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Using Persistent Volumes as Virtual Machine disks

Author: Fabian Deutsch <[email protected]>

Introduction

Virtual Machines are able to have disks attached. They are not always required, but have some value if you need to persist data.

Kubernetes provides persistent storage through Persistent Volumes and Claims.

The purpose of this document is to describe the mechanism to use Persistent Volumes as a backing store for Virtual Machine disks.

Use-case

The primary use-case is to attach regular (writable) disks to Virtual Machines which are backed by Persistent Volumes.

API

This section describes how Persistent Volumes are referenced in the VM Resource type.

In general the referencing is aligned with how pods are consuming Persistent Volume Claims as described here

Today the VM.spec.domain reflects much of libvirt's domain xml specification. To communicate the new storage type through the API, an additional disk type PersistentVolumeClaim is accepted. In the case of a PersistentVolumeClaim type The disk/source/name attribute is used to name the claim to use.

Example with the following PV and PVC:

# For the sake of completeness a volume and claim
kind: PersistentVolume
metadata:
  name: pv001
  labels:
    release: "stable"
spec:
  capacity:
    storage: 5Gi
  iscsi:
    targetPortal: example.com:3260
    iqn: iqn.2013-07.com.example:iscsi-nopool/
    lun: 0
---
kind: PersistentVolumeClaim
metadata:
  name: disk-01
spec:
  resource:
    requests:
      storage: 4Gi
  selector:
    matchLabels:
release: "stable"

this is used by the VM in the following way:

kind: VirtualMachine
spec:
  domain:
    devices:
      disks:
      - type: PersistentVolumeClaim
        source:
          name: disk-01
        target:
          bus: scsi
          device: sda

Implementation

Flow

  1. User adds an existing PersistentVolumeClaim as described above to the VM instance.
  2. The VM Pod is getting scheduled on a host, the virt-handler identifies the Claim, translates it into the corresponding libvirt representation and includes it in the domain xml.

Note: The virt-controller does not do anything with the claim.

Note: In this flow, the claim is not used by the pod, the claim is only used by the virt-handler to identify the connection details to the storage.

Because VMs only accept block storage as disks, the handler can only accept claims which are backed by block storage types. Currently only iSCSI support is implemented.

virt-handler behavior

Once a VM is scheduled on a host, the virt-handler will transform the VM Spec into a libvirt domain xml.

During this transformation, every disk which is of type PersistentVolumeClaim will be converted into an adequate/equivalent libvirt disk type.

To achieve this, the virt-handler needs to look at volume attached to a claim, to identify the type and connection details. Note that the claim itself will contain the connection details if it is associated with a volume. In the case that the claim is not associated with a volume the launch of the VM will fail. If the type is iSCSI, then it can take the connection details and transform them into the correct libvirtd representation.

Example for an iSCSI volume:

  <disk type='network' device='disk'>
    <driver name='qemu' type='raw'/>
    <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-nopool/2'>
      <host name='example.com' port='3260'/>
    </source>
  </disk>

In the snippet above, most of the snippet is hard-coded, only the values for disk/source/name, disk/host/name, and disk/host/port are populated with the values from the Persistent Volume Claim.

Once the VM starts up, qemu will be connecting to the target to connect the disk.