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Block Storage Overview

Block storage allows a single pod to mount storage. This guide shows how to create a simple, multi-tier web application on Kubernetes using persistent volumes enabled by Rook.

Prerequisites

This guide assumes a Rook cluster as explained in the Quickstart.

Provision Storage

Before Rook can provision storage, a StorageClass and CephBlockPool CR need to be created. This will allow Kubernetes to interoperate with Rook when provisioning persistent volumes.

Note

This sample requires at least 1 OSD per node, with each OSD located on 3 different nodes.

Each OSD must be located on a different node, because the failureDomain is set to host and the replicated.size is set to 3.

Save this StorageClass definition as storageclass.yaml:

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apiVersion: ceph.rook.io/v1
kind: CephBlockPool
metadata:
  name: replicapool
  namespace: rook-ceph
spec:
  failureDomain: host
  replicated:
    size: 3
---
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
   name: rook-ceph-block
# Change "rook-ceph" provisioner prefix to match the operator namespace if needed
provisioner: rook-ceph.rbd.csi.ceph.com
parameters:
    # clusterID is the namespace where the rook cluster is running
    clusterID: rook-ceph
    # Ceph pool into which the RBD image shall be created
    pool: replicapool

    # (optional) mapOptions is a comma-separated list of map options.
    # For krbd options refer
    # https://docs.ceph.com/docs/master/man/8/rbd/#kernel-rbd-krbd-options
    # For nbd options refer
    # https://docs.ceph.com/docs/master/man/8/rbd-nbd/#options
    # mapOptions: lock_on_read,queue_depth=1024

    # (optional) unmapOptions is a comma-separated list of unmap options.
    # For krbd options refer
    # https://docs.ceph.com/docs/master/man/8/rbd/#kernel-rbd-krbd-options
    # For nbd options refer
    # https://docs.ceph.com/docs/master/man/8/rbd-nbd/#options
    # unmapOptions: force

    # RBD image format. Defaults to "2".
    imageFormat: "2"

    # RBD image features
    # Available for imageFormat: "2". Older releases of CSI RBD
    # support only the `layering` feature. The Linux kernel (KRBD) supports the
    # full complement of features as of 5.4
    # `layering` alone corresponds to Ceph's bitfield value of "2" ;
    # `layering` + `fast-diff` + `object-map` + `deep-flatten` + `exclusive-lock` together
    # correspond to Ceph's OR'd bitfield value of "63". Here we use
    # a symbolic, comma-separated format:
    # For 5.4 or later kernels:
    #imageFeatures: layering,fast-diff,object-map,deep-flatten,exclusive-lock
    # For 5.3 or earlier kernels:
    imageFeatures: layering

    # The secrets contain Ceph admin credentials.
    csi.storage.k8s.io/provisioner-secret-name: rook-csi-rbd-provisioner
    csi.storage.k8s.io/provisioner-secret-namespace: rook-ceph
    csi.storage.k8s.io/controller-expand-secret-name: rook-csi-rbd-provisioner
    csi.storage.k8s.io/controller-expand-secret-namespace: rook-ceph
    csi.storage.k8s.io/node-stage-secret-name: rook-csi-rbd-node
    csi.storage.k8s.io/node-stage-secret-namespace: rook-ceph

    # Specify the filesystem type of the volume. If not specified, csi-provisioner
    # will set default as `ext4`. Note that `xfs` is not recommended due to potential deadlock
    # in hyperconverged settings where the volume is mounted on the same node as the osds.
    csi.storage.k8s.io/fstype: ext4

# Delete the rbd volume when a PVC is deleted
reclaimPolicy: Delete

# Optional, if you want to add dynamic resize for PVC.
# For now only ext3, ext4, xfs resize support provided, like in Kubernetes itself.
allowVolumeExpansion: true

If you've deployed the Rook operator in a namespace other than "rook-ceph", change the prefix in the provisioner to match the namespace you used. For example, if the Rook operator is running in the namespace "my-namespace" the provisioner value should be "my-namespace.rbd.csi.ceph.com".

Create the storage class.

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kubectl create -f deploy/examples/csi/rbd/storageclass.yaml

Note

As specified by Kubernetes, when using the Retain reclaim policy, any Ceph RBD image that is backed by a PersistentVolume will continue to exist even after the PersistentVolume has been deleted. These Ceph RBD images will need to be cleaned up manually using rbd rm.

Consume the storage: Wordpress sample

We create a sample app to consume the block storage provisioned by Rook with the classic wordpress and mysql apps. Both of these apps will make use of block volumes provisioned by Rook.

Start mysql and wordpress from the deploy/examples folder:

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kubectl create -f mysql.yaml
kubectl create -f wordpress.yaml

Both of these apps create a block volume and mount it to their respective pod. You can see the Kubernetes volume claims by running the following:

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kubectl get pvc

Example Output: kubectl get pvc

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NAME             STATUS    VOLUME                                     CAPACITY   ACCESSMODES   AGE
mysql-pv-claim   Bound     pvc-95402dbc-efc0-11e6-bc9a-0cc47a3459ee   20Gi       RWO           1m
wp-pv-claim      Bound     pvc-39e43169-efc1-11e6-bc9a-0cc47a3459ee   20Gi       RWO           1m

Once the wordpress and mysql pods are in the Running state, get the cluster IP of the wordpress app and enter it in your browser:

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kubectl get svc wordpress

Example Output: kubectl get svc wordpress

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NAME        CLUSTER-IP   EXTERNAL-IP   PORT(S)        AGE
wordpress   10.3.0.155   <pending>     80:30841/TCP   2m

You should see the wordpress app running.

If you are using Minikube, the Wordpress URL can be retrieved with this one-line command:

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echo http://$(minikube ip):$(kubectl get service wordpress -o jsonpath='{.spec.ports[0].nodePort}')

Note

When running in a vagrant environment, there will be no external IP address to reach wordpress with. You will only be able to reach wordpress via the CLUSTER-IP from inside the Kubernetes cluster.

Consume the storage: Toolbox

With the pool that was created above, we can also create a block image and mount it directly in a pod. See the Direct Block Tools topic for more details.

Teardown

To clean up all the artifacts created by the block demo:

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kubectl delete -f wordpress.yaml
kubectl delete -f mysql.yaml
kubectl delete -n rook-ceph cephblockpools.ceph.rook.io replicapool
kubectl delete storageclass rook-ceph-block

Advanced Example: Erasure Coded Block Storage

If you want to use erasure coded pool with RBD, your OSDs must use bluestore as their storeType. Additionally the nodes that are going to mount the erasure coded RBD block storage must have Linux kernel >= 4.11.

Attention

This example requires at least 3 bluestore OSDs, with each OSD located on a different node.

The OSDs must be located on different nodes, because the failureDomain is set to host and the erasureCoded chunk settings require at least 3 different OSDs (2 dataChunks + 1 codingChunks).

To be able to use an erasure coded pool you need to create two pools (as seen below in the definitions): one erasure coded and one replicated.

Attention

This example requires at least 3 bluestore OSDs, with each OSD located on a different node.

The OSDs must be located on different nodes, because the failureDomain is set to host and the erasureCoded chunk settings require at least 3 different OSDs (2 dataChunks + 1 codingChunks).

Erasure Coded CSI Driver

The erasure coded pool must be set as the dataPool parameter in storageclass-ec.yaml It is used for the data of the RBD images.

Node Loss

If a node goes down where a pod is running where a RBD RWO volume is mounted, the volume cannot automatically be mounted on another node. The node must be guaranteed to be offline before the volume can be mounted on another node.

Configure CSI-Addons

Deploy csi-addons controller and enable csi-addons sidecar as mentioned in the CSI Addons guide.

Handling Node Loss

Warning

Automated node loss handling is currently disabled, please refer to the manual steps to recover from the node loss. We are actively working on a new design for this feature. For more details see the tracking issue.

When a node is confirmed to be down, add the following taints to the node:

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kubectl taint nodes <node-name> node.kubernetes.io/out-of-service=nodeshutdown:NoExecute
kubectl taint nodes <node-name> node.kubernetes.io/out-of-service=nodeshutdown:NoSchedule

After the taint is added to the node, Rook will automatically blocklist the node to prevent connections to Ceph from the RBD volume on that node. To verify a node is blocklisted:

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kubectl get networkfences.csiaddons.openshift.io
NAME           DRIVER                       CIDRS                     FENCESTATE   AGE   RESULT
minikube-m02   rook-ceph.rbd.csi.ceph.com   ["192.168.39.187:0/32"]   Fenced       20s   Succeeded

The node is blocklisted if the state is Fenced and the result is Succeeded as seen above.

Node Recovery

If the node comes back online, the network fence can be removed from the node by removing the node taints:

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kubectl taint nodes <node-name> node.kubernetes.io/out-of-service=nodeshutdown:NoExecute-
kubectl taint nodes <node-name> node.kubernetes.io/out-of-service=nodeshutdown:NoSchedule-