These upgrade settings control the level of parallelism and the level of
disruption caused by an upgrade.
maxUnavailable controls the number of nodes that can be simultaneously
unavailable.
maxSurge controls the number of additional nodes that can be added to the
node pool temporarily for the time of the upgrade to increase the number of
available nodes.
(maxUnavailable + maxSurge) determines the level of parallelism (how many
nodes are being upgraded at the same time).
Note: upgrades inevitably introduce some disruption since workloads need to
be moved from old nodes to new, upgraded ones. Even if maxUnavailable=0,
this holds true. (Disruption stays within the limits of
PodDisruptionBudget, if it is configured.)
Consider a hypothetical node pool with 5 nodes having maxSurge=2,
maxUnavailable=1. This means the upgrade process upgrades 3 nodes
simultaneously. It creates 2 additional (upgraded) nodes, then it brings
down 3 old (not yet upgraded) nodes at the same time. This ensures that
there are always at least 4 nodes available.
Protobuf type google.container.v1.NodePool.UpgradeSettings
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