Technotes for future me



On-disk files in a container are ephemeral, which presents some problems for non-trivial applications when running in containers. One problem is the loss of files when a container crashes. The kubelet restarts the container but with a clean state. A second problem occurs when sharing files between containers running together in a Pod.


Docker has a concept of volumes volumes, though it is somewhat looser and less managed. A Docker volume is a directory on disk or in another container. Docker provides volume drivers, but the functionality is somewhat limited.

Kubernetes supports many types of volumes. A Pod can use any number of volume types simultaneously. Ephemeral volume types have a lifetime of a pod, but persistent volumes exist beyond the lifetime of a pod. Consequently, a volume outlives any containers that run within the pod, and data is preserved across container restarts. When a pod ceases to exist, Kubernetes destroys ephemeral volumes; however, Kubernetes does not destroy persistent volumes.

At its core, a volume is just 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.

Types of Volumes


An emptyDir volume is first created when a Pod is assigned to a node, and exists as long as that Pod is running on that node. As the name says, the emptyDir volume is initially empty. All containers in the Pod can read and write the same files in the emptyDir volume, though that volume can be mounted at the same or different paths in each container. When a Pod is removed from a node for any reason, the data in the emptyDir is deleted permanently.

Note: A container crashing does not remove a Pod from a node. The data in an emptyDir volume is safe across container crashes.

Some uses for an emptyDir are:

scratch space, such as for a disk-based merge sort
checkpointing a long computation for recovery from crashes
holding files that a content-manager container fetches while a webserver container serves the data

Depending on your environment, emptyDir volumes are stored on whatever medium that backs the node such as disk or SSD, or network storage. However, if you set the emptyDir.medium field to "Memory", Kubernetes mounts a tmpfs (RAM-backed filesystem) for you instead. While tmpfs is very fast, be aware that unlike disks, tmpfs is cleared on node reboot and any files you write count against your container’s memory limit.

emptyDir configuration example

apiVersion: v1
kind: Pod
  name: test-pd
  - image:
    name: test-container
    - mountPath: /cache
      name: cache-volume
  - name: cache-volume
    emptyDir: {}


Last updated on 19 Mar 2021
Published on 18 Mar 2021
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