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JIRA:

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Motivation

Support for multiple log directories per broker, aka JBOD (Just a Bunch Of Disks) came in KIP-112 and since then JBOD has been an important feature in Kafka, allowing it to run on large deployments with multiple local disks.

To ensure availability, when a partition leader fails, the controller should elect a new leader from one of the other in-sync replicas. But the controller does not check whether each leader is correctly performing its duties, instead the controller simply assumes that each broker is working correctly if it is still an active member of the cluster. In KRaft, cluster membership is based on timely heartbeat requests sent by each broker to the active controller. In ZooKeeper, cluster membership is based on an ephemeral zNode under /brokers/ids.

In KRaft mode, when a single log directory fails, the broker will be unable to be either a leader or a follower for any partitions in that log directory, but the controller will have no signal that it needs to update leadership and ISR for the replicas in that log directory, as the broker will continue to send heartbeat request.

In ZooKeeper mode when a log directory fails, the broker sends a notification to the controller which then sends a full LeaderAndIsr request to the broker, listing all the partitions for all log directories for that broker. The controller relies on per-partition error results from the broker to update leadership and ISR for the replicas in the failed log directory. Without this notification, the partitions with leadership on that log directory will not get a new leader assigned and would remain unavailable.

Support for KRaft in JBOD, was proposed and accepted back in KIP-589 — with a new RPC from the broker to the controller indicating the affected topic partitions in a failed log directory — but the implementation was never merged and concerns were raised with possible large requests from the broker to the controller.

KIP-833 was accepted, with plans to mark KRaft as production ready and deprecate ZooKeeper mode, but JBOD is still a missing feature in KRaft. This KIP aims to provide support for JBOD in KRaft, while avoiding any RPC having to list all the partitions in a log directory.

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