Parallelize the eviction of pods with volumes

[timuthy]

How to categorize this issue?

/area performance
/kind enhancement
/priority 3

What would you like to be added:
MCM should provide a knob to configure the degree of parallel evictions for pods with volumes.

Why is this needed:
#262 established a serial eviction of pods with volumes to make the overall node drain process faster, esp. for cloud providers where many parallel detach/attach operations lead to rate limits and huge back-offs.

On some infrastructures and to some degree, a parallel eviction for pods with volumes may lead to a beneficial performance boost. Today, shoot clusters with many nodes often need a considerable amount of time to perform rolling updates. We see this aspect being one of the root causes that can be improved.

[timuthy]

Any opinion @gardener/mcm-maintainers?

[elankath]

Hi Tim, we can support this though I am doubtful whether we should make it configurable in the shoot YAML as it can possibly lead to severe degradation if operator configures a high value and then a fair amount of effort diagnosing/trouble-shooting such issues.

However, I think we can introduce a fixed degree of parallelism in evicting Pods with PVs after relevant testing of the behaviour on problematic providers like Azure. Now that we have implemented #781, today we wait for all volumes to be detached from the Node before proceeding to VM deletion. Hence those edge cases where still attached volumes cause the attach/detach controller to go into timeouts, is ameliorated.

[timuthy]

Thanks for the feedback @elankath. It wasn't meant to be an option for shoot owners. The degree of parallelism can also be configured by Gardenlet via its config.

[elankath]

That fine. A "hidden knob" like a CLI option parallel-eviction-limit=X should be OK.