Liveness and Readiness Probes¶
It is possible to configure Liveness and Readiness Probes in a similar fashion like it is possible to configure Liveness and Readiness Probes on Containers.
Liveness Probes will effectively stop the VirtualMachineInstance if they fail, which will allow higher level controllers, like VirtualMachine or VirtualMachineInstanceReplicaSet to spawn new instances, which will hopefully be responsive again.
Readiness Probes are an indicator for Services and Endpoints if the VirtualMachineInstance is ready to receive traffic from Services. If Readiness Probes fail, the VirtualMachineInstance will be removed from the Endpoints which back services until the probe recovers.
Watchdogs focus on ensuring that an Operating System is still responsive. They
complement the probes which are more workload centric. Watchdogs require kernel
support from the guest and additional tooling like the commonly used watchdog
binary.
Exec probes are Liveness or Readiness probes specifically intended for VMs.
These probes run a command inside the VM and determine the VM ready/live state based
on its success.
For running commands inside the VMs, the qemu-guest-agent package is used.
A command supplied to an exec probe will be wrapped by virt-probe in the
operator and forwarded to the guest.
Define a HTTP Liveness Probe¶
The following VirtualMachine configures a HTTP Liveness Probe
via spec.template.spec.livenessProbe.httpGet, which will query port 1500 of the
VirtualMachine, after an initial delay of 120 seconds. The
VirtualMachine itself installs and runs a minimal HTTP server on
port 1500 via cloud-init.
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
name: example-http-liveness-probe
spec:
runStrategy: Always
template:
spec:
domain:
devices:
disks:
- disk:
bus: virtio
name: containerdisk
- disk:
bus: virtio
name: cloudinit
rng: {}
resources:
requests:
memory: 1Gi
livenessProbe:
initialDelaySeconds: 120
periodSeconds: 20
httpGet:
port: 1500
timeoutSeconds: 10
terminationGracePeriodSeconds: 0
volumes:
- name: containerdisk
containerDisk:
image: quay.io/containerdisks/centos-stream:9
- name: cloudinit
cloudInitNoCloud:
userData: |-
#cloud-config
password: centos
user: centos
chpasswd: { expire: False }
bootcmd:
- ["sudo", "dnf", "install", "-y", "nmap-ncat"]
- ["sudo", "systemd-run", "--unit=httpserver", "nc", "-klp", "1500", "-e", '/usr/bin/echo -e HTTP/1.1 200 OK\\nContent-Length: 12\\n\\nHello World!']
Define a TCP Liveness Probe¶
The following VirtualMachine configures a TCP Liveness Probe via
spec.template.spec.livenessProbe.tcpSocket, which will query port 1500 of the
VirtualMachine, after an initial delay of 120 seconds. The
VirtualMachine itself installs and runs a minimal HTTP server on
port 1500 via cloud-init.
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
name: example-tcp-liveness-probe
spec:
runStrategy: Always
template:
spec:
domain:
devices:
disks:
- disk:
bus: virtio
name: containerdisk
- disk:
bus: virtio
name: cloudinit
rng: {}
resources:
requests:
memory: 1Gi
livenessProbe:
initialDelaySeconds: 120
periodSeconds: 20
tcpSocket:
port: 1501
timeoutSeconds: 10
terminationGracePeriodSeconds: 0
volumes:
- name: containerdisk
containerDisk:
image: quay.io/containerdisks/centos-stream:9
- name: cloudinit
cloudInitNoCloud:
userData: |-
#cloud-config
password: centos
user: centos
chpasswd: { expire: False }
bootcmd:
- ["sudo", "dnf", "install", "-y", "nmap-ncat"]
- ["sudo", "systemd-run", "--unit=httpserver", "nc", "-klp", "1500", "-e", '/usr/bin/echo -e HTTP/1.1 200 OK\\nContent-Length: 12\\n\\nHello World!']
Define Readiness Probes¶
Readiness Probes are configured in a similar way like liveness probes.
Instead of spec.template.spec.livenessProbe, spec.template.spec.readinessProbe
needs to be filled:
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
name: example-readiness-probe
spec:
runStrategy: Always
template:
spec:
domain:
devices:
disks:
- disk:
bus: virtio
name: containerdisk
- disk:
bus: virtio
name: cloudinit
rng: {}
resources:
requests:
memory: 1Gi
readinessProbe:
initialDelaySeconds: 120
periodSeconds: 20
timeoutSeconds: 10
failureThreshold: 3
successThreshold: 3
httpGet:
port: 1500
terminationGracePeriodSeconds: 0
volumes:
- name: containerdisk
containerDisk:
image: quay.io/containerdisks/centos-stream:9
- name: cloudinit
cloudInitNoCloud:
userData: |-
#cloud-config
password: centos
user: centos
chpasswd: { expire: False }
bootcmd:
- ["sudo", "dnf", "install", "-y", "nmap-ncat"]
- ["sudo", "systemd-run", "--unit=httpserver", "nc", "-klp", "1500", "-e", '/usr/bin/echo -e HTTP/1.1 200 OK\\nContent-Length: 12\\n\\nHello World!']
Note that in the case of Readiness Probes, it is also possible to set a
failureThreshold and a successThreashold to only flip between ready
and non-ready state if the probe succeeded or failed multiple times.
Dual-stack considerations¶
Some context is needed to understand the limitations imposed by a dual-stack network configuration on readiness - or liveness - probes. Users must be fully aware that a dual-stack configuration is currently only available when using a masquerade binding type. Furthermore, it must be recalled that accessing a VM using masquerade binding type is performed via the pod IP address; in dual-stack mode, both IPv4 and IPv6 addresses can be used to reach the VM.
Dual-stack networking configurations have a limitation when using HTTP / TCP
probes - you cannot probe the VMI by its IPv6 address. The reason for this
is the host field for both the
HTTP
and
TCP
probe actions default to the pod's IP address, which is currently always the
IPv4 address.
Since the pod's IP address is not known before creating the VMI, it is not possible to pre-provision the probe's host field.
Defining a Watchdog¶
A watchdog is a more VM centric approach where the responsiveness of the
Operating System is focused on. One can configure the i6300esb watchdog
device:
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
name: example-watchdog
spec:
runStrategy: Always
template:
spec:
domain:
devices:
watchdog:
name: mywatchdog
i6300esb:
action: "poweroff"
disks:
- disk:
bus: virtio
name: containerdisk
- disk:
bus: virtio
name: cloudinit
rng: {}
resources:
requests:
memory: 1Gi
terminationGracePeriodSeconds: 0
volumes:
- name: containerdisk
containerDisk:
image: quay.io/containerdisks/fedora:latest
- name: cloudinit
cloudInitNoCloud:
userData: |-
#cloud-config
password: fedora
user: fedora
chpasswd: { expire: False }
packages:
busybox
The example above configures it with the poweroff action. It defines what will
happen if the OS can't respond anymore. Other possible actions are reset
and shutdown. The VM in this example will have the device exposed
as /dev/watchdog. This device can then be used by the watchdog
binary. For example, if root executes this command inside the VM:
The watchdog will send a heartbeat every two seconds to /dev/watchdog and
after four seconds without a heartbeat the defined action will be executed. In
this case a hard poweroff.
Defining Guest-Agent Ping Probes¶
Guest-Agent probes are based on qemu-guest-agent guest-ping. This will ping
the guest and return an error if the guest is not up and running. To easily
define this on VM spec, specify guestAgentPing: {} in VM's
spec.template.spec.readinessProbe. virt-controller will translate this
into a corresponding command wrapped by virt-probe.
Note: You can only define one of the type of probe, i.e. guest-agent exec or ping probes.
Important: If the qemu-guest-agent is not installed and enabled inside the VM, the probe will fail. Many images don't enable the agent by default so make sure you either run one that does or enable it.
Make sure to provide enough delay and failureThreshold for the VM and the agent to be online.
In the following example the Fedora image does have qemu-guest-agent available
by default. Nevertheless, in case qemu-guest-agent is not installed, it will be
installed and enabled via cloud-init as shown in the example below. Also,
cloud-init assigns the proper SELinux context, i.e. virt_qemu_ga_exec_t, to the
/tmp/healthy.txt file. Otherwise, SELinux will deny the attempts to open the
/tmp/healthy.txt file causing the probe to fail.
apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
name: example-agent-readiness-probe
spec:
runStrategy: Always
template:
spec:
domain:
devices:
disks:
- name: containerdisk
disk:
bus: virtio
- name: cloudinitdisk
disk:
bus: virtio
rng: {}
resources:
requests:
memory: 1Gi
readinessProbe:
exec:
command: ["cat", "/tmp/healthy.txt"]
failureThreshold: 10
initialDelaySeconds: 120
periodSeconds: 10
timeoutSeconds: 5
terminationGracePeriodSeconds: 180
volumes:
- name: containerdisk
containerDisk:
image: quay.io/containerdisks/fedora
- name: cloudinitdisk
cloudInitNoCloud:
userData: |
#cloud-config
chpasswd:
expire: false
password: password
user: fedora
packages:
qemu-guest-agent
runcmd:
- ["touch", "/tmp/healthy.txt"]
- ["sudo", "chcon", "--type", "virt_qemu_ga_exec_t", "/tmp/healthy.txt"]
- ["sudo", "systemctl", "enable", "--now", "qemu-guest-agent"]
Note that, in the above example if SELinux is not installed in your container
disk image, the command chcon should be removed from the VM manifest shown.
Otherwise, the chcon command will fail.
The .status.ready field will switch to true indicating that probes are
returning successfully:
Additionally, the following command can be used inside the VM to watch the incoming qemu-ga commands: