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Exercise 4 - Storage Policies

In this exercise, we will learn how to modify and use Storage Classes and Storage Policies.

Quick Review of a Storage Class and Policies

A StorageClass provides a way for administrators to describe the “classes” of storage they offer. Different classes might map to quality-of-service levels, or to backup/snapshot policies, or to arbitrary policies determined by the DevOps architects. Kubernetes itself is unopinionated about what classes represent.

You can define policies based on the type of application at the StorageClass (a Kubernetes Kind) level. This exercise explains when to add a storage policy to your OpenEBS cluster and how to use the same.

A storage policy states the desired behavior of an OpenEBS volume. Storage policies can be created, updated, and deleted in a running OpenEBS cluster through corresponding operations on StorageClass. Cluster administrators can update storage policies independent of the cluster. Once a storage policy is installed, users can create and access it’s objects with kubectl commands on StorageClass.

Storage policies are meant to be created per team, workload, storage controller, and so on that fits your requirement. Since OpenEBS storage controllers (i.e. jiva or cStor) run from within a container, a custom storage policy can be created and set against a particular storage controller instance that meets the demands of the application (which consumes the storage exposed from the storage controller instance). You can now define policies based on the type of application at the StorageClass level. Following are some of the properties that can be customized at the default level in the openebs-storageclasses.yaml file.

Types of Storage Policies

OpenEBS supports several types of Storage Policies:

  • openebs.io/jiva-replica-count
  • openebs.io/jiva-replica-image
  • openebs.io/jiva-controller-image
  • openebs.io/storage-pool
  • openebs.io/capacity
  • openebs.io/fstype
  • openebs.io/volume-monitor

Replica Count Policy

You can specify the jiva replica count using the openebs.io/jiva-replica-count property. In the following example, the jiva-replica-count is specified as 1. Hence, a single replica is created.

  1. Create an empty openebs-mystrorageclass1.yaml YAML file and edit:

    nano openebs-mystorageclass1.yaml

  2. Add the following content:

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
    name: openebs-mystorageclass
provisioner: openebs.io/provisioner-iscsi
parameters:
    openebs.io/jiva-replica-count: "1"

Replica Image Policy

You can specify the jiva replica image using the openebs.io/jiva-replica-image property. Jiva replica image is an OpenEBS provided docker image for its storage engine.

  1. Following is a sample intent that makes use of replica image policy:

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
    name: mysql
provisioner: openebs.io/provisioner-iscsi
parameters:
    openebs.io/jiva-replica-count: "1"
    openebs.io/capacity: "1G"
    openebs.io/jiva-replica-image: "openebs/jiva:0.6.0"

Controller Image Policy

You can specify the jiva controller image using the openebs.io/jiva-controller-image property.

  1. Edit your storageclass file to add the new parameter:

    kind: StorageClass
metadata:
    name: mysql
provisioner: openebs.io/provisioner-iscsi
parameters:
    openebs.io/jiva-replica-count: "1"
    openebs.io/capacity: "1G"
    openebs.io/jiva-replica-image: "openebs/jiva:0.6.0"
    openebs.io/jiva-controller-image: "openebs/jiva:0.6.0"

Storage Pool Policy

A storage pool provides a persistent path for an OpenEBS volume. It can be a directory on any of the following.

  • host-os or
  • mounted disk/LUN/network share

You must define the storage pool as a Kubernetes Custom Resource (CR) before using it as a Storage Pool policy.

  1. Following is a sample Kubernetes custom resource definition for a storage pool that we have applied in Exercise 2:

    cd ~/openebs/k8s
cat openebs-config.yaml

    openebs-config.yaml sets the default storagepool to path /var/openebs

    apiVersion: openebs.io/v1alpha1
kind: StoragePool
metadata:
    name: default
    type: hostdir
spec:
    path: "/var/openebs"

  2. Let's create a new storagepool CR. Copy the openebs-config.yaml to openebs-mystoragepool.yaml and modify name and path parameters as follows:

    apiVersion: openebs.io/v1alpha1
kind: StoragePool
metadata:
    name: sp-mntdir
    type: hostdir
spec:
    path: "/mnt/openebs"

  3. This storage pool custom resource can now be used in a storageclass as follows (Example file: openebs-mystrorageclass4.yaml):

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
    name: openebs-mystorageclass
provisioner: openebs.io/provisioner-iscsi
parameters:
    openebs.io/jiva-replica-count: "3"
    openebs.io/capacity: "3G"
    openebs.io/jiva-replica-image: "openebs/jiva:0.6.0"
    openebs.io/storage-pool: "sp-mntdir"

Capacity and FS Type Policy

By default OpenEBS comes with ext4 file system. However, you can specify the file system policy for a particular volume using openebs.io/fstype property:

  1. Following is a sample setting used in openebs-mongodb storageclass:

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
   name: openebs-mongodb
provisioner: openebs.io/provisioner-iscsi
parameters:
  openebs.io/storage-pool: "test-mntdir"
  openebs.io/jiva-replica-count: "1"
  openebs.io/volume-monitor: "true"
  openebs.io/capacity: 5G
  openebs.io/fstype: "xfs"

Volume Monitoring Policy

You can specify the monitoring policy for a particular volume using openebs.io/volume-monitor property.

  1. The following Kubernetes storageclass sample uses the Volume Monitoring policy:

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
    name: sc-percona-monitor
provisioner: openebs.io/provisioner-iscsi
parameters:
    openebs.io/jiva-replica-image: "openebs/jiva:0.6.0"
    openebs.io/volume-monitor: "true"

Continue to Exercise 5 - Configuring Storage Disaggregation with OpenEBS