title: Why do .index files exist in the kafka-log directory?
body: <p>I just made a new topic, and haven't produced any message yet.
  A file named in <code>00000000000000000000.index</code> was created in the directory <code>/tmp/kafka-logs-1/topicname-0/</code>, and the size of that file is really big.
  I opened that binary file in vi, and the contents are only "0000 0000 0000 0000..."
  What does this mean? What is this index file about?</p>