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

LocalPV-LVM Thin Provisioning

Table of Contents

Summary

This proposal charts out the workflow details to support creation of thin provisioned volumes.

Motivation

Goals

  • Able to provision thin volume in user specified VolumeGroup.

Non Goals

  • Reclaiming a space from VolumeGroup if thin pool doesn't contain volumes.

Proposal

User Stories

  • Thin provisioned volume will occupy storage only on demand and it will help a lot save cost on storage.

Implementation Details

  • User/Admin has to set thinProvision parameter to "yes" under storageclass parameters which informs driver to create thin provisioned volume.
  • 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 appropriate node based on scheduling attributes(like topology information, matching VG name and available capacity) and creates LVM volume resource by setting Spec.ThinProvision to yes along with other properties.
  • Once the LVMVolume resource is created corresponding node LVM volume controller reconcile LVM volume resource in the following way:
    • LVM controller will check Spec.ThinProvision field, if the field is set then controller will perform following operations:
      • Fetch information about existence of thin pool matching with volume group(<vgname>_thinpool).
        • If no such pool found then controller will create new pool with min(volume_request_size, VG_available_size) size along with thin volume. Command used to create thin pool & volume: lvcreate -L <min_pool_size> -T lvmvg/lvmvg_thinpool -V <volume_size> -n <volume_name> -y.
        • If there is a thin pool with <vg_name>_thinpool name then controller will create thin volume. Command used to create thin volume: lvcreate -T lvmvg/lvmvg_thinpool -V <volume_size> -n <volume_name> -y
      • If thin volume creation is successfull then controller will LVM volume resource as Ready.
  • After watching Ready status CSI driver will return success response to CreateVolume gRPC request.

Usage details

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

Test Plan

  • Provision an application on thin volume with a capacity less than underlying VG size and verify volume accessibility from application.
  • Provision an application on thin volume with a capacity greater than underlying VG size and verify volume accessibility from application.
  • Provision multiple thin volumes with capacities greater than underlying VG size and verify volume accessibility from applications.
  • Deprovision thin volume and verify that space should be reclaimed from thin pool.

Graduation Criteria

All testcases mentioned in Test Plan section need to be automated

Drawbacks

NA

Alternatives

The current implementation will create a new thin pool if there doesn't exist pool with name <vg_name>_thinpool in specified VolumeGroup(VG). This approach requires additional parameters to reclaim the space when there are no volumes exist on the pool and also adds complexities for day-2-operations like thin volume expansions(case where pool doesn't have enough space).

To mitigate above limitations we can hand over thin pool creation to the administrator (or) user. Once admin/user creates a thin pool user can specify pool name (or) regex of name in storage class(new parameter). Below is an example of a storage class with thin pool parameter

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: openebs-thin-lvm
provisioner: local.csi.openebs.io
parameters:
  storage: "lvm"
  volgroup: "lvmvg"
  thinPool: "$lvm.*pool$"
  • Once the user provisions a PVC by referring to the above storage class, external provisioner will send CreateVolume request to the CSI driver. Now, CSI driver will figure out nodes based on the requirement and creates LVMVolume on it.
  • Corresponding LVM controller will get an event and creates a thin volume on user specified pool.

Since we are not the creator of thin pool, so it is not necessary to support all features on it.

Limitations:

  • This approach will increase manual steps for day-1 operations, like creating a thin pool by figuring out an appropriate volume group(assume in larger environments it is not possible).
  • User has to manually reclaim the space by deleting a thin pool(if pool doesn't contain any volumes).
  • Application teams has to request admin for creation of multiple storage classes to provision thin volumes.

Conclusion Since alternative approach requires more steps to perform day1 operations it was decided to continue with current approach.