OKD supports FlexVolume, an out-of-tree plug-in that uses an executable model to interface with drivers.

To use storage from a back-end that does not have a built-in plug-in, you can extend OKD through FlexVolume drivers and provide persistent storage to applications.

Pods interact with FlexVolume drivers through the flexvolume in-tree plugin.

Additional References

About FlexVolume drivers

A FlexVolume driver is an executable file that resides in a well-defined directory on all nodes in the cluster. OKD calls the FlexVolume driver whenever it needs to mount or unmount a volume represented by a PersistentVolume with flexVolume as the source.

Attach and detach operations are not supported in OKD for FlexVolume.

FlexVolume driver example

The first command-line argument of the FlexVolume driver is always an operation name. Other parameters are specific to each operation. Most of the operations take a JavaScript Object Notation (JSON) string as a parameter. This parameter is a complete JSON string, and not the name of a file with the JSON data.

The FlexVolume driver contains:

  • All flexVolume.options.

  • Some options from flexVolume prefixed by kubernetes.io/, such as fsType and readwrite.

  • The content of the referenced secret, if specified, prefixed by kubernetes.io/secret/.

FlexVolume driver JSON input example
{
	"fooServer": "192.168.0.1:1234", (1)
        "fooVolumeName": "bar",
	"kubernetes.io/fsType": "ext4", (2)
	"kubernetes.io/readwrite": "ro", (3)
	"kubernetes.io/secret/<key name>": "<key value>", (4)
	"kubernetes.io/secret/<another key name>": "<another key value>",
}
1 All options from flexVolume.options.
2 The value of flexVolume.fsType.
3 ro/rw based on flexVolume.readOnly.
4 All keys and their values from the secret referenced by flexVolume.secretRef.

OKD expects JSON data on standard output of the driver. When not specified, the output describes the result of the operation.

FlexVolume driver default output example
{
	"status": "<Success/Failure/Not supported>",
	"message": "<Reason for success/failure>"
}

Exit code of the driver should be 0 for success and 1 for error.

Operations should be idempotent, which means that the mounting of an already mounted volume should result in a successful operation.

Installing FlexVolume drivers

FlexVolume drivers that are used to extend OKD are executed only on the node. To implement FlexVolumes, a list of operations to call and the installation path are all that is required.

Prerequisites
  • FlexVolume drivers must implement these operations:

    init

    Initializes the driver. It is called during initialization of all nodes.

    • Arguments: none

    • Executed on: node

    • Expected output: default JSON

    mount

    Mounts a volume to directory. This can include anything that is necessary to mount the volume, including finding the device and then mounting the device.

    • Arguments: <mount-dir> <json>

    • Executed on: node

    • Expected output: default JSON

    unmount

    Unmounts a volume from a directory. This can include anything that is necessary to clean up the volume after unmounting.

    • Arguments: <mount-dir>

    • Executed on: node

    • Expected output: default JSON

    mountdevice

    Mounts a volume’s device to a directory where individual Pods can then bind mount.

This call-out does not pass "secrets" specified in the FlexVolume spec. If your driver requires secrets, do not implement this call-out.

  • Arguments: <mount-dir> <json>

  • Executed on: node

  • Expected output: default JSON

    unmountdevice

    Unmounts a volume’s device from a directory.

  • Arguments: <mount-dir>

  • Executed on: node

  • Expected output: default JSON

    • All other operations should return JSON with {"status": "Not supported"} and exit code 1.

Procedure

To install the FlexVolume driver:

  1. Ensure that the executable file exists on all nodes in the cluster.

  2. Place the executable file at the volume plug-in path: /etc/kubernetes/kubelet-plugins/volume/exec/<vendor>~<driver>/<driver>.

For example, to install the FlexVolume driver for the storage foo, place the executable file at: /etc/kubernetes/kubelet-plugins/volume/exec/openshift.com~foo/foo.

Consuming storage using FlexVolume drivers

Each PersistentVolume object in OKD represents one storage asset in the storage back-end, such as a volume.

Procedure
  • Use the PersistentVolume object to reference the installed storage.

Persistent volume object definition using FlexVolume drivers example
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv0001 (1)
spec:
  capacity:
    storage: 1Gi (2)
  accessModes:
    - ReadWriteOnce
  flexVolume:
    driver: openshift.com/foo (3)
    fsType: "ext4" (4)
    secretRef: foo-secret (5)
    readOnly: true (6)
    options: (7)
      fooServer: 192.168.0.1:1234
      fooVolumeName: bar
1 The name of the volume. This is how it is identified through persistent volume claims or from Pods. This name can be different from the name of the volume on back-end storage.
2 The amount of storage allocated to this volume.
3 The name of the driver. This field is mandatory.
4 The file system that is present on the volume. This field is optional.
5 The reference to a secret. Keys and values from this secret are provided to the FlexVolume driver on invocation. This field is optional.
6 The read-only flag. This field is optional.
7 The additional options for the FlexVolume driver. In addition to the flags specified by the user in the options field, the following flags are also passed to the executable:
"fsType":"<FS type>",
"readwrite":"<rw>",
"secret/key1":"<secret1>"
...
"secret/keyN":"<secretN>"

Secrets are passed only to mount or unmount call-outs.