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https://issues.apache.org/jira/browse/KAFKA-21285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18127479#comment-18127479
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Gergely Harmadás commented on KAFKA-21285:
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This seems like a genuine bug. As mentioned in the description Kafka has some
restrictions on certain config value changes. While the broker is running,
going from {{num.io.threads}} 16->8->6 pass the validation check "{_}value
should be at least half the current value{_}". However after the restart the
situation changes:
# After startup the broker first operates with the static config from
server.properties
# In the meantime the broker looks at the metadata log and tries to replay it
on top of the latest checkpoint
# When the replay is finished all intermediate edits are collapsed to their
final values. (change num.io.threads->8 is lost)
# Since we only have the final value (6) DynamicConfigPublisher tries to
update num.io.threads 16-> 6 which fails the "{_}value should be at least half
the current value{_}". check.
> Dynamic configs less than half the value in server.properties will not take
> effect after restart in kraft mode
> --------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-21285
> URL: https://issues.apache.org/jira/browse/KAFKA-21285
> Project: Kafka
> Issue Type: Bug
> Components: config
> Affects Versions: 4.3.1
> Reporter: yuzhou
> Assignee: Gergely Harmadás
> Priority: Major
>
> Dynamic configs less than half the value in server.properties will not take
> effect after restart in kraft mode
> * first, start kafka kraft mode, in server.properties, add
> "num.io.threads=16"
> * then, modify config from 16 to 8 then to 6
> bin/kafka-configs.sh --bootstrap-server localhost:9092 --entity-type brokers
> --entity-default --alter --add-config num.io.threads=8
> bin/kafka-configs.sh --bootstrap-server localhost:9092 --entity-type brokers
> --entity-default --alter --add-config num.io.threads=6
> * finally, restart broker.
> after restart, log show:
> [2026-10-15 15:45:10,838] INFO [DynamicConfigPublisher controller id=1]
> Updating cluster configuration : num.io.threads -> 6,min.insync.replicas -> 1
> (kafka.server.metadata.DynamicConfigPublisher)
> ...
> [2026-10-15 15:45:11,274] ERROR Failed to update broker configuration with
> configs :
> {advertised.listeners=PLAINTEXT://localhost:9092,CONTROLLER://localhost:9093,
> broker.id=1, controller.listener.names=CONTROLLER,
> controller.quorum.bootstrap.servers=localhost:9093,
> inter.broker.listener.name="PLAINTEXT",
> listener.security.protocol.map="CONTROLLER:PLAINTEXT,PLAINTEXT:PLAINTEXT,SSL:SSL,SASL_PLAINTEXT:SASL_PLAINTEXT,SASL_SSL:SASL_SSL",
> listeners=PLAINTEXT://:9092,CONTROLLER://:9093,
> log.dirs=/tmp/kraft-combined-logs, log.retention.check.interval.ms=300000,
> log.retention.hours=168, log.segment.bytes=1073741824, min.insync.replicas=1,
> node.id=1, num.io.threads=6, num.network.threads=3, num.partitions=1,
> num.recovery.threads.per.data.dir=2, offsets.topic.replication.factor=1,
> process.roles=broker,controller, share.coordinator.state.topic.min.isr=1,
> share.coordinator.state.topic.replication.factor=1,
> socket.receive.buffer.bytes=102400, socket.request.max.bytes=104857600,
> socket.send.buffer.bytes=102400, transaction.state.log.min.isr=1,
> transaction.state.log.replication.factor=1}
> (kafka.server.DynamicBrokerConfig)
> org.apache.kafka.common.config.ConfigException: Dynamic thread count update
> validation failed for num.io.threads=6, value should be at least half the
> current value 16
> at
> org.apache.kafka.server.DynamicThreadPool.validateReconfiguration(DynamicThreadPool.java:51)
> at
> kafka.server.BrokerDynamicThreadPool.validateReconfiguration(DynamicBrokerConfig.scala:647)
> at
> kafka.server.DynamicBrokerConfig.$anonfun$processReconfiguration$3(DynamicBrokerConfig.scala:466)
> at
> java.base/java.util.concurrent.CopyOnWriteArrayList.forEach(CopyOnWriteArrayList.java:807)
> at
> kafka.server.DynamicBrokerConfig.processReconfiguration(DynamicBrokerConfig.scala:464)
> at
> kafka.server.DynamicBrokerConfig.updateCurrentConfig(DynamicBrokerConfig.scala:437)
> at
> kafka.server.DynamicBrokerConfig.$anonfun$updateDefaultConfig$1(DynamicBrokerConfig.scala:293)
> at org.apache.kafka.server.util.LockUtils.inLock(LockUtils.java:96)
> at org.apache.kafka.server.util.LockUtils.inWriteLock(LockUtils.java:115)
> at
> kafka.server.DynamicBrokerConfig.updateDefaultConfig(DynamicBrokerConfig.scala:288)
> at
> kafka.server.BrokerConfigHandler.processConfigChanges(ConfigHandler.scala:150)
> at
> kafka.server.metadata.DynamicConfigPublisher.$anonfun$onMetadataUpdate$5(DynamicConfigPublisher.scala:81)
> at scala.Option.foreach(Option.scala:437)
> at
> kafka.server.metadata.DynamicConfigPublisher.$anonfun$onMetadataUpdate$2(DynamicConfigPublisher.scala:74)
> at java.base/java.util.HashMap$KeySet.forEach(HashMap.java:1008)
> at
> kafka.server.metadata.DynamicConfigPublisher.$anonfun$onMetadataUpdate$1(DynamicConfigPublisher.scala:57)
> at
> kafka.server.metadata.DynamicConfigPublisher.$anonfun$onMetadataUpdate$1$adapted(DynamicConfigPublisher.scala:56)
> at scala.Option.foreach(Option.scala:437)
> at
> kafka.server.metadata.DynamicConfigPublisher.onMetadataUpdate(DynamicConfigPublisher.scala:56)
> at
> kafka.server.metadata.BrokerMetadataPublisher.onMetadataUpdate(BrokerMetadataPublisher.scala:190)
> at
> org.apache.kafka.image.loader.MetadataLoader.initializeNewPublishers(MetadataLoader.java:332)
> at
> org.apache.kafka.image.loader.MetadataLoader.lambda$scheduleInitializeNewPublishers$0(MetadataLoader.java:288)
> at
> org.apache.kafka.queue.KafkaEventQueue$EventContext.run(KafkaEventQueue.java:134)
> at
> org.apache.kafka.queue.KafkaEventQueue$EventHandler.handleEvents(KafkaEventQueue.java:217)
> at
> org.apache.kafka.queue.KafkaEventQueue$EventHandler.run(KafkaEventQueue.java:188)
> at java.base/java.lang.Thread.run(Thread.java:840)
>
> the actcual num.io.threads is 16 , expected 6
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