Introduction aux Volumes Kubernetes

Description

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Volumes
Kubernetes volumes provide a way for containers in a pods to access and share data via the filesystem. There are different kinds of volume that you can use for different purposes, such as:

populating a configuration file based on a ConfigMap or a Secret
providing some temporary scratch space for a pod
sharing a filesystem between two different containers in the same pod
sharing a filesystem between two different pods (even if those Pods run on different nodes)
durably storing data so that it stays available even if the Pod restarts or is replaced
passing configuration information to an app running in a container, based on details of the Pod the container is in (for example: telling a sidecar container what namespace the Pod is running in)
providing read-only access to data in a different container image
Data sharing can be between different local processes within a container, or between different containers, or between Pods.

Why volumes are important
Data persistence: On-disk files in a container are ephemeral, which presents some problems for non-trivial applications when running in containers. One problem occurs when a container crashes or is stopped, the container state is not saved so all of the files that were created or modified during the lifetime of the container are lost. After a crash, kubelet restarts the container with a clean state.

Shared storage: Another problem occurs when multiple containers are running in a Pod and need to share files. It can be challenging to set up and access a shared filesystem across all of the containers.

The Kubernetes volume abstraction can help you to solve both of these problems.

Before you learn about volumes, PersistentVolumes and PersistentVolumeClaims, you should read up about Pods and make sure that you understand how Kubernetes uses Pods to run containers.

How volumes work
Kubernetes supports many types of volumes. A Pod can use any number of volume types simultaneously. Ephemeral volume types have a lifetime linked to a specific Pod, but persistent volumes exist beyond the lifetime of any individual pod. When a pod ceases to exist, Kubernetes destroys ephemeral volumes; however, Kubernetes does not destroy persistent volumes. For any kind of volume in a given pod, data is preserved across container restarts.

At its core, a volume is a directory, possibly with some data in it, which is accessible to the containers in a pod. How that directory comes to be, the medium that backs it, and the contents of it are determined by the particular volume type used.

To use a volume, specify the volumes to provide for the Pod in .spec.volumes and declare where to mount those volumes into containers in .spec.containers[*].volumeMounts.