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The changes to storage formatting simply ensure the existence of two new fields of in an existing metadata file – meta.properties – at the log directory roots. The new fields are ignored by earlier versions of Kafka.

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• Partition reassignment across directories and across brokers involves different API calls — AlterPartitionReassignments and AlterReplicaLogDirs. Whilst reassigning partitions across brokers into a specific log directory is already possible, it involves an intricate sequence of calls previous prior calls to AlterReplicaLogDirs and expecting errors as a successful result. Once this work is done we can consolidate these two API calls by extending AlterPartitionReassignments to allow target log directories to be specified and deprecate AlterReplicaLogDirs. This can be done as part of a future KIP.
• The only way to know which log directory UUID corresponds to which log directory path is by reading the meta.properties  files in each broker. A future KIP should expand the DescribeLogDirs RPC response to include log directory UUIDs along with the system path for each log directory.
• Partition initialization can be optimized, by having the controller preselect a log directory for new partitions. This would avoid having to wait for the broker to send a AssignReplicasToDirs request to indicate the chosen log directory before it is safe for the broker to assume leadership of the partition. Maybe the controller could also take available storage in each log directory into account if the broker indicates the available storage space for each log directory as part of broker registration. This may be be proposed in a future KIP.

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