If you enable auto.leader.rebalance.enable (which is on by default), and you have a cluster with many partitions, there is a severe amount of replication downtime following a restart. This causes `UnderReplicatedPartitions` to fire, and replication is paused.
This is because the current automated leader rebalance mechanism changes leaders for all imbalanced partitions at once, instead of doing it gradually. This effectively stops all replica fetchers in the cluster (assuming there are enough imbalanced partitions), and restarts them. This can take minutes on busy clusters, during which no replication is happening and user data is at risk. Clients with acks=-1 also see issues at this time, because replication is effectively stalled.
To quote Todd Palino from the mailing list:
There is an admin CLI command to trigger the preferred replica election manually. There is also a broker configuration “auto.leader.rebalance.enable” which you can set to have the broker automatically perform the PLE when needed. DO NOT USE THIS OPTION. There are serious performance issues when doing so, especially on larger clusters. It needs some development work that has not been fully identified yet.
This setting is extremely useful for smaller clusters, but with high partition counts causes the huge issues stated above.
One potential fix could be adding a new configuration for the number of partitions to do automated leader rebalancing for at once, and stop once that number of leader rebalances are in flight, until they're done. There may be better mechanisms, and I'd love to hear if anybody has any ideas.