Volumes
As we know, containers, which create the Pods, are ephemeral in nature. All data stored inside a container is deleted if the container crashes. However, kubelet will restart it with a clean state, which means that it will not have any of the old data.
To overcome this problem, Kubernetes uses Volumes. A Volume is essentially a directory backed by a storage medium. The storage medium and its content are determined by the Volume Type.
In Kubernetes, a Volume is attached to a Pod and shared among the containers of that Pod. The Volume has the same life span as the Pod, and it outlives the containers of the Pod - this allows data to be preserved across container restarts.
VolumeTypes
A directory which is mounted inside a Pod is backed by the underlying Volume Type. A Volume Type decides the properties of the directory, like size, content, etc. Some of the Volume Types are:
- emptyDir
An empty Volume is created for the Pod as soon as it is scheduled on the Worker Node. The Volume’s life is tightly coupled with the Pod. If the Pod dies, the content of emptyDir is deleted forever.
hostPath With the hostPath Volume Type, we can share a directory from the host to the Pod. If the Pod dies, the content of the Volume is still available on the host.
awsElasticBlockStore With the awsElasticBlockStore Volume Type, we can mount an AWS EBS Volume into a Pod. nfs With nfs, we can mount an NFS share into a Pod.
iscsi With iscsi, we can mount an iSCSI share into a Pod.
secret With the secret Volume Type, we can pass sensitive information, such as passwords, to Pods. We will take a look at an example in a later chapter.
persistentVolumeClaim We can attach a Persistent Volume to a Pod using a persistentVolumeClaim.
Persistent Volumes
Persistent Volume subsystem, provides APIs for users and administrators to manage and consume storage. To manage the Volume, it uses the PersistentVolume (PV) API resource type, and to consume it, it uses the PersistentVolumeClaim (PVC) API resource type.
Managing storage is a distinct problem from managing compute. The PersistentVolume subsystem provides an API for users and administrators that abstracts details of how storage is provided from how it is consumed. To do this we introduce two new API resources: PersistentVolume and PersistentVolumeClaim.
A PersistentVolume (PV)
is a piece of storage in the cluster that has been provisioned by an administrator. It is a resource in the cluster just like a node is a cluster resource. PVs are volume plugins like Volumes, but have a lifecycle independent of any individual pod that uses the PV. This API object captures the details of the implementation of the storage, be that NFS, iSCSI, or a cloud-provider-specific storage system.
A PersistentVolumeClaim (PVC)
is a request for storage by a user. It is similar to a pod. Pods consume node resources and PVCs consume PV resources. Pods can request specific levels of resources (CPU and Memory). Claims can request specific size and access modes (e.g., can be mounted once read/write or many times read-only).
After a successful bind, the PersistentVolumeClaim resource can be used in a Pod.
While PersistentVolumeClaims allow a user to consume abstract storage resources, it is common that users need PersistentVolumes with varying properties, such as performance, for different problems. Cluster administrators need to be able to offer a variety of PersistentVolumes that differ in more ways than just size and access modes, without exposing users to the details of how those volumes are implemented. For these needs there is the StorageClass resource
A StorageClass provides a way for administrators to describe the “classes” of storage they offer.
Different classes might map to quality-of-service levels, or to backup policies, or to arbitrary policies determined by the cluster administrators.
Kubernetes itself is unopinionated about what classes represent. This concept is sometimes called “profiles” in other storage systems.
Lets follow a basic example outlined below https://kubernetes.io/docs/tasks/configure-pod-container/configure-persistent-volume-storage/
Lets build a Wordpress site backed by MySQL DB.
Please clone the repo or use kubectl create/apply -f
TASK
- Remember the Addressbook application you created via docker-compose? Try to run run that on kubernetes with PV, Secrets, Deployments, Services