Volume Expansion

Status

Overview

A storage provider that allows volume expansion after creation, may choose to implement volume expansion either via a control-plane CSI RPC call or via node CSI RPC call or both as a two step process.

Implementing Volume expansion functionality

To implement volume expansion the CSI driver MUST:

1. Implement VolumeExpansion plugin capability.
2. Implement EXPAND_VOLUME controller capability or implement EXPAND_VOLUME node capability or both.

ControllerExpandVolume RPC call can be made when volume is ONLINE or OFFLINE depending on VolumeExpansion plugin capability. Where ONLINE and OFFLINE means:

1. ONLINE : Volume is currently published or available on a node.
2. OFFLINE : Volume is currently not published or available on a node.

NodeExpandVolume RPC call on the other hand - always requires volume to be published or staged on a node (and hence ONLINE). For block storage file systems - NodeExpandVolume is typically used for expanding the file system on the node, but it can be also used to perform other volume expansion related housekeeping operations on the node.

For details, see the CSI spec.

Deploying volume expansion functionality

The Kubernetes CSI development team maintains external-resizer Kubernetes CSI Sidecar Containers. This sidecar container implements the logic for watching the Kubernetes API for Persistent Volume claim edits and issuing ControllerExpandVolume RPC call against a CSI endpoint and updating PersistentVolume object to reflect new size.

This sidecar is needed even if CSI driver does not have EXPAND_VOLUME controller capability, in this case it performs a NO-OP expansion and updates PersistentVolume object. NodeExpandVolume is always called by Kubelet on the node.

For more details, see external-resizer.

Enabling Volume expansion for CSI volumes in Kubernetes

To expand a volume if permitted by the storage class, users just need to edit the persistent volume claim object and request more storage.

In Kubernetes 1.14 and 1.15, this feature was in alpha status and required enabling the following feature gate:

--feature-gates=ExpandCSIVolumes=true

Also in Kubernetes 1.14 and 1.15, online expansion had to be enabled explicitly:

--feature-gates=ExpandInUsePersistentVolumes=true

external-resizer and kubelet add appropriate events and conditions to persistent volume claim objects indicating progress of volume expansion operations.