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From Jun Rao <jun...@gmail.com>
Subject Re: Partition reassignment reversed
Date Tue, 02 Dec 2014 22:27:39 GMT
Did you run the --verify option (
http://kafka.apache.org/documentation.html#basic_ops_restarting) to check
if the reassignment process completes? Also, what version of Kafka are you
using?

Thanks,

Jun

On Mon, Dec 1, 2014 at 7:16 PM, Andrew Jorgensen <
ajorgensen@twitter.com.invalid> wrote:

> I unfortunately do not have any specific logs from these events but I will
> try and describe the events as accurately as possible to give an idea of
> the problem I saw.
>
> The odd behavior manifested itself when I bounced all of the kafka
> processes on each of the servers in a 12 node cluster. A few weeks prior I
> did a partition reassignment to add four new kafka brokers to the cluster.
> This cluster has 4 topics on it each with 350 partitions each, a retention
> policy of 6 hours, and a replication factor of 1. Originally I attempted to
> run a migration on all of the topics and partitions adding the 4 new nodes
> using the partition reassignment tool. This seemed to cause a lot of
> network congestion and according to the logs some of the nodes were having
> trouble talking to each other. The network congestion lasted for the
> duration of the migration and began to get better toward the end. After the
> migration I confirmed that data was being stored and served from the new
> brokers. Today I bounced each of the kafka processes on each of the brokers
> to pick up a change made to the log4j properties. After bouncing one
> processes I started seeing some strange errors on the four newer broker
> nodes that looked like:
>
> kafka.common.NotAssignedReplicaException: Leader 10 failed to record
> follower 7's position 0 for partition [topic-1,185] since the replica 7 is
> not recognized to be one of the assigned replicas 10 for partition
> [topic-2,185]
>
> and on the older kafka brokers the errors looked like:
>
> [2014-12-01 17:06:04,268] ERROR [ReplicaFetcherThread-0-12], Error for
> partition [topic-1,175] to broker 12:class kafka.common.UnknownException
> (kafka.server.ReplicaFetcherThread)
>
> I proceeded to bounce the rest of the kafka processes and after bouncing
> the rest the errors seemed to stop. It wasn’t until a few hours later I
> noticed that the amount of data stored on the 4 new kafka brokers had
> dropped off significantly. When I ran a describe for the topics in the
> errors it was clear that the assigned partitions had been reverted to a
> state prior to the original migration to add the 4 new brokers. I am unsure
> of why bouncing the kafka process would cause the state in zookeeper to get
> overwritten given that it had seemed to have been working for the last few
> weeks until the process was restarted. My hunch is that the controller
> keeps some state about the world pre-reassignment and removes that state
> after it detects that the reassignment happened successfully. In this case
> the network congestion on each of the brokers caused the controller not to
> get notified when all the reassignments were completed and thus kept the
> pre-assignement state around. When the process was bounced it read from
> zookeeper to get this state and reverted the existing scheme to the
> pre-assignment state. Has this behavior been observed before? Does this
> sound like a logical understanding of what happened in this case?
>
> --
> Andrew Jorgensen
> @ajorgensen

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