Managing storage is a distinct problem from managing compute resources. {product-title} leverages the Kubernetes persistent volume (PV) framework to allow administrators to provision persistent storage for a cluster. Using persistent volume claims (PVCs), developers can request PV resources without having specific knowledge of the underlying storage infrastructure.
PVCs are specific to a project and are created and used by developers as a means to use a PV. PV resources on their own are not scoped to any single project; they can be shared across the entire {product-title} cluster and claimed from any project. After a PV has been bound to a PVC, however, that PV cannot then be bound to additional PVCs. This has the effect of scoping a bound PV to a single namespace (that of the binding project).
PVs are defined by a PersistentVolume API object, which represents a piece
of existing networked storage in the cluster that has been provisioned by an
administrator. It is a resource in the cluster just like a node is a cluster
resource. PVs are volume plug-ins like Volumes, but have a lifecycle
independent of any individual
pod that uses the PV. PV
objects capture the details of the implementation of the storage, be that NFS,
iSCSI, or a cloud-provider-specific storage system.
|
Important
|
High-availability of storage in the infrastructure is left to the underlying storage provider. |
PVCs are defined by a PersistentVolumeClaim API object, which represents a
request for storage by a developer. It is similar to a pod in that pods consume
node resources and PVCs consume PV resources. For example, pods can request
specific levels of resources (e.g., CPU and memory), while PVCs can request
specific storage capacity and access
modes (e.g, they can be mounted once read/write or many times read-only).
PVs are resources in the cluster. PVCs are requests for those resources and also act as claim checks to the resource. The interaction between PVs and PVCs have the following lifecycle.
A cluster administrator creates some number of PVs. They carry the details of the real storage that is available for use by cluster users. They exist in the API and are available for consumption.
A user creates a PersistentVolumeClaim with a specific amount of storage
requested and with certain access modes. A control loop in the master watches
for new PVCs, finds a matching PV (if possible), and binds them together. The
user will always get at least what they asked for, but the volume may be in
excess of what was requested. To minimize the excess, {product-title} binds to
the smallest PV that matches all other criteria.
Claims remain unbound indefinitely if a matching volume does not exist. Claims are bound as matching volumes become available. For example, a cluster provisioned with many 50Gi volumes would not match a PVC requesting 100Gi. The PVC can be bound when a 100Gi PV is added to the cluster.
Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a pod. For those volumes that support multiple access modes, the user specifies which mode is desired when using their claim as a volume in a pod.
Once a user has a claim and that claim is bound, the bound PV belongs to the
user for as long as they need it. Users schedule pods and access their claimed
PVs by including a persistentVolumeClaim in their pod’s volumes block. See
below for syntax details.
When a user is done with a volume, they can delete the PVC object from the API which allows reclamation of the resource. The volume is considered "released" when the claim is deleted, but it is not yet available for another claim. The previous claimant’s data remains on the volume which must be handled according to policy.
The reclaim policy of a PersistentVolume tells the cluster what to do with
the volume after it is released. Currently, volumes can either be retained or
recycled.
Retention allows for manual reclamation of the resource. For those volume
plug-ins that support it, recycling performs a basic scrub on the volume (e.g.,
rm -rf /<volume>/*) and makes it available again for a new claim.
Each PV contains a spec and status, which is the specification and
status of the volume.
apiVersion: v1
kind: PersistentVolume
metadata:
name: pv0003
spec:
capacity:
storage: 5Gi
accessModes:
- ReadWriteOnce
persistentVolumeReclaimPolicy: Recycle
nfs:
path: /tmp
server: 172.17.0.2Generally, a PV will have a specific storage capacity. This is set using the
PV’s capacity attribute. See the
Kubernetes
Resource Model to understand the units expected by capacity.
Currently, storage capacity is the only resource that can be set or requested. Future attributes may include IOPS, throughput, etc.
A PersistentVolume can be mounted on a host in any way supported by the
resource provider. Providers will have different capabilities and each PV’s
access modes are set to the specific modes supported by that particular volume.
For example, NFS can support multiple read/write clients, but a specific NFS PV
might be exported on the server as read-only. Each PV gets its own set of access
modes describing that specific PV’s capabilities.
Claims are matched to volumes with similar access modes. The only two matching criteria are access modes and size. A claim’s access modes represent a request. Therefore, the user may be granted more, but never less. For example, if a claim requests RWO, but the only volume available was an NFS PV (RWO+ROX+RWX), the claim would match NFS because it supports RWO.
Direct matches are always attempted first. The volume’s modes must match or contain more modes than you requested. The size must be greater than or equal to what is expected. If two types of volumes (NFS and iSCSI, for example) both have the same set of access modes, then either of them will match a claim with those modes. There is no ordering between types of volumes and no way to choose one type over another.
All volumes with the same modes are grouped, then sorted by size (smallest to largest). The binder gets the group with matching modes and iterates over each (in size order) until one size matches.
The access modes are:
| Access Mode | CLI Abbreviation | Description |
|---|---|---|
ReadWriteOnce |
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The volume can be mounted as read-write by a single node. |
ReadOnlyMany |
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The volume can be mounted read-only by many nodes. |
ReadWriteMany |
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The volume can be mounted as read-write by many nodes. |
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Important
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A volume’s For example, a GCE Persistent Disk has |
The current recycling policies are:
| Recycling Policy | Description |
|---|---|
Retain |
Manual reclamation |
Recycle |
Basic scrub (e.g, |
|
Note
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Currently, only NFS and HostPath support the 'Recycle' recycling policy. |
A volumes can be found in one of the following phases:
| Phase | Description |
|---|---|
Available |
A free resource that is not yet bound to a claim. |
Bound |
The volume is bound to a claim. |
Released |
The claim has been deleted, but the resource is not yet reclaimed by the cluster. |
Failed |
The volume has failed its automatic reclamation. |
The CLI shows the name of the PVC bound to the PV.
Each PVC contains a spec and status, which is the specification and
status of the claim.
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: myclaim
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 8GiClaims use the same conventions as volumes when requesting storage with specific access modes.
Claims, like pods, can request specific quantities of a resource. In this case, the request is for storage. The same resource model applies to both volumes and claims.
Pods access storage by using the claim as a volume. Claims must exist in the
same namespace as the pod using the claim. The cluster finds the claim in the
pod’s namespace and uses it to get the PersistentVolume backing the claim.
The volume is then mounted to the host and into the pod:
kind: Pod
apiVersion: v1
metadata:
name: mypod
spec:
containers:
- name: myfrontend
image: dockerfile/nginx
volumeMounts:
- mountPath: "/var/www/html"
name: mypd
volumes:
- name: mypd
persistentVolumeClaim:
claimName: myclaim