Set minAvailable:1 to unblock node upgrades

[imjasonh]

Fixes #3654

cc @nikhil-thomas

/kind bug

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Release Notes

Modify webhook PodDisruptionBudget minAvailable to 1, so node upgrades aren't blocked

[vdemeester]

/meow

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[bobcatfish]

/lgtm

[nikhil-thomas]

@imjasonh may be im missing how pdb works. 🤓 I am wondering whether this will be enough in our case as we have set the webhook replica count to 1 (

pipeline/config/webhook.yaml

Line 34 in 7a204f0

replicas: 1

)

so my concern is that if we set minAvailable: 1 in pdb where the replica count is 1 in the deployment spec, wouldn't it still block an eviction/drain.

[nikhil-thomas]

i experimented with minAvailable: 1

what i did

1. create pdb with minAvailable: 1 = 1
2. use `kubectl drain command

$ kubectl drain ip-10-0.us-east-2.compute.internal
evicting pod openshift-pipelines/tekton-pipelines-webhook-84cf757556-js76b
error when evicting pod "tekton-pipelines-webhook-84cf757556-js76b" (will retry after 5s): Cannot evict pod as it would violate the pod's disruption budget.

3. then i set the minAvailable: 0 in pdb. this avolided the drain block

$ kubectl drain ip-10-0.us-east-2.compute.internal --ignore-daemonsets                                                                                       130 ↵
node/ip-10-0-177-72.us-east-2.compute.internal cordoned
... 
evicting pod tekton-pipelines/tekton-pipelines-controller-68f85bfbd8-jwrz9
evicting pod tekton-pipelines/tekton-pipelines-webhook-76c87bfb9c-dbmxj

i don't fully understand, how autoscaller, pdb and replica count in a deployment mainfest work together.
Im just trying to make sure this patch will solve the problem for us. :)

observed same result on

• Kubernetes (Server Version: v1.18.12-gke.1210)
• OpenShift (Server Version: 4.6.18, Kubernetes Version: v1.19.0+f173eb4