Kubernetes - Volumes with EmptyDir and Memory

There will be always cases where we need to save data on to a drive or disk. Pods when stopped will destroy the data stored inside the container. Node reboots also clear any data from the Ram based disks. There are many times where we need  some shared spacer have containers that process data and hand it off to another container before they die.

Kubernetes provides us a way to manage storage for containers. A Volume is a directory accessible to all running containers in the pod, with a guarantee that the data is preserved. In this article we will see how we can create volumes and attach them to the pods. For the demo we will use NFS as our additional drive.

Depending the type of volumes, we differentiate the types of volumes as,

1. Node-local volumes, such as emptyDir or hostPath
2. Generic networked volumes, such as nfs, glusterfs, or cephfs
3. Cloud provider–specific volumes, such as awsElasticBlockStore,  azureDisk, or gcePersistentDisk
4. Special-purpose volumes, such as secret or gitRepo

Empty directory  - One of the easiest ways to achieve improved persistence amid container crashes and data sharing within a pod is to use the emptydir volume. This volume type can be used with either the storage volumes of the node machine itself or an optional RAM disk for higher performance.

emptyDir volume is created when the pod is assigned to the node and exists until the pod runs on that node.As the name says  , the volume is initially empty and pod can read and write contents to the volume.

Volume with Directory Type -

[root@manja17-I13330 kubenetes-config]# cat emptyDir-mysql.yml

apiVersion: v1

kind: Pod

metadata:

  name: "mysql"

  labels:

   name: "lbl-k8s-mysql"

spec:

  containers:

   - image: docker.io/jagadesh1982/testing-service

     name:  "testing"

     imagePullPolicy: IfNotPresent

     ports:

      - containerPort: 3306

     volumeMounts:

       - mountPath: /data-mount

         name: data

  volumes:

    - name: data

      emptyDir: {}

Login to the container and see how it goes,

[root@manja17-I13330 kubenetes-config]# kubectl exec mysql -c testing -it bash

root@mysql:/usr/src/app# cd /data-mount

root@mysql:/data-mount# pwd

/data-mount

root@mysql:/data-mount# touch hello

root@mysql:/data-mount# echo "hello world" >> ./hello

root@mysql:/data-mount# exit

exit

Volume with Memory Type - By default emptyDir volumes are stored on whatever medium is backing the node. It that is an SSD disk the node restarts will still have the data available. There is another type of backend “memory” which tells k8 to mount a tmpfs ( Ram backed file System ). While this is fast , any node restart clears the contents.

Lets create a Volume type with

[root@manja17-I13330 kubenetes-config]# cat emptyDir-memory-pod.yml

apiVersion: v1

kind: Pod

metadata:

  name: "mysql"

  labels:

   name: "lbl-k8s-mysql"

spec:

  containers:

   - image: docker.io/jagadesh1982/testing-service

     name:  "testing"

     imagePullPolicy: IfNotPresent

     ports:

      - containerPort: 3306

     volumeMounts:

       - mountPath: /data-mount

         name: data

  volumes:

    - name: data

      emptyDir:

        medium: Memory

[root@manja17-I13330 kubenetes-config]# kubectl exec mysql -c testing -it bash

root@mysql:/# cd data-mount/

root@mysql:/data-mount# pwd

/data-mount

root@mysql:/# exit

Exit

In the next Volumes we will see how to mount a NFS location to the container.