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Using volumes to persist container data - Working with containers | Nodes | OpenShift Container Platform 4.7
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Using volumes to persist container data

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

Understanding volumes

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.

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:

Table 1. 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.

Listing volumes and volume mounts in a pod

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

Procedure

To list volumes:

$ 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

Adding volumes to a pod

You can add volumes and volume mounts to a pod.

Procedure

To add a volume, a volume mount, or both to pod templates:

$ oc set volume <object_type>/<name> --add [options]
Table 2. 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

  • 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

  • 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

  • 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"
            }}'

Updating volumes and volume mounts in a pod

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

Procedure

Updating existing volumes 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

  • 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

Removing volumes and volume mounts from a pod

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

Procedure

To remove a volume from pod templates:

$ oc set volume <object_type>/<name> --remove [options]
Table 3. 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 allows you to share one volume for multiple uses in a single pod using the volumeMounts.subPath property to specify a subPath value inside a volume instead of the volume’s root.

Procedure

  1. 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

  2. Specify the subPath:

    Example Pod spec with subPath parameter
    apiVersion: v1
kind: Pod
metadata:
  name: my-site
spec:
    containers:
    - name: mysql
      image: mysql
      volumeMounts:
      - mountPath: /var/lib/mysql
        name: site-data
        subPath: mysql (1)
    - name: php
      image: php
      volumeMounts:
      - mountPath: /var/www/html
        name: site-data
        subPath: html (2)
    volumes:
    - name: site-data
      persistentVolumeClaim:
        claimName: my-site-data
    1 Databases are stored in the mysql folder.
    2 HTML content is stored in the html folder.