As I research, I know that the relationship between the PV and the PVC is one to one. How about the StorageClass, can one StorageClass can be mapped to multiple couple PV + PVC ?
Persistent Volume — low level representation of a storage volume.
Persistent Volume Claim — binding between a Pod and Persistent Volume.
Storage Class — allows for dynamic provisioning of Persistent Volumes.
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 policies, or to arbitrary policies determined by the cluster administrators. Kubernetes itself is unopinionated about what classes represent. This concept is sometimes called "profiles" in other storage systems
Storage Classes (SC)
StorageClass allows dynamic provisioning of Persistent Volumes, when PVC claims it.
StorageClass abstracts underlying storage provider.
StorageClass is used in conjunction with PVC that allow Pods to dynamically request a new storage.
StorageClass use provisioners that are specific to the storage platform or cloud provider to give Kubernetes access to the physical storage.
Each storage backend has own provisioner. Storage Backend is defined in the StorageClass component via provisioner attribute.
Related
I have a PV alpha-pv in the kubernetes cluster and have created a PVC matching the PV specs. The PV uses the Storage Class: slow. However, when I check the existence of Storage Class in Cluster there is no Storage Class existing and still my PVC was Bound to the PV.
How is this Possible when the Storage Class referred in the PV/PVC does not exists in the cluster?
If I don't mention the Storage Class in PVC, I get error message stating Storage Class Set. There is already an existing PV in the cluster which has RWO access modes, 1Gi storage size and with the Storage class named slow. But on checking the Storage Class details, there is no Storage Class resource in cluster.
If I add the Storage Class name slow in my PVC mysql-alpha-pvc, then the PVC binds to the PV. But I'm not clear how this happens when the Storage Class referred in PV/PVC named slow doesn't exist in the cluster.
Short answer
It depends.
Theory
One of the main purpose of using a storageClass is dynamic provisioning. That means that persistent volumes will be automatically provisioned once persistent volume claim requests for the storage: immediately or after pod using this PVC is created. See Volume binding mode.
Also:
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 policies, or to arbitrary
policies determined by the cluster administrators. Kubernetes itself
is unopinionated about what classes represent. This concept is
sometimes called "profiles" in other storage systems.
Reference.
How it works
If for instance kubernetes is used in cloud (Google GKE, Azure AKS or AWS EKS), they have already had predefined storageClasses, for example this is from Google GKE:
$ kubectl get storageclasses
NAME PROVISIONER RECLAIMPOLICY VOLUMEBINDINGMODE ALLOWVOLUMEEXPANSION AGE
premium-rwo pd.csi.storage.gke.io Delete WaitForFirstConsumer true 27d
standard (default) kubernetes.io/gce-pd Delete Immediate true 27d
standard-rwo pd.csi.storage.gke.io Delete WaitForFirstConsumer true 27d
So you can create PVC's and refer to storageClass, PV will be created for you.
Another scenario which you faced is you can create PVC and PV with any custom storageClassName only for binding purposes. Usually it's used for testing something locally. This is also called static provisioning.
In this case you can create "fake" storage class which won't exist in kubernetes server.
Please see an example with such type of binding:
It defines the StorageClass name manual for the PersistentVolume,
which will be used to bind PersistentVolumeClaim requests to this
PersistentVolume.
Useful links:
Kubernetes storage classes
Kubernetes dynamic provisioning
Kubernetes persistent volumes
Hello I already faced the same challenge but solved,
Please Make sure :
Your PVC configuration ( RW mode, Size, Name) is matching what is in the PV configuration
Claim name in your Deployment is equal to your PVC
Scale your deployment to (0) then to (1) you will find that it is
working smoothly
if you are facing any challenges you could run ( kubectl get events ) to know what is the blocker.
I have longhorn installed in my Kubernetes cluster for the local node storage. But I also have external storage mounted as seperate storageClass. So I have 2 storageClasses. Is there a solution to use both of them at the safe time for a pvc, a bit like RAID0 uses to different harddrives, to use the storage of both of them?
No, you only should give one storageClass name in a PVC. As far the k8s doc, Each StorageClass contains the fields provisioner, parameters, and reclaimPolicy, which are used when a PersistentVolume belonging to the class needs to be dynamically provisioned.
Also, StorageClasses are the foundation of dynamic provisioning, allowing cluster administrators to define abstractions for the underlying storage platform. Users simply refer to a StorageClass by name in the PersistentVolumeClaim (PVC) using the “storageClassName” parameter.
I am wondering what does "capacity" mean for Kubernetes persistent volume claim. Does that mean Kubernetes will schedule a pod with the volume you specified (e.g. 20 GB) on your cluster and that 20 GB space will be isolated or does it automatically scale down if you didn't fully use the 20 GB.
From the docs:
A PersistentVolumeClaim (PVC) is a request for storage by a user. It
is similar to a Pod. Pods consume node resources and PVCs consume PV
resources.
So, a PVC is not an actual storage but a request for the storage. A Persistent Volume (PV) is the actual storage:
A PersistentVolume (PV) is a piece of storage in the cluster that has
been provisioned by an administrator or dynamically provisioned using
Storage Classes.
A PVC resource gets bound to the actual PV resource if there is a matching PV available. The physical storage can then be used by the pods by using the reference to the PVC in the pods spec persistentVolumeClaim field under volumes.
I am wondering what does "capacity" mean for Kubernetes persistent
volume claim.
The PVC resource do not have a capacity field but has a storage field which implies that this PVC resource will bound to a PV object if the matching capacity on the PV object has at least the amount of storage requested by the PVC object if provisioned statically.
PVC can also be configured to allow dynamic provision which implies the actual storage will be provisioned as part of the PVC creation if there is not a matching PV already in the cluster.
Does that mean Kubernetes will schedule a pod with the volume you
specified (e.g. 20 GB) on your cluster and that 20 GB space will be
isolated or does it automatically scale down if you didn't fully use
the 20 GB.
If you have configured dynamic provisioning for the cluster and create a PVC object then a matching PV object is created internally and the PVC gets bound to the same. A pod needs to reference the PVC object to actually use it. Further, storage resources do not get resized automatically based on the actual consumption. Once created they will be persisted until deleted explicitly and they have the lifecycle independent of the pods.
I've been working with Kubernetes for quite a while, but still often got confused about Volume, PersistentVolume and PersistemtVolumeClaim. It would be nice if someone could briefly summarize the difference of them.
Volume - For a pod to reference a storage that is external , it needs volume spec. This volume can be from configmap, from secrets, from persistantvolumeclaim, from hostpath etc
PeristentVolume - It is representation of a storage that is made avaliable. The plugins for cloud provider enable to create this resource.
PeristentVolumeClaim - This claims specific resources and if the persistent volume is avaliable in namespaces match the claim requirement, then claim get tied to that Peristentvolume
At this point this PVC/PV aren't used. Then in Pod spec, pod makes use of claim as volumes and then the storage is attached to Pod
These are all in a Kubernetes application context. Too keep applications portable between different Kubernetes platforms, it is good to abstract away the infrastructure from the application. Here I will explain the Kubernetes objects that belongs to Application config and also to the Platform config. If your application runs on both e.g. GCP and AWS, you will need two sets of platform configs, one for GCP and one for AWS.
Application config
Volume
A pod may mount volumes. The source for volumes can be different things, e.g. a ConfigMap, Secret or a PersistentVolumeClaim
PersistentVolumeClaim
A PersistentVolumeClaim represents a claim of a specific PersistentVolume instance. For portability this claim can be for a specific StorageClass, e.g. SSD.
Platform config
StorageClass
A StorageClass represents PersistentVolume type with specific properties. It can be e.g. SSD. But the StorageClass is different on each platform, e.g. one definition on AWS, Azure, another on GCP or on Minikube.
PersistentVolume
This is a specific volume on the platform. And it may be different on platforms, e.g. awsElasticBlockStore or gcePersistentDisk. This is the instance that holds the actual data.
Minikube example
See Configure a Pod to Use a PersistentVolume for Storage for a full example on how to use PersistentVolume, StorageClass and Volume for a Pod using Minikube and a hostPath.
Kubernetes Dynamic Volume Provisioning gives a handy way to supply pods with dynamically-allocated storage volumes. For example, NFS Provisioner transparently spins up an NFS server and exposes that storage to client pods with Kubernetes volume interface, on-demand.
But how efficient is that? Does provisioner introduce another network protocol layer to communicate with client pod/container, in addition to NFS client-server communication? Or client pod/container talks directly to NFS server once the persistent volume claim was fulfilled?
As mentioned in the official documentation when you consider to allocate Persistent volumes to the Pods in the cluster there is a requirement to specify StorageClass in order to find appropriate provisioner (volume plugin) for the storage provider. StorageClass defines all the necessary parameters have to be passed to the storage provider and what provisioner: should be selected in Kubernetes API apiVersion: storage.k8s.io/v1 for the successful creation of PersistentVolume which corresponds to PersistentVolumeClaim request.
Find a list of the provisioners supported internally by Kubernetes here.
However, you are not limited only with internal volume plugins which are already included in provisioner: kubernetes.io module, but there are a lot of external provisioners which can be used for some specific scenarios, look at kubernetes-incubator/external-storage project.