aiven-anton opened a new pull request, #22751:
URL: https://github.com/apache/kafka/pull/22751
In order to support node replacements during KRaft migration, and for a
cluster that has such replacements to remain possible to rollback, the storage
formatting tool must be able to produce a storage directory that can be rolled
back. Currently this is not the case, and the tool always produces a
meta.properties file with version=1.
When rollback is triggered on such a cluster that has had node replacements,
this results in a broker crash-loop with the below log.
```
Jul 02 14:56:32 kafka[25156]: [2026-07-02 14:56:32,647] ERROR Exiting Kafka
due to fatal exception during startup. (kafka.Kafka$)
java.lang.RuntimeException: Found unexpected
version in /srv/kafka/meta.properties. ZK-based brokers that are not migrating
only support version 0 (which is implicit when the `version` field is missing).
at
org.apache.kafka.metadata.properties.MetaPropertiesEnsemble.verify(MetaPropertiesEnsemble.java:489)
at
kafka.server.KafkaServer.startup(KafkaServer.scala:258)
at kafka.Kafka$.main(Kafka.scala:112)
at kafka.Kafka.main(Kafka.scala)
Jul 02 14:56:32 kafka[25156]: [2026-07-02 14:56:32,650] INFO shutting down
(kafka.server.KafkaServer)
```
This commit remedies this by introducing a new flag to the formatter tool
that makes it create a meta.properties with version=0 and compatible with
rollback. This matches what the broker itself does during a normal from-ZK
migration.
Documentation is updated both in the KRaft migration section and in the
generic section for provisioning nodes.
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