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This document describes ephemeral volumes in Kubernetes. Familiarity with volumes is suggested, in particular PersistentVolumeClaim and PersistentVolume.
Some application need additional storage but don't care whether that data is stored persistently across restarts. For example, caching services are often limited by memory size and can move infrequently used data into storage that is slower than memory with little impact on overall performance.
Other applications expect some read-only input data to be present in files, like configuration data or secret keys.
Ephemeral volumes are designed for these use cases. Because volumes follow the Pod's lifetime and get created and deleted along with the Pod, Pods can be stopped and restarted without being limited to where some persistent volume is available.
Ephemeral volumes are specified inline in the Pod spec, which simplifies application deployment and management.
Types of ephemeral volumes
Kubernetes supports several different kinds of ephemeral volumes for different purposes:
- emptyDir: empty at Pod startup, with storage coming locally from the kubelet base directory (usually the root disk) or RAM
- configMap, downwardAPI, secret: inject different kinds of Kubernetes data into a Pod
- CSI ephemeral volumes: similar to the previous volume kinds, but provided by special CSI drivers which specifically support this feature
- generic ephemeral volumes, which can be provided by all storage drivers that also support persistent volumes
secret are provided as
They are managed by kubelet on each node.
CSI ephemeral volumes must be provided by third-party CSI storage drivers.
Generic ephemeral volumes can be provided by third-party CSI storage drivers, but also by any other storage driver that supports dynamic provisioning. Some CSI drivers are written specifically for CSI ephemeral volumes and do not support dynamic provisioning: those then cannot be used for generic ephemeral volumes.
The advantage of using third-party drivers is that they can offer functionality that Kubernetes itself does not support, for example storage with different performance characteristics than the disk that is managed by kubelet, or injecting different data.
CSI ephemeral volumes
Kubernetes v1.16 [beta]
This feature requires the
CSIInlineVolume feature gate
to be enabled. It is enabled by default starting with Kubernetes 1.16.
Conceptually, CSI ephemeral volumes are similar to
secret volume types: the storage is managed locally on each
node and is created together with other local resources after a Pod has been
scheduled onto a node. Kubernetes has no concept of rescheduling Pods
anymore at this stage. Volume creation has to be unlikely to fail,
otherwise Pod startup gets stuck. In particular, storage capacity
aware Pod scheduling is not
supported for these volumes. They are currently also not covered by
the storage resource usage limits of a Pod, because that is something
that kubelet can only enforce for storage that it manages itself.
Here's an example manifest for a Pod that uses CSI ephemeral storage:
kind: Pod apiVersion: v1 metadata: name: my-csi-app spec: containers: - name: my-frontend image: busybox:1.28 volumeMounts: - mountPath: "/data" name: my-csi-inline-vol command: [ "sleep", "1000000" ] volumes: - name: my-csi-inline-vol csi: driver: inline.storage.kubernetes.io volumeAttributes: foo: bar
volumeAttributes determine what volume is prepared by the
driver. These attributes are specific to each driver and not
standardized. See the documentation of each CSI driver for further
CSI driver restrictions
CSI ephemeral volumes allow users to provide
directly to the CSI driver as part of the Pod spec. A CSI driver
volumeAttributes that are typically restricted to
administrators is NOT suitable for use in an inline ephemeral volume.
For example, parameters that are normally defined in the StorageClass
should not be exposed to users through the use of inline ephemeral volumes.
Cluster administrators who need to restrict the CSI drivers that are allowed to be used as inline volumes within a Pod spec may do so by:
volumeLifecycleModesin the CSIDriver spec, which prevents the driver from being used as an inline ephemeral volume.
- Using an admission webhook to restrict how this driver is used.
Generic ephemeral volumes
Kubernetes v1.23 [stable]
Generic ephemeral volumes are similar to
emptyDir volumes in the
sense that they provide a per-pod directory for scratch data that is
usually empty after provisioning. But they may also have additional
- Storage can be local or network-attached.
- Volumes can have a fixed size that Pods are not able to exceed.
- Volumes may have some initial data, depending on the driver and parameters.
- Typical operations on volumes are supported assuming that the driver supports them, including snapshotting, cloning, resizing, and storage capacity tracking.
kind: Pod apiVersion: v1 metadata: name: my-app spec: containers: - name: my-frontend image: busybox:1.28 volumeMounts: - mountPath: "/scratch" name: scratch-volume command: [ "sleep", "1000000" ] volumes: - name: scratch-volume ephemeral: volumeClaimTemplate: metadata: labels: type: my-frontend-volume spec: accessModes: [ "ReadWriteOnce" ] storageClassName: "scratch-storage-class" resources: requests: storage: 1Gi
Lifecycle and PersistentVolumeClaim
The key design idea is that the parameters for a volume claim are allowed inside a volume source of the Pod. Labels, annotations and the whole set of fields for a PersistentVolumeClaim are supported. When such a Pod gets created, the ephemeral volume controller then creates an actual PersistentVolumeClaim object in the same namespace as the Pod and ensures that the PersistentVolumeClaim gets deleted when the Pod gets deleted.
That triggers volume binding and/or provisioning, either immediately if
the StorageClass uses immediate volume binding or when the Pod is
tentatively scheduled onto a node (
binding mode). The latter is recommended for generic ephemeral volumes
because then the scheduler is free to choose a suitable node for
the Pod. With immediate binding, the scheduler is forced to select a node that has
access to the volume once it is available.
In terms of resource ownership,
a Pod that has generic ephemeral storage is the owner of the PersistentVolumeClaim(s)
that provide that ephemeral storage. When the Pod is deleted,
the Kubernetes garbage collector deletes the PVC, which then usually
triggers deletion of the volume because the default reclaim policy of
storage classes is to delete volumes. You can create quasi-ephemeral local storage
using a StorageClass with a reclaim policy of
retain: the storage outlives the Pod,
and in this case you need to ensure that volume clean up happens separately.
While these PVCs exist, they can be used like any other PVC. In particular, they can be referenced as data source in volume cloning or snapshotting. The PVC object also holds the current status of the volume.
Naming of the automatically created PVCs is deterministic: the name is
a combination of Pod name and volume name, with a hyphen (
-) in the
middle. In the example above, the PVC name will be
my-app-scratch-volume. This deterministic naming makes it easier to
interact with the PVC because one does not have to search for it once
the Pod name and volume name are known.
The deterministic naming also introduces a potential conflict between different Pods (a Pod "pod-a" with volume "scratch" and another Pod with name "pod" and volume "a-scratch" both end up with the same PVC name "pod-a-scratch") and between Pods and manually created PVCs.
Such conflicts are detected: a PVC is only used for an ephemeral volume if it was created for the Pod. This check is based on the ownership relationship. An existing PVC is not overwritten or modified. But this does not resolve the conflict because without the right PVC, the Pod cannot start.
Enabling the GenericEphemeralVolume feature allows users to create PVCs indirectly if they can create Pods, even if they do not have permission to create PVCs directly. Cluster administrators must be aware of this. If this does not fit their security model, they should use an admission webhook that rejects objects like Pods that have a generic ephemeral volume.
The normal namespace quota for PVCs still applies, so even if users are allowed to use this new mechanism, they cannot use it to circumvent other policies.
Ephemeral volumes managed by kubelet
CSI ephemeral volumes
- For more information on the design, see the Ephemeral Inline CSI volumes KEP.
- For more information on further development of this feature, see the enhancement tracking issue #596.
Generic ephemeral volumes
- For more information on the design, see the Generic ephemeral inline volumes KEP.