Storage in Kubernetes

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Using Pod Volumes

• Pod Volumes are a part of the Pod specification and have the storage reference hard coded in the Pod manifest
• This is not bad, but it doesn’t allow for flexible storage allocation
• Pod Volumes can be used for any storage type
• Also, the ConfigMap can be used to mount Pod Volumes

Example how simple share storage .

 

Managing Persistent Volumes

• PersistentVolumes (PV) are an API resource that represents specific storage
• PVs can be created manually, or automatically using StorageClass and storage provisioners
• Pods do not connect to PVs directly, but indirectly using PersistentVolumeClaim (PVC)

 

Configuring PersistentVolumeClaim

• PVCs allows Pods to connect to any type of storage that is provided at a
  specific site
• Site-specific storage needs to be created as a PersistentVolume, either manually or automatically using StorageClass
• Behind StorageClass a storage provisioner is required

Now let’s use pv in the pod:

 

StorageClass

• StorageClass is an API resource that allows storage to be automatically
  provisioned
• StorageClass can also be used as a property that connects PVC and PV without using an actual StorageClass resource
• Multiple StorageClass resources can co-exist in the same cluster to provide access to different types of storage
• For automatic working, one StorageClass must be set as default
  • kubectl patch storageclass mysc -p '{"metadata": {"annotations":{"storageclass.kubernetes.io/is-default-class":"true"}}}'

Using StorageClass

• To enable automatic provisioning, StorageClass needs a backing storage provisioner
• In the PV and PVC definition, a storageClass property can be set to connect to a specific StorageClass which is useful if multiple StorageClass resources are available
• If the storageClass property is not set, the PVC will get storage from the default StorageClass
• If also no default StorageClass is set, the PVC will get stuck in a status of Pending

Using an NFS Storage Provisioner

• The Storage Provisioner works with a StorageClass to automatically provide storage
• It runs as a Pod in the Kubernetes cluster, provided with access control configured through Roles, RoleBindings, and ServiceAccounts
• Once operational, you don’t have to manually create PersistentVolumes anymore

Requirements

• To create a storage provisioner, access permissions to the API are required
• Roles and RoleBindings are created to provide these permissions
• A ServiceAccount is created to connect the Pod to the appropriate RoleBinding

Configuring a Storage Provisioner

• On control:
  • Ubuntu: sudo apt install -y nfs-server
  • RHEL/Centos: sudo dnf install -y nfs-utils
• On other nodes:
  • Debian/Ubuntu: sudo apt install nfs-client
  • RHEL/Centos:  sudo dnf install nfs-utils nfs4-acl-tools 
• On control:
  • sudo mkdir /nfsexport
  • sudo sh -c 'echo "/nfsexport *(rw,no_root_squash)" > /etc/exports'
  • sudo systemctl restart nfs-server
• On other nodes: showmount -e control

 

ConfigMap

• A ConfigMap is an API resource used to store site-specific data
• A Secret is a base64 encoded ConfigMap
• ConfigMaps are used to store either environment variables, startup parameters or configuration files
• When a Configuration File is used in a ConfigMap or Secret, it is mounted as a volume to provide access to its contents

Creating a ConfigMap

• echo "hello world" > index.html
• kubectl create cm webindex --from-file=index.html
• kubectl describe cm webindex
• kubectl create deploy webserver --image=nginx
• kubectl edit deploy webserver

Let’s checkout how to use ConfigMap as a volume.

Now edit the deployment webserver and  add volume in the spec section in the same level as container section:

Let’s test it:

That proves that ConfigMap has sucessfully been mounted.

Lab: Setting up Storage

• Create a PersistentVolume, using the HostPath storage type to access the directory/storage
• Create a file /storage/index.html, containing the text "hello lab4"
• Run a Pod that uses an Nginx image and mounts the HostPath storage on the directory /usr/share/nginx/html
• On the running Pod, use kubectl exec to verify the existence of the file /usr/share/nginx/html

Solution:

The example yaml files you can find in the kubernetes documentation:

https://kubernetes.io/docs/home/ -> persistent volume -> Configure a Pod to Use a PersistentVolume for Storage