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https://issues.apache.org/jira/browse/KAFKA-4084?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17085948#comment-17085948
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Evan Williams commented on KAFKA-4084:
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[~sql_consulting]
Many thanks for that!
I've just started trying to implement the patch, and just wanted to confirm a
few things:
# Do I *replace* the entire contents of /usr/share/java/kafka (where my Kafka
JAR's are), with the 96 JAR's from the tar file ? Or do I just *copy them in
addition* to the existing JAR's ? I tried replacing them all, however kafka was
just crashing on start.
# In regards to the patched JAR's, I see that they have "2.4.1" in the
filenames, ie: kafka-clients-2.4.1-SNAPSHOT.jar
Whereas my existing JARS's have 5.4.1. - will that effect things, when I do the
rolling restarts ? ie, mistmatching versions.
> automated leader rebalance causes replication downtime for clusters with too
> many partitions
> --------------------------------------------------------------------------------------------
>
> Key: KAFKA-4084
> URL: https://issues.apache.org/jira/browse/KAFKA-4084
> Project: Kafka
> Issue Type: Bug
> Components: controller
> Affects Versions: 0.8.2.2, 0.9.0.0, 0.9.0.1, 0.10.0.0, 0.10.0.1
> Reporter: Tom Crayford
> Priority: Major
> Labels: reliability
> Fix For: 1.1.0
>
>
> 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:
> bq. 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.
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