Raw Block Volume Feature


Status Min K8s Version Max K8s Version external-provisioner Version external-attacher Version
Alpha 1.11 1.13 0.4 0.4
Alpha 1.13 1.13 1.0 1.0
Beta 1.14 - 1.1+ 1.1+


This page documents how to implement raw block volume support to a CSI Driver.

A block volume is a volume that will appear as a block device inside the container. A mounted (file) volume is volume that will be mounted using a specified file system and appear as a directory inside the container.

The CSI spec supports both block and mounted (file) volumes.

Implementing Raw Block Volume Support in Your CSI Driver

CSI doesn't provide a capability query for block volumes, so COs will simply pass through requests for block volume creation to CSI plugins, and plugins are allowed to fail with the InvalidArgument GRPC error code if they don't support block volumes. Kubernetes doesn't make any assumptions about which CSI plugins support blocks and which don't, so users have to know if any given storage class is capable of creating block volumes.

The difference between a request for a mounted (file) volume and a block volume is the VolumeCapabilities field of the request. Note that this field is an array and the created volume must support ALL of the capabilities requested, or else return an error. If the AccessType method of a VolumeCapability VolumeCapability_Block, then the capability is requesting a raw block volume. Unlike mount volumes, block volumes don't have any specific capabilities that need to be validated, although access modes still apply.

Block volumes are much more likely to support multi-node flavors of VolumeCapability_AccessMode_Mode than mount volumes, because there's no file system state stored on the node side that creates any technical impediments to multi-attaching block volumes. While there may still be good reasons to prevent multi-attaching block volumes, and there may be implementations that are not capable of supporting multi-attach, you should think carefully about what makes sense for your driver.

CSI plugins that support both mount and block volumes must be sure to check the capabilities of all CSI RPC requests and ensure that the capability of the request matches the capability of the volume, to avoid trying to do file-system-related things to block volumes and block-related things to file system volumes. The following RPCs specify capabilities that must be validated:

  • CreateVolume() (multiple capabilities)
  • ControllerPublishVolume()
  • ValidateVolumeCapabilities() (multiple capabilities)
  • GetCapacity() (see below)
  • NodeStageVolume()
  • NodePublishVolume()

Also, CSI plugins that implement the optional GetCapacity() RPC should note that that RPC includes capabilities too, and if the capacity for mount volumes is not the same as the capacity for block volumes, that needs to be handled in the implementation of that RPC.

Q: Can CSI plugins support only block volumes and not mount volumes? A: Yes! This is just the reverse case of supporting mount volumes only. Plugins may return InvalidArgument for any creation request with an AccessType of VolumeCapability_Mount.

Differences Between Block and Mount Volumes

The main difference between block volumes and mount volumes is the expected result of the NodePublish(). For mount volumes, the CO expects the result to be a mounted directory, at TargetPath. For block volumes, the CO expects there to be a device file at TargetPath. The device file can by a bind-mounted device from the hosts /dev file system, or it can be a device node created at that location using mknod().

It's desirable but not required to expose an unfiltered device node. For example, CSI plugins based on technologies that implement SCSI protocols should expect that pods consuming the block volumes they create may want to send SCSI commands to the device. This is something that should "just work" by default (subject to container capabilities) so CSI plugins should avoid anything that would break this kind of use case. The only hard requirement is that the device implements block reading/writing however.

For plugins with the RPC_STAGE_UNSTAGE_VOLUME capability, the CO doesn't care exactly what is placed at the StagingTargetPath, but it's worth noting that some CSI RPCs are allowed to pass the plugin either a staging path or a publish path, so it's important to think carefully about how NodeStageVolume() is implemented, knowing that either path could get used by the CO to refer to the volume later on. This is made more challenging because the CSI spec says that StagingTargetPath is always a directory even for block volumes.

Sidecar Deployment

The raw block feature requires the external-provisioner and external-attacher sidecars to be deployed.

Kubernetes Cluster Setup

The BlockVolume and CSIBlockVolume feature gates need to be enabled on all Kubernetes masters and nodes.

  • TODO: detail how Kubernetes API raw block fields get mapped to CSI methods/fields.