container-empty-disks.md

Provide temporary empty disks

Motivation

KubeVirt in combination with use-cases like the cluster-autoscaler can be used to scale and provision vm pools. With the VirtualMachineInstanceReplicaSet and cloud-init stateless pools can be instantiated and provisioned. Many of the used cloud images for these use-cases are booted with small disks with no additional free space, and/or run in read-only mode. A common pattern to store temporary data which should survive restarts or temporary data which is too big to fit in memory on temporary disks. The emptyDir in k8s provides a similar functionality.

To provide a similar functionality for our VirtualMachineInstances an emptyDisk volume type will be introduced.

Implementation

A new volume type emptyDisk is introduced. The only parameter it supports is capacity. The emptyDisk will be placed by virt-launcher in an emptyDir, to make sure that the kubelet can take care of clean-up and that ephemeral-store resource definitions apply to the emptyDisks too.

The definition look like this:

// EmptyDisk represents a temporary disk which shares the vms lifecycle
type EmptyDiskSource struct {
	// Capacity of the sparse disk.
	Capacity resource.Quantity `json:"capacity"`
}

A usage example:

apiVersion: kubevirt.io/v1alpha2
kind: VirtualMachineInstance
metadata:
  name: testvm
spec:
  terminationGracePeriodSeconds: 0
  domain:
    resources:
      requests:
        memory: 512M
    devices:
      disks:
      - name: disk1
        volumeName: containerDisk
        disk:
          dev: vda
      - name: disk2
        volumeName: emptyDisk
        disk:
          dev: vdb
  volumes:
    - name: containerDisk
      containerDisk:
        image: kubevirt/cirros-container-disk-demo:devel
    - name: emptyDisk
      emptyDisk:
        capacity: 20G

This example will boot cirros (which traditionally boots read-only) and attach a 20 GB temporary disk which can be used to write temporary data.

How and if this will be tied into the pods resources.requests.ephemeral-storage or resources.limits.ephemeral-storage is not yet clear and not part of this implementation.