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• normal topic KRaft metadata;
• batch metadata may be stored on broker disk depending on the batch coordinator implementation (e.g. in a Kafka topic);
• brokers may require some limited amount of disk space to perform certain operations like object compaction;
• caching in the read path may optionally use broker disk instead of memory.
  

It is also worth to mention mentioning that Diskless topics are not meant to change the Kafka Storage API, but to have a separate request processing that takes care of the access to remote storage.

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