Using volumes to persist container data¶

You can use volumes to persist the data used by the containers in a pod. A volume is directory, accessible to the containers in a pod, where data is stored for the life of the pod.

Files in a container are ephemeral. As such, when a container crashes or stops, the data is lost.

Understanding volumes¶

You can provide persistent or temporary storage for your applications by mounting volumes to pods and containers. Using volumes ensures that data remains available even if a container restarts by backing the file system with host-local or network-attached storage.

Volumes are mounted file systems available to pods and their containers which may be backed by a number of host-local or network attached storage endpoints. Containers are not persistent by default; on restart, their contents are cleared.

To ensure that the file system on the volume contains no errors and, if errors are present, to repair them when possible, OpenShift Container Platform invokes the fsck utility prior to the mount utility. This occurs when either adding a volume or updating an existing volume.

The simplest volume type is emptyDir, which is a temporary directory on a single machine. Administrators may also allow you to request a persistent volume that is automatically attached to your pods.

Note

emptyDir volume storage may be restricted by a quota based on the pod’s FSGroup, if the FSGroup parameter is enabled by your cluster administrator.

Working with volumes using the OpenShift Container Platform CLI¶

You can use the CLI command oc set volume to add and remove volumes and volume mounts for any object that has a pod template like replication controllers or deployment configs. You can also list volumes in pods or any object that has a pod template.

The oc set volume command uses the following general syntax:

$ oc set volume <object_selection> <operation> <mandatory_parameters> <options>

Object selection

Specify one of the following for the object_selection parameter in the oc set volume command:

Object Selection

Syntax
Description
Example
`_<object_type> <name>_`
Selects `<name>` of type `_<object_type>_`.
`deploymentConfig registry`
`_<object_type>/<name>_`
Selects `<name>` of type `_<object_type>_`.
`deploymentConfig/registry`
`_<object_type>_` `--selector=_<object_label_selector>_`
Selects resources of type `_<object_type>_` that matched the given label selector.
`deploymentConfig` `--selector="name=registry"`
`_<object_type>_ --all`
Selects all resources of type `_<object_type>_`.
`deploymentConfig --all`
`-f` or `--filename=_<file_name>_`
File name, directory, or URL to file to use to edit the resource.
`-f registry-deployment-config.json`

Operation: Specify --add or --remove for the operation parameter in the oc set volume command.
Mandatory parameters: Any mandatory parameters are specific to the selected operation and are discussed in later sections.
Options: Any options are specific to the selected operation and are discussed in later sections.

About listing volumes and volume mounts in a pod¶

You can list volumes and volume mounts in pods or pod templates.

You can list volumes by running the following command:

$ oc set volume <object_type>/<name> [options]

List volume supported options:

Option
Description
Default
`--name`
Name of the volume.

`-c, --containers`
Select containers by name. It can also take wildcard `'*'` that matches any character.
`'*'`

For example:

To list all volumes for pod p1:
```
$ oc set volume pod/p1
```
To list volume v1 defined on all deployment configs:
```
$ oc set volume dc --all --name=v1
```

About adding volumes to a pod¶

You can add volumes and volume mounts to a pod. Volumes persist the data used by the containers, even if container crashes or stops.

You can add a volume, a volume mount, or both to pod templates by running the following command:

$ oc set volume <object_type>/<name> --add [options]

Supported Options for Adding Volumes

Option
Description
Default
`--name`
Name of the volume.
Automatically generated, if not specified.
`-t, --type`
Name of the volume source. Supported values: `emptyDir`, `hostPath`, `secret`, `configmap`, `persistentVolumeClaim` or `projected`.
`emptyDir`
`-c, --containers`
Select containers by name. It can also take wildcard `'*'` that matches any character.
`'*'`
`-m, --mount-path`
Mount path inside the selected containers. Do not mount to the container root, `/`, or any path that is the same in the host and the container. This can corrupt your host system if the container is sufficiently privileged, such as the host `/dev/pts` files. It is safe to mount the host by using `/host`.

`--path`
Host path. Mandatory parameter for `--type=hostPath`. Do not mount to the container root, `/`, or any path that is the same in the host and the container. This can corrupt your host system if the container is sufficiently privileged, such as the host `/dev/pts` files. It is safe to mount the host by using `/host`.

`--secret-name`
Name of the secret. Mandatory parameter for `--type=secret`.

`--configmap-name`
Name of the configmap. Mandatory parameter for `--type=configmap`.

`--claim-name`
Name of the persistent volume claim. Mandatory parameter for `--type=persistentVolumeClaim`.

`--source`
Details of volume source as a JSON string. Recommended if the desired volume source is not supported by `--type`.

`-o, --output`
Display the modified objects instead of updating them on the server. Supported values: `json`, `yaml`.

`--output-version`
Output the modified objects with the given version.
`api-version`

For example:

To add a new volume source emptyDir to the registry DeploymentConfig object:

$ oc set volume dc/registry --add

Tip

You can alternatively apply the following YAML to add the volume:

Sample deployment config with an added volume

kind: DeploymentConfig
apiVersion: apps.openshift.io/v1
metadata:
  name: registry
  namespace: registry
spec:
  replicas: 3
  selector:
    app: httpd
  template:
    metadata:
      labels:
        app: httpd
    spec:
      volumes:
        - name: volume-pppsw
          emptyDir: {}
      containers:
        - name: httpd
          image: >-
            image-registry.openshift-image-registry.svc:5000/openshift/httpd:latest
          ports:
            - containerPort: 8080
              protocol: TCP

where:

spec.template.spec.volumes: Specifies the volume source emptyDir.

To add volume v1 with secret secret1 for replication controller r1 and mount inside the containers at /data:

$ oc set volume rc/r1 --add --name=v1 --type=secret --secret-name='secret1' --mount-path=/data

Tip

You can alternatively apply the following YAML to add the volume:

Sample replication controller with added volume and secret

kind: ReplicationController
apiVersion: v1
metadata:
  name: example-1
  namespace: example
spec:
  replicas: 0
  selector:
    app: httpd
    deployment: example-1
    deploymentconfig: example
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: httpd
        deployment: example-1
        deploymentconfig: example
    spec:
      volumes:
        - name: v1
          secret:
            secretName: secret1
            defaultMode: 420
      containers:
        - name: httpd
          image: >-
            image-registry.openshift-image-registry.svc:5000/openshift/httpd:latest
          volumeMounts:
            - name: v1
              mountPath: /data

where:

spec.template.spec.volumes: Specifies the volume and secret.
spec.template.spec.containers.volumeMounts: Specifies the container mount path.

To add existing persistent volume v1 with claim name pvc1 to deployment configuration dc.json on disk, mount the volume on container c1 at /data, and update the DeploymentConfig object on the server:

$ oc set volume -f dc.json --add --name=v1 --type=persistentVolumeClaim \
  --claim-name=pvc1 --mount-path=/data --containers=c1

Tip

You can alternatively apply the following YAML to add the volume:

Sample deployment config with persistent volume added

kind: DeploymentConfig
apiVersion: apps.openshift.io/v1
metadata:
  name: example
  namespace: example
spec:
  replicas: 3
  selector:
    app: httpd
  template:
    metadata:
      labels:
        app: httpd
    spec:
      volumes:
        - name: volume-pppsw
          emptyDir: {}
        - name: v1
          persistentVolumeClaim:
            claimName: pvc1
      containers:
        - name: httpd
          image: >-
            image-registry.openshift-image-registry.svc:5000/openshift/httpd:latest
          ports:
            - containerPort: 8080
              protocol: TCP
          volumeMounts:
            - name: v1
              mountPath: /data

where:

spec.template.spec.volumes.name.v1: Specifies the persistent volume claim named pvc1.
spec.template.spec.containers.volumeMounts: Specifies the container mount path.

To add a volume v1 based on Git repository $**$https://github.com/namespace1/project1$**$ with revision 5125c45f9f563 for all replication controllers:

$ oc set volume rc --all --add --name=v1 \
  --source='{"gitRepo": {
                "repository": "https://github.com/namespace1/project1",
                "revision": "5125c45f9f563"
            }}'

About updating volumes and volume mounts in a pod¶

You can modify the volumes and volume mounts in a pod.

You can update existing volumes by using the --overwrite option:

$ oc set volume <object_type>/<name> --add --overwrite [options]

For example:

To replace existing volume v1 for replication controller r1 with existing persistent volume claim pvc1:

$ oc set volume rc/r1 --add --overwrite --name=v1 --type=persistentVolumeClaim --claim-name=pvc1

Tip

You can alternatively apply the following YAML to replace the volume:

Sample replication controller with persistent volume claim named pvc1

kind: ReplicationController
apiVersion: v1
metadata:
  name: example-1
  namespace: example
spec:
  replicas: 0
  selector:
    app: httpd
    deployment: example-1
    deploymentconfig: example
  template:
    metadata:
      labels:
        app: httpd
        deployment: example-1
        deploymentconfig: example
    spec:
      volumes:
        - name: v1
          persistentVolumeClaim:
            claimName: pvc1
      containers:
        - name: httpd
          image: >-
            image-registry.openshift-image-registry.svc:5000/openshift/httpd:latest
          ports:
            - containerPort: 8080
              protocol: TCP
          volumeMounts:
            - name: v1
              mountPath: /data

The spec.template.spec.volumes stanza sets the persistent volume claim to pvc1.

To change the DeploymentConfig object d1 mount point to /opt for volume v1:

$ oc set volume dc/d1 --add --overwrite --name=v1 --mount-path=/opt

Tip

You can alternatively apply the following YAML to change the mount point:

Sample deployment config with mount point set to opt.

kind: DeploymentConfig
apiVersion: apps.openshift.io/v1
metadata:
  name: example
  namespace: example
spec:
  replicas: 3
  selector:
    app: httpd
  template:
    metadata:
      labels:
        app: httpd
    spec:
      volumes:
        - name: volume-pppsw
          emptyDir: {}
        - name: v2
          persistentVolumeClaim:
            claimName: pvc1
        - name: v1
          persistentVolumeClaim:
            claimName: pvc1
      containers:
        - name: httpd
          image: >-
            image-registry.openshift-image-registry.svc:5000/openshift/httpd:latest
          ports:
            - containerPort: 8080
              protocol: TCP
          volumeMounts:
            - name: v1
              mountPath: /opt

The spec.template.spec.containers.volumeMounts stanza sets the mount point to /opt.

About removing volumes and volume mounts from a pod¶

You can remove a volume or volume mount from a pod.

You can remove a volume from a pod template by running the following command:

$ oc set volume <object_type>/<name> --remove [options]

Supported options for removing volumes

Option
Description
Default
`--name`
Name of the volume.

`-c, --containers`
Select containers by name. It can also take wildcard `'*'` that matches any character.
`'*'`
`--confirm`
Indicate that you want to remove multiple volumes at once.

`-o, --output`
Display the modified objects instead of updating them on the server. Supported values: `json`, `yaml`.

`--output-version`
Output the modified objects with the given version.
`api-version`

For example:

To remove a volume v1 from the DeploymentConfig object d1:
```
$ oc set volume dc/d1 --remove --name=v1
```
To unmount volume v1 from container c1 for the DeploymentConfig object d1 and remove the volume v1 if it is not referenced by any containers on d1:
```
$ oc set volume dc/d1 --remove --name=v1 --containers=c1
```
To remove all volumes for replication controller r1:
```
$ oc set volume rc/r1 --remove --confirm
```

Configuring volumes for multiple uses in a pod¶

You can configure a volume to share one volume for multiple uses in a single pod by using the volumeMounts.subPath property to specify a subPath value inside a volume instead of the volume’s root.

Note

You cannot add a subPath parameter to an existing scheduled pod.

Procedure

To view the list of files in the volume, run the oc rsh command:

$ oc rsh <pod>

Example output

sh-4.2$ ls /path/to/volume/subpath/mount
example_file1 example_file2 example_file3

Specify the subPath:

Example Pod spec with subPath parameter

apiVersion: v1
kind: Pod
metadata:
  name: my-site
spec:
    securityContext:
      runAsNonRoot: true
      seccompProfile:
        type: RuntimeDefault
    containers:
    - name: mysql
      image: mysql
      volumeMounts:
      - mountPath: /var/lib/mysql
        name: site-data
        subPath: mysql
      securityContext:
        allowPrivilegeEscalation: false
        capabilities:
          drop: [ALL]
    - name: php
      image: php
      volumeMounts:
      - mountPath: /var/www/html
        name: site-data
        subPath: html
      securityContext:
        allowPrivilegeEscalation: false
        capabilities:
          drop: [ALL]
    volumes:
    - name: site-data
      persistentVolumeClaim:
        claimName: my-site-data

where:

spec.containers.volumeMounts.subPath.mysql: Specifies that databases are stored in the mysql folder.
spec.containers.volumeMounts.subPath.html: Specifies that HTML content is stored in the html folder.