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One important use for this file is  is the current directory structure does not allow us to reload the broker's view of topic ID on startup (perhaps after a failure). It is necessary to persist this file to disk so this information can be reloaded. 

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It will be easy to update the file to include more fields in the future. This may assist with tooling purposes like mapping topic IDs to topic names.

In the JBOD mode, a partition's data can be moved from one disk to another. The partition metadata file would be copied during this process.

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