Chapter 10. Software-defined Storage and Storage Management for Containers

Question 1

What is Software-defined storage and what aims it to do?

Answer 1

• represents storage virtualization
• aims to seperate the underlying storage hardware from the software that manages and provisions it

Question 2

What is the benefit of Software-defined Storage and what additional features can it provide?

Answer 2

• physical hardware from several sources can be combined, as a single storage pool
• additional: resiliency features like replication, erasure-coding or snapshots of the pooled resources

Question 3

What access methods does SDN allow?

Answer 3

• File
• Block
• Object

Question 4

What are examples for Software-Defined Storage implementations?

Answer 4

• Ceph
• Gluster
• MinIO
• OpenEBS
• TrueNAS

Question 5

What is Ceph and what does it support?

Answer 5

• implementation for SDN
• open source distributed storage system
• provides object, filesystem and block storage in a single unified storage cluster
• flexible, highly reliable, easy to manage

Question 6

What are volumes in Kubernetes and what are they used for?

Answer 6

• essentially a directory backed by a storage medium
• storage medium and its content are determined by the volume typ
• Kubernetes uses volumes to attach external storage to containers managed by pods

Question 7

In what relations is a volume to containers and how long-lived are they?

Answer 7

• volume linked to a pod
• shared among containers of that pod
• volume lifetime same as pod
• data remains between container restarts
• deletion of pod looses all data as well

Question 8

What is the new approach for storage plugins in Kubernetes?

Answer 8

• based on third party drivers implementing Container Storage Interface

Question 9

What does the new storage plugins approach in Kubernetes mean?

Answer 9

• CSI (Container Storage Interface) requires the Kubernetes Operators to download and install the desired CSI driver when needed

Question 10

What are some of the Volume Types supported by Kubernetes?

Answer 10

• azure Disk, to mount azure Data Disk on containers of a pod
• azuFile
• awsElasticBlockStorage
• cephfs: mount a CephFS volume on containers of a pod
• configMap: attack decoupled storage object, encapsulates configuration data, scripts, possibly entire filesystems
• hostPath: share a directory from the host with containers of a pod
• persistentVolumeClaim
• secret: encoded sensitive informations, such as password, keys, certificates, tokens etc

Question 11

What does the persistent volume subsystem enable?

Answer 11

• provides APIs to manage and consume storage
• for management of volumes PersistentVolume (PV) is used
• for consumation of volumes PersistentVolumeClaim (PVC) is used

Question 12

What are Persistent volumes (PVs) in Kubernetes? How are they provisioned?

Answer 12

• piece of storage in the cluster
• provisioned either by an administrator (statically) or dynamically using Storage Classes
• resource in a cluster just like node is as cluster resource
• are Volume plugins like regular volumes
• but lifecycle is independent of any inidividual pods that use PV
• API object captures details of the implementation of the storage

Question 13

How does the dynamic storage provisioning of PV in Kubernetes work abstractly?

Answer 13

• by using the Storage Class resource
• via a PersistentVolumeClaim a user sends a request for dynamic PV creation, which gets wired to the StorageClass resource

Question 14

What is a Persistent Volume Claime (PVC) in Kubernetes?

Answer 14

• request for storage by a user
• users request for PV resource based on size, access modes and volume type
• once a suitable PV is found, it is bound to PVC
• after a successful bind, the PVC can be used in a Pod to allow the containers access to the PV

Question 15

What are the steps for PVC-request by users in Kubernetes?

Answer 15

1. Dynamic or static provisioning of PVs
2. user requests a claim (PVC)
3. Once suitable PV is found, PV is bound to PVC
4. after successful bind the PVC is mounted to a pod to allow containers access to the PV

Question 16

What happens to a PV after a user is done with their activities?

Answer 16

• when the pod is deleted, the PVC may be detached from the PV
• thus the PV is released for future use
• once released the PV can either be deleted, retained, or recycled for future usage
• based on the reclaim policy the user has defined on the PV

Question 17

What benefit do open source Storage interfaces, that are not available as Kubernetes native storage plugins have?

Answer 17

• no vendor-lock-in
• decoupling of applications from underlying storage infrastructure
• ability to move clusters between infrastructure without impacting the application or need for manual configuration

Question 18

What are options for open source storage interfaces?

Answer 18

• Rook
• Longhorn

Question 19

What is Container Storage Interface (CSI) and what are its goals?

Answer 19

• standardization of volume interfaces
• aim: same volume plugin works with different container orchestrators out of the box

Question 20

What is the role of CSI?

Answer 20

• maintain the CSI specification
• maintain the protobuf, a protocol buffer language and platform-neutral, that serialized structured data

Question 21

What is the goal of the CSI specification?

Answer 21

• define APIs for dynamic provisioning, attaching, mounting, consumption and snapshot management of storage volumes
• additionally: define plugin configuration steps to be taken by container orchestrator, together with deployment configuration options