A Kubernetes volume is essentially a directory accessible to all containers running in a pod. In contrast to the container-local filesystem, the data in volumes is preserved across container restarts. The medium backing a volume and its contents are determined by the volume type:
- node-local types such as
emptyDirorhostPath - file-sharing types such as
nfs - cloud provider-specific types like
awsElasticBlockStore,azureDisk, orgcePersistentDisk - distributed file system types, for example
glusterfsorcephfs - special-purpose types like
secret,gitRepo
A special type of volume is PersistentVolume, which we will cover elsewhere.
Let’s create a pod
with two containers that use an emptyDir volume to exchange data:
kubectl apply -f https://raw.githubusercontent.com/openshift-evangelists/kbe/main/specs/volumes/pod.yaml
kubectl describe pod sharevol
Name: sharevol
Namespace: default
...
Volumes:
xchange:
Type: EmptyDir (a temporary directory that shares a pod's lifetime)
Medium:
We first exec into one of the containers in the pod, c1, check the volume mount
and generate some data:
kubectl exec -it sharevol -c c1 -- bash
mount | grep xchange
/dev/vda3 on /tmp/xchange type xfs (rw,relatime,seclabel,attr2,inode64,rjquota)
echo 'some data' > /tmp/xchange/data
exit the pod to continue
exit
When we now exec into c2, the second container running in the pod, we can see
the volume mounted at /tmp/data and are able to read the data created in the
previous step:
kubectl exec -it sharevol -c c2 -- bash
mount | grep /tmp/data
/dev/vda3 on /tmp/data type xfs (rw,relatime,seclabel,attr2,inode64,prjquota)
cat /tmp/data/data/
cat: /tmp/data/data/: Not a directory
cat /tmp/data/data
some data
Note that in each container you need to decide where to mount the volume and
that for emptyDir you currently can not specify resource consumption limits.
return to continue
exit
You can remove the pod with:
kubectl delete pod/sharevol
As already described, this will destroy the shared volume and all its contents.