doc/java-broker/src/docbkx/runtime/Java-Broker-Runtime-Flow-To-Disk.xml

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<section xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="Java-Broker-Runtime-Flow-To-Disk">
  <title>Flow to Disk</title>
  <para>Flow to disk limits the amount of <link linkend="Java-Broker-Runtime-Memory">direct memory</link>
    that can be occupied by messages. Once this
    limit is reached any new transient messages will be
    transferred to disk. Newly arriving transient messages will continue to go to the disk until the
    cumulative size of all messages falls below the limit once again.</para>
  <para>By default the Broker makes 40% of the max direct available memory for messages. This memory is
    divided between all the queues across all virtual hosts defined on the Broker with a percentage
    calculated according to their current queue size. These calculations are refreshed periodically
    by the housekeeping cycle.</para>
  <para>For example if there are two queues, one containing 75MB and the second 100MB messages
    respectively and the Broker has 1GB direct memory with the default of 40% available for messages.
    The first queue will have a target size of 170MB and the second 230MB. Once 400MB is taken by
    messages, messages will begin to flow to disk. New messages will cease to flow to disk when
    their cumulative size falls beneath 400MB.</para>
  <para>Flow to disk is configured by Broker context variable
      <literal>broker.flowToDiskThreshold</literal>. It is expressed as a size in bytes and defaults
    to 40% of the JVM maximum heap size.</para>
  <para>Periodically, the log message <link linkend="Java-Broker-Appendix-Operation-Logging-Message-VHT-1008">VHT-1008</link>
    will be issued if messages are newly flowed to disk indicating the total amount of bytes evacuated from memory
    during the lifetime of the VirtualHost.</para>
</section>