Chapter 30 of 47 · Storage, part 1

Volumes: emptyDir and hostPath

Anything a container writes to its own filesystem disappears when the container is replaced. A volume is a folder that Kubernetes mounts into the container, kept somewhere else. Different volume types keep data for different lengths of time. This chapter covers the two simplest; the next ones cover storage that truly lasts.

What survives what?

Write a file, then crash the container, delete the Pod, or move it to another node. Watch which places keep the file:

Event	Container fs	emptyDir	hostPath	PersistentVolume
Container crashes and restarts	✗ lost	✓	✓	✓
Pod deleted	✗	✗ lost	✓ (on that node)	✓
Pod lands on another node	✗	✗	✗ (data stays on the old node)	✓ follows the Pod

How volumes are written

Always two parts: declare the volume once at Pod level, then mount it into each container that needs it:

spec:
  volumes:
    - name: shared
      emptyDir: {}
  containers:
    - name: posts
      image: micro-posts:1.0
      volumeMounts:
        - name: shared
          mountPath: /var/log/app
    - name: log-shipper
      image: fluent-bit:3.0
      volumeMounts:
        - name: shared
          mountPath: /logs
          readOnly: true

`volumes`

Declared under the Pod spec, next to containers. Gives the volume a name and a type (here emptyDir). The type decides where data lives and how long.

emptyDir: scratch space for one Pod

An empty folder created when the Pod starts, deleted when the Pod is removed. It survives container restarts.

Good for

Sharing files between containers in a Pod (logs, generated config)
A cache or temp files that can be rebuilt
Large sorts or uploads being processed

Not for

Anything you must keep: orders, uploads, database files
Sharing between different Pods (each Pod has its own)

volumes:
  - name: cache
    emptyDir:
      sizeLimit: 500Mi        # Pod is evicted if it writes more
  - name: fast-scratch
    emptyDir:
      medium: Memory          # RAM-backed (tmpfs): very fast, counts against memory limit

hostPath: a folder on the node

Mounts a real folder from the node's own disk into the Pod.

volumes:
  - name: node-logs
    hostPath:
      path: /var/log
      type: Directory          # must already exist (DirectoryOrCreate creates it)

Avoid it for your apps.

Tied to one node: if the Pod moves, its data stays behind (see the simulator).
Security risk: a Pod that mounts / or the container runtime socket can take over the whole node. Many clusters forbid hostPath for normal workloads.
Files may be created as root and clash between Pods on the same node.

Legitimate uses: node-level agents that must see the node itself, such as log collectors reading /var/log or monitoring agents, usually run as a DaemonSet (one Pod per node). Also handy for quick experiments on a single-node minikube.

The volume family

Type	Data lives	Chapter
`emptyDir`	As long as the Pod	this one
`hostPath`	On one node's disk	this one
`configMap`, `secret`	Read-only files from those objects	18, 19
`persistentVolumeClaim`	Independent of Pods; real disks	31, 32
`ephemeral`, `csi`	Real storage, but only for the Pod's lifetime	33

Recap

A volume is declared under spec.volumes and mounted with volumeMounts.
emptyDir: per Pod, survives container restarts, deleted with the Pod. Great for sharing and scratch.
hostPath: a node folder. Node-bound and risky; for node agents and experiments only.
For data that must outlive Pods, use PersistentVolumes (next chapter).

Quick check

Your posts app stores uploaded images in an emptyDir. Users say images vanish after every deploy. Why?

Show answer

A deploy (rolling update) replaces the Pods, and each new Pod gets a new, empty emptyDir. Uploads need a PersistentVolume, or better, object storage like S3.

Volumes: common questions

When is an emptyDir volume deleted?

When the Pod is removed from its node. It survives container restarts inside the same Pod.