external HD to the powered USB hub, see where it is located by using the fdisk command:

sudo fdisk -l        

A huge amount of output will result. The external hard drive will usually be at the end of this output. Mine is located at /dev/sda1.

Next, create a mount point and mount your hard drive. In the examples below, our mount point is /mnt/nfs and the hard drive is located at /dev/sda1:

sudo mkdir /mnt/nfs
sudo chown -R pi:pi /mnt/nfs 
sudo mount /dev/sda1 /mnt/nfs       

Step 3: Export new filesystem

The /etc/exports file allows us to define which folders/files we wish to share with others, and who we wish to share them with. Determine the IPs of all the worker nodes on your network by visiting 192.168.1.1. In this example, our master node has the IP 192.168.1.123 and our three worker nodes have the IPs 192.168.1.124, 192.168.1.125 and 192.168.1.126 respectively.

Modify the /etc/exports file to include the following lines:

/mnt/nfs 192.168.1.124(rw,sync,no_subtree_check)
/mnt/nfs 192.168.1.125(rw,sync,no_subtree_check)
/mnt/nfs 192.168.1.126(rw,sync,no_subtree_check)

This indicates that each of our worker nodes have read/write access to the shared drive located at /mnt/nfs, and all writes to the hd on the master will be commited immediately to the network.

Let’s restart some of the key services, and make this file visible on our network:

sudo /etc/init.d/rpcbind restart
sudo /etc/init.d/nfs-kernel-server restart
sudo exportfs -r

Now, log in on one of your worker nodes (say, 192.168.1.124):

ssh 192.168.1.124

We should be able to create our mount point and mount the hard drive:

sudo mkdir /mnt/nfs
sudo chown -R pi:pi /mnt/nfs 
sudo mount 192.168.1.123:/mnt/nfs /mnt/nfs       

You can check if it mounted correctly by using the ls command:

ls /mnt/nfs

If doing the above on a worker node shows you the contents of your harddrive, celebrate! All that is needed to repeat this process on all worker nodes.

Step 4: Automate the mounting

When you reboot the machines, the hard drive will no longer be mounted. You can attempt to automate this process by editing the /etc/fstab file. However, if you ran into issues like I did, keep in mind that you can always script the process.

The sequence of commands that must always be run on the master are:

sudo mount /dev/sda1 /mnt/nfs
sudo /etc/init.d/rpcbind restart
sudo /etc/init.d/nfs-kernel-server restart

The sequence of commands that must always be run on each worker are:

sudo /etc/init.d/nfs-common restart
sudo mount 192.168.1.123:/mnt/nfs /mnt/nfs

Troubleshooting

There are a number of problems you may face during setup. I’ve compiled a list of the most common issues, and ways to rectify them.

• Check and make sure that all the ip addresses in /etc/exports are valid. If not, fix with the appropriate IPs.
• Be sure to run the exportfs -r command once you update the /etc/exports file!
• Ensure that you can ssh into the worker nodes from the master nodes without needing a password. If you are asked to provide a password, that means the host’s ssh keys were not added to the worker’s set of authorized keys. Reboot the worker node!
• Note that the rpcbind and nfs-kernel-server have to be restarted each time. They do not start up automatically on the Raspberry Pi!
• If it’s taking too long to build images for the worker nodes, consider using a smaller SD card to create the worker node image (I recommend 8GB). This will yield 8GB worker node images, which can be formatted onto a larger SD card. You can then expand the size of the image to fit the SD card. See the set up tutorial for more instructions.