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title authors owners creation-date last-updated status
LVM-LocalPV Shared Volume
@pawanpraka1
@kmova
2021-06-16
2021-06-16
Implemented

LVM-LocalPV Shared Volume

Table of Contents

Summary

This proposal charts out the workflow details to support sharing of LVM volume between different instances of applications on the same node.

Motivation

Goals

  • Able to access the LVM volume from multiple application instances on the same node.

Non-Goals

  • Accessing LVM volume from different nodes other than where LVM is accessible.

Proposal

Implementation Details

  • User/Admin has to set shared parameter to "yes" under storageclass parameters to make volume to be available in more than one application instance.
  • During volume provisioning time external-provisioner will read all key-value pairs that are specified under referenced storageclass and pass information to CSI driver as payload for CreateVolume gRPC request.
  • After receiving the CreateVolume request CSI driver will pick the appropriate node based on scheduling attributes(like topology information, matching VG name, available capacity) and creates LVM volume resource with node information as well as other attributes like VG name, desired capacity information, and also property of volume that states whether it can be accessed between multiple instances or not.
  • After creation of LVM volume resource corresponding LVM volume controller gets an event and create LVM volume on desired volume group and marks LVM volume resource status as Ready.
  • After watching Ready status CSI driver will return success response to CreateVolume gRPC request.

How can be volume shared between multiple instances

  • During application mount time i.e NodePublishVolume request CSI driver will verify whether underlying LVM volume is already mounted or not. If volume is already mounted then driver will verify whether target path is same as mounted path if both are different then driver will allow mounting of volume only when spec.shared is set else an error will be returned. In case if the target path and mounted path same then success response will be returned.
// VolumeInfo defines LVM info
type VolumeInfo struct {

	// OwnerNodeID is the Node ID where the volume group is present which is where
	// the volume has been provisioned.
	// OwnerNodeID can not be edited after the volume has been provisioned.
	OwnerNodeID string `json:"ownerNodeID"`

	// VolGroup specifies the name of the volume group where the volume has been created.
	VolGroup string `json:"volGroup"`

	// VgPattern specifies the regex to choose volume groups where volume
	// needs to be created.
	VgPattern string `json:"vgPattern"`

	// Capacity of the volume
	Capacity string `json:"capacity"`

	// Shared specifies whether the volume can be shared among multiple pods.
	// If it is not set to "yes", then the LVM LocalPV Driver will not allow
	// the volumes to be mounted by more than one pods.
	Shared string `json:"shared,omitempty"`

	// ThinProvision specifies whether logical volumes can be thinly provisioned.
	// If it is set to "yes", then the LVM LocalPV Driver will create
	// thinProvision i.e. logical volumes that are larger than the available extents.
	ThinProvision string `json:"thinProvision,omitempty"`
}

In above structure, Shared value controls shared property of volume.

Usage details

  1. User/Admin can configure shared value to yes under storageclass parameter.
  apiVersion: storage.k8s.io/v1
  kind: StorageClass
  metadata:
    name: nvme-lvmsc
  allowVolumeExpansion: true
  provisioner: local.csi.openebs.io
  parameters:
    volgroup: "lvmvg"
    shared: "yes"

Note: If a shared field is unspecified then defaults to No that means volume can't be shared among multiple instances.

Test Plan

  • Provision multiple application instances by setting a shared property to yes and verify that all application instances should get into a running state.
  • Provision multiple application instances by setting a shared property to No and verify that among multiple instances, only one should be in running state.
  • Provision single application instance without specifying any shared property and verify that application should be able to access the volume.

Graduation Criteria

All testcases mentioned in Test Plan section need to be automated

Drawbacks

NA

Alternatives

NA