Some Dockerfile have a VOLUME command.
What happens when such containers are deployed in Kubernetes, but no kubernetes volume are provided: no persistent volume (PV), nor persistent volume claim (PVC) ?
Where are the file stored ?
Is the volume persistent ?
For exemple, Dockerfile image for Docker's library/postgreSQL container image has:
VOLUME /var/lib/postgresql/dataThe stable/postgresql helm charts won't always create a PV:
kind: StatefulSet
### SNIP SNIP ###
containers:
- name: {{ template "postgresql.fullname" . }}
image: {{ template "postgresql.image" . }}
### SNIP SNIP ###
volumeMounts:
{{ if .Values.persistence.enabled }}
- name: data
mountPath: {{ .Values.persistence.mountPath }}
subPath: {{ .Values.persistence.subPath }}
{{- end }}
### SNIP SNIP ###
{{- if and .Values.persistence.enabled .Values.persistence.existingClaim }}
- name: data
persistentVolumeClaim:
{{- with .Values.persistence.existingClaim }}
claimName: {{ tpl . $ }}
{{- end }}
{{- else if not .Values.persistence.enabled }}
- name: data
emptyDir: {}
{{- else if and .Values.persistence.enabled (not .Values.persistence.existingClaim) }}
volumeClaimTemplates:
- metadata:
name: data
{{- with .Values.persistence.annotations }}
annotations:
{{- range $key, $value := . }}
{{ $key }}: {{ $value }}
{{- end }}
{{- end }}
spec:
accessModes:
{{- range .Values.persistence.accessModes }}
- {{ . | quote }}
{{- end }}
resources:
requests:
storage: {{ .Values.persistence.size | quote }}
{{ include "postgresql.storageClass" . }}
{{- end }}(answer based on what I observed with Rancher 2.4, kubernetes 1.17 using Docker)
Short answer:
a Docker volume is created in /var/lib/docker/volumes/... and removed when the pods is stopped or redeployed.
Long answer:
Kubernetes doesn't seems to have any knowledge of the volume the containers / in the Dockerfile. No Kubernetes objects seems to be created.
When Kubernetes tell Docker daemon to start a container, Docker creates a volume (like docker volume create) and attach the created volume (unless kubernete's provides a volume to mount).
Where are the file stored ?
The file is a regular Docker volume (see docker volume below)
Is the volume persistent ?
No, the docker volume is removed when the pod is removed (like if you ran docker rm $mycontainer --volumes)
docker inspect 6ce5f52186d4 | grep '"Driver": "local"' -A5 -B5 {
"Type": "volume",
"Name": "679135a23430ceea6adb8d89e04c6baf9da33239a83ecb4c4ec3263e2c925d39",
"Source": "/var/lib/docker/volumes/679135a23430ceea6adb8d89e04c6baf9da33239a83ecb4c4ec3263e2c925d39/_data",
"Destination": "/var/lib/postgresql/data",
"Driver": "local",
"Mode": "",
"RW": true,
"Propagation": ""
}
$ docker volume ls
DRIVER VOLUME NAME
local 679135a23430ceea6adb8d89e04c6baf9da33239a83ecb4c4ec3263e2c925d39 $ du -hs /var/lib/docker/volumes/679135a23430ceea6adb8d89e04c6baf9da33239a83ecb4c4ec3263e2c925d39/
51.4M /var/lib/docker/volumes/679135a23430ceea6adb8d89e04c6baf9da33239a83ecb4c4ec3263e2c925d39/