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Overview

Rook is an open-source distributed filesystem designed for use under Kubernetes, and is only supported on Kubernetes 1.7 or higher.

This document serves as a high-level overview or introduction to some basic concepts and patterns surrounding Rook.

All of this is explained in much better detail by their official documentation: https://rook.github.io/docs/rook/master/

Source code is also available here: https://github.com/rook/rook

Prerequisites

Minimum Version: Kubernetes v1.7 or higher is supported by Rook.

If you are using dataDirHostPath to persist rook data on kubernetes hosts, make sure your host has at least 5GB of space available on the specified path.

You will also need to set up RBAC, and ensure that the Flex volume plugin has been configured.

Setting up RBAC

On Kubernetes 1.7+, you will need to configure Rook to use RBAC appropriately.

See https://rook.github.io/docs/rook/master/rbac.html

Flexvolume Configuration

The Rook agent requires setup as a Flex volume plugin to manage the storage attachments in your cluster. See the Flex Volume Configuration topic to configure your Kubernetes deployment to load the Rook volume plugin.

Getting Started

Now that we've examined each of the pieces, let's zoom out and see what we can do with the whole cluster.

For the quickest quick start, check out the Rook QuickStart guide: https://rook.github.io/docs/rook/master/quickstart.html

Getting Started without an Existing Kubernetes cluster

The easiest way to deploy a new Kubernetes cluster with Rook support on OpenStack (Nebula / SDSC) is to use the https://github.com/nds-org/kubeadm-terraform repository.

This may work for other cloud providers as well, but has not yet been thoroughly tested.

Getting Started on an Existing Kubernetes cluster

If you’re feeling lucky, a simple Rook cluster can be created with the following kubectl commands. For the more detailed install, skip to the next section to deploy the Rook operator.

kubectl create -f https://raw.githubusercontent.com/rook/rook/master/cluster/examples/kubernetes/rook-operator.yaml
kubectl create -f https://raw.githubusercontent.com/rook/rook/master/cluster/examples/kubernetes/rook-cluster.yaml

After the cluster is running, you can create block, object, or file storage to be consumed by other applications in your cluster.

For a more detailed look at the deployment process, see below.

Deploy the Rook Operator

The first step is to deploy the Rook system components, which include the Rook agent running on each node in your cluster as well as Rook operator pod.

kubectl create -f https://raw.githubusercontent.com/rook/rook/master/cluster/examples/kubernetes/rook-operator.yaml
# verify the rook-operator and rook-agents pods are in the `Running` state before proceeding
kubectl -n rook-system get pod

You can also deploy the operator with the Rook Helm Chart.

Restart Kubelet (Kubernetes 1.7.x only)

For versions of Kubernetes prior to 1.8, the Kubelet process on all nodes will require a restart after the Rook operator and Rook agents have been deployed. As part of their initial setup, the Rook agents deploy and configure a Flexvolume plugin in order to integrate with Kubernetes’ volume controller framework. In Kubernetes v1.8+, the dynamic Flexvolume plugin discovery will find and initialize our plugin, but in older versions of Kubernetes a manual restart of the Kubelet will be required.

Create a Rook Cluster

Now that the Rook operator and agent pods are running, we can create the Rook cluster. For the cluster to survive reboots, make sure you set the dataDirHostPath property. For more settings, see the documentation on configuring the cluster.

Save the cluster spec as rook-cluster.yaml:

apiVersion: v1
kind: Namespace
metadata:
  name: rook
---
apiVersion: rook.io/v1alpha1
kind: Cluster
metadata:
  name: rook
  namespace: rook
spec:
  dataDirHostPath: /var/lib/rook
  storage:
    useAllNodes: true
    useAllDevices: false
    storeConfig:
      storeType: bluestore
      databaseSizeMB: 1024
      journalSizeMB: 1024

Create the cluster:

kubectl create -f rook-cluster.yaml


Use kubectl to list pods in the rook namespace. You should be able to see the following pods once they are all running:

$ kubectl -n rook get pod
NAME                              READY     STATUS    RESTARTS   AGE
rook-ceph-mgr0-1279756402-wc4vt   1/1       Running   0          5m
rook-ceph-mon0-jflt5              1/1       Running   0          6m
rook-ceph-mon1-wkc8p              1/1       Running   0          6m
rook-ceph-mon2-p31dj              1/1       Running   0          6m
rook-ceph-osd-0h6nb               1/1       Running   0          5m

Monitoring Your Rook Cluster

A glimpse into setting up Prometheus for monitoring Rook: https://rook.github.io/docs/rook/master/monitoring.html

Advanced Configuration

Advanced Configuration options are also documented here: https://rook.github.io/docs/rook/master/advanced-configuration.html

Cluster Teardown

See https://rook.github.io/docs/rook/master/teardown.html for thorough steps on destroying / cleaning up your Rook cluster

Components

Rook runs a number of smaller microservices that run on different nodes in your Kubernetes cluster:

  • The Rook Operator + API
  • Ceph Managers / Monitors / OSDs
  • Rook Agents

The Rook Operator

The Rook operator is a simple container that has all that is needed to bootstrap and monitor the storage cluster. 

The operator will start and monitor ceph monitor pods and a daemonset for the Object Storage Devices (OSDs).

The operator manages custom resource definitions (CRDs) for pools, object stores (S3/Swift), and file systems by initializing the pods and other artifacts necessary to run the services.

The operator will monitor the storage daemons to ensure the cluster is healthy.

The operator will also watch for desired state changes requested by the api service and apply the changes.

The Rook operator also creates the Rook agents as a daemonset, which runs a pod on each node. 

Ceph Managers / Monitors / OSDs

The operator will start and monitor ceph monitor pods and a daemonset for the OSDs, which provides basic Reliable Autonomic Distributed Object Store (RADOS) storage. 

The operator will monitor the storage daemons to ensure the cluster is healthy.

Ceph monitors (aka "Ceph mons") will be started or failed over when necessary, and other adjustments are made as the cluster grows or shrinks. 

Rook Agents

Each agent is a pod deployed on a different Kubernetes node, which configures a Flexvolume plugin that integrates with Kubernetes’ volume controller framework.

All storage operations required on the node are handled such as attaching network storage devices, mounting volumes, and formatting the filesystem.

Storage

Rook provides three types of storage to the Kubernetes cluster:


  • Block Storage: Mount storage to a single pod
  • Object Storage: Expose an S3 API to the storage cluster for applications to put and get data that is accessible from inside or outside the Kubernetes cluster
  • Shared File System: Mount a file system that can be shared across multiple pods

Custom Resource Definitions

Rook also allows you to create and manage your storage cluster through custom resource definitions (CRDs). Each type of resource has its own CRD defined.

  • Cluster: A Rook cluster provides the basis of the storage platform to serve block, object stores, and shared file systems.
  • Pool: A pool manages the backing store for a block store. Pools are also used internally by object and file stores.
  • Object Store: An object store exposes storage with an S3-compatible interface.
  • File System: A file system provides shared storage for multiple Kubernetes pods.

Shared Storage Example

Shamelessly stolen from https://rook.github.io/docs/rook/master/filesystem.html

Prerequisites

This guide assumes you have created a Rook cluster as explained in the main Kubernetes guide

Multiple File Systems Not Supported

By default only one shared file system can be created with Rook. Multiple file system support in Ceph is still considered experimental and can be enabled with the environment variable ROOK_ALLOW_MULTIPLE_FILESYSTEMS defined in rook-operator.yaml.

Please refer to cephfs experimental features page for more information.

Create the File System

Create the file system by specifying the desired settings for the metadata pool, data pools, and metadata server in the FilesystemCRD. In this example we create the metadata pool with replication of three and a single data pool with erasure coding. For more options, see the documentation on creating shared file systems.

Save this shared file system definition as rook-filesystem.yaml:

apiVersion: rook.io/v1alpha1
kind: Filesystem
metadata:
  name: myfs
  namespace: rook
spec:
  metadataPool:
    replicated:
      size: 3
  dataPools:
    - erasureCoded:
       dataChunks: 2
       codingChunks: 1
  metadataServer:
    activeCount: 1
    activeStandby: true

The Rook operator will create all the pools and other resources necessary to start the service. This may take a minute to complete.

# Create the file system
$ kubectl create -f rook-filesystem.yaml


# To confirm the file system is configured, wait for the mds pods to start
$ kubectl -n rook get pod -l app=rook-ceph-mds
NAME                                      READY     STATUS    RESTARTS   AGE
rook-ceph-mds-myfs-7d59fdfcf4-h8kw9       1/1       Running   0          12s
rook-ceph-mds-myfs-7d59fdfcf4-kgkjp       1/1       Running   0          12s

In this example, there is one active instance of MDS which is up, with one MDS instance in standby-replay mode in case of failover:To see detailed status of the file system, start and connect to the Rook toolbox. A new line will be shown with ceph status for the mds service.

$ ceph status                                                                                                                                              
  ...
  services:
    mds: myfs-1/1/1 up {[myfs:0]=mzw58b=up:active}, 1 up:standby-replay

Consume the Shared File System: K8s Registry Sample

As an example, we will start the kube-registry pod with the shared file system as the backing store. Save the following spec as kube-registry.yaml:

apiVersion: v1
kind: ReplicationController
metadata:
  name: kube-registry-v0
  namespace: kube-system
  labels:
    k8s-app: kube-registry
    version: v0
    kubernetes.io/cluster-service: "true"
spec:
  replicas: 3
  selector:
    k8s-app: kube-registry
    version: v0
  template:
    metadata:
      labels:
        k8s-app: kube-registry
        version: v0
        kubernetes.io/cluster-service: "true"
    spec:
      containers:
      - name: registry
        image: registry:2
        resources:
          limits:
            cpu: 100m
            memory: 100Mi
        env:
        - name: REGISTRY_HTTP_ADDR
          value: :5000
        - name: REGISTRY_STORAGE_FILESYSTEM_ROOTDIRECTORY
          value: /var/lib/registry
        volumeMounts:
        - name: image-store
          mountPath: /var/lib/registry
        ports:
        - containerPort: 5000
          name: registry
          protocol: TCP
      volumes:
      - name: image-store
        flexVolume:
          driver: rook.io/rook
          fsType: ceph
          options:
            fsName: myfs # name of the filesystem specified in the filesystem CRD.
            clusterNamespace: rook # namespace where the Rook cluster is deployed
            # by default the path is /, but you can override and mount a specific path of the filesystem by using the path attribute
            # path: /some/path/inside/cephfs

You now have a docker registry which is HA with persistent storage.

Kernel Version Requirement

If the Rook cluster has more than one filesystem and the application pod is scheduled to a node with kernel version older than 4.7, inconsistent results may arise since kernels older than 4.7 do not support specifying filesystem namespaces.


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