Improve documentation for Kafka zero-copy
Improve documentation for Kafka zero-copy. Kafka combines pagecache and zero-copy to greatly improve message consumption efficiency. But zero-copy only works in PlaintextTransportLayer.
Reviewers: Divij Vaidya Guozhang Wang
<p>
This combination of pagecache and sendfile means that on a Kafka cluster where the consumers are mostly caught up you will see no read activity on the disks whatsoever as they will be serving data entirely from cache.
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+ TLS/SSL libraries operate at the user space (in-kernel <code>SSL_sendfile</code> is currently not supported by Kafka). Due to this restriction, <code>sendfile</code> could not be used when transport layer enables SSL protocol. For enabling
+ SSL configuration, refer to <code>security.protocol</code> and <code>security.inter.broker.protocol</code>
+ <p>
For more background on the sendfile and zero-copy support in Java, see this <a href="https://developer.ibm.com/articles/j-zerocopy/">article</a>.
<h4 class="anchor-heading"><a id="design_compression" class="anchor-link"></a><a href="#design_compression">End-to-end Batch Compression</a></h4>
