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Using file-system based PersistentVolumes as virtual disk backends

Introduction

So far Kubernetes PersistentVolumes are remote file-systems attached to a pod at runtime. This is currently the only way of exposing remote or local storage to pods, through a file-system. Despite the fact that storage is exposed as file-systems to pod, KubeVirt has no mechanism to use these plain file-system based PVs to act as backends for virtual disks.

Goals

This proposal is about adding a simple mechanism to allow using file-system based PersistentVolumes for storing file images backing virtual disks.

The focus is primarily on providing the right cluster levels mechanics to enable future changes for improving usability, performance, or other characteristics.

Non-Goals

  • Provide a totally automatic solution
  • Cover snapshots explicitly

Use-case

The primary use-case is to allow a VMI to use PVs as backend storage for their virtual disks.

Design Overview

The mechanism proposed in this proposal is leveraging Kubernetes to attach the remote storage as a file-system to a container, to use this file-system to store a disk image to act as a backend storage to the VMI's disk.

To keep the story compatability between the block and file-system storage, this proposal assumes that the file-system based storage will also just support a 1:1 mapping between the virtual disk and volume. This aligns with the fact that a block volume can also just back a single virtual disk.

API

The general API to use PersistentVolume claims as virtual disk backends was introduced with direct PV proposal.

The change suggested by this proposal, does not require any API change.

To use a PV as a virtual disk backend, a user needs to create a claim for the required PV, this claim is then used as a disk source for a virtual disk.

An example:

kind: VMI
spec:
  domain:
    devices:
      disks:
      - name: root-disk
        volumeName: my-fs-store
  volumes:
  - name: my-fs-store
    persistentVolumeClaim: my-fs-claim

Here the user attaches the PersistentVolumeClaim my-fs-claim as a disk to a VMI.

Storage Type Inference

The system needs to know if the referenced volume needs to be treated as a block or file-system volume. Since Kubernetes 1.9 this information can be inferred from the existing PV metadata.

Implementation

Volume layout

A file-system based volume will contain only a single image file, this file must be named disk.img. The format of the file must be raw.

The file-system layout of a mounted volume then looks like:

/disk.img

In future we might want to add additional files to carry metadata, but the limit of a single image file per volume must not be changed.

Additional Notes

Virtfs

Virtfs might allow us to directly use file-systems as a backing store for virtual machines. This API design should not contradict with this use-case, but it will probably depend on the driver field mentioned above to signal the system how the PV has to be consumed (mounted).