Configuration as Filesystem

By using filesystem, configMaps are shared through virtiofs. In contrast with using disks for sharing configMaps, filesystem allows you to dynamically propagate changes on configMaps to VMIs (i.e. the VM does not need to be rebooted).

Limitation

Currently, VMIs cannot be live migrated since virtiofs does not support live migration.

Example of configMap:

apiVersion: v1
kind: ConfigMap
metadata:
  name: app-config
data:
  DATABASE: staging
  USERNAME: admin
  PASSWORD: secret

Example of VM using configMap as a filesystem:

apiVersion: kubevirt.io/v1
kind: VirtualMachineInstance
metadata:
  labels:
    special: vmi-fedora
  name: vmi-fedora
spec:
  domain:
    devices:
      filesystems:
        - name: config-fs
          virtiofs: {}
      disks:
      - disk:
          bus: virtio
        name: containerdisk
    machine:
      type: ""
    resources:
      requests:
        memory: 1024M
  terminationGracePeriodSeconds: 0
  volumes:
  - name: containerdisk
    containerDisk:
      image: quay.io/containerdisks/fedora:latest
  - cloudInitNoCloud:
      userData: |-
        #cloud-config
        chpasswd:
          expire: false
        password: fedora
        user: fedora
        bootcmd:
          # mount the ConfigMap
          - "sudo mkdir /mnt/app-config"
          - "sudo mount -t virtiofs config-fs /mnt/app-config"
    name: cloudinitdisk
  - configMap:
      name: app-config
    name: config-fs

To login and verify the VM run:

[fedora@vmi-fedora ~]$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda5       4.0G  461M  3.1G  13% /
devtmpfs        4.0M     0  4.0M   0% /dev
tmpfs           450M     0  450M   0% /dev/shm
tmpfs           180M  720K  179M   1% /run
tmpfs           450M     0  450M   0% /tmp
/dev/vda2       966M   61M  840M   7% /boot
/dev/vda3       100M   12M   89M  12% /boot/efi
/dev/vda5       4.0G  461M  3.1G  13% /home
config-fs       9.8G  910M  8.4G  10% /mnt/app-config
tmpfs            90M  4.0K   90M   1% /run/user/1000
[fedora@vmi-fedora ~]$ ls -lrth /mnt/app-config
total 0
lrwxrwxrwx. 1 root 107 15 Jan 15 16:20 USERNAME -> ..data/USERNAME
lrwxrwxrwx. 1 root 107 15 Jan 15 16:20 PASSWORD -> ..data/PASSWORD
lrwxrwxrwx. 1 root 107 15 Jan 15 16:20 DATABASE -> ..data/DATABASE