Mount Disks

In Kubernetes, volumes are directories avaliable to containers to use as filesystem. Volumes persist beyond the lifecycle of pods.

There are many types of Volumes. Kalm simplifies the typical usecase by representing them as "Disks".

Let's create an application with 2 containers, and share a Disk between them.

• create an application
• create a component called comp1 with centos:7 as the image
• one caviat is that containers must have a running process, or it will be terminated immediately after creation. Therefore even though we don't need the container to do anything, we still need to give it a command to stay alive:
• enter /bin/bash -c 'sleep 100000' for the command, basically telling it to sleep

Now lets create and mount a disk

• Go to the Disks tab
• Click Add Disk
• Select Create and mount disk in the Type dropdown
• enter /tmp/folder1 for Mount Path
• select standard for Storage Class
• enter 0.1 (Gi) for Size
• click Update Component

After the component is created, open a shell and navigate to the mounted disk, and create a file representing application data.

cd /tmp/folder1

echo "Some Data" > mydata

Now let's create a second component, identical to the first one:

• create a second component called comp2 with centos:7 as the image
• enter /bin/bash -c 'sleep 100000' for the command

This time, let's mount the existing disk we created above with comp1

• Go to the Disks tab
• Click Add Disk
• Select Mount an existing disk in the Type dropdown
• enter /tmp/folder2 for Mount Path
• select the option that starts with pvc-comp1-... in the Claim Name dropdown
• click Update Component

Let's delete comp1

After comp2 is created, open a shell to the pod and navigate to the mounted disk

cd /tmp/folder2

Let's check to see if the file we created by shelling into comp1's container exists here

cat mydata

Hey it seems the data created while we were attached to comp1 (which no longer exists) is avaliable in comp2.

Recap

In this guide, we created two components attached to the same Disk, and shown that data is shared across the two volumes, and the disk survives the deletion of components.