docs/topics/impala_data_cache.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="data_cache">

  <title>Data Cache for Remote Reads</title>

  <conbody>

    <p>
      When Impala compute nodes and its storage are not co-located, the network bandwidth
      requirement goes up as the network traffic includes the data fetch as well as the
      shuffling exchange traffic of intermediate results.
    </p>

    <note>
      This is an experimental feature in Impala 3.3 and is not generally supported.
    </note>

    <p>
      To mitigate the pressure on the network, you can enable the compute nodes to cache the
      working set read from remote filesystems, such as, remote HDFS data node, S3, ABFS, ADLS.
    </p>

    <p>
      To enable remote data cache, set the <codeph>--data_cache</codeph> Impala Daemon start-up
      flag as below:
    </p>

<codeblock>--data_cache=<varname>dir1</varname>,<varname>dir2</varname>,<varname>dir3</varname>,...:<varname>quota</varname></codeblock>

    <p>
      The flag is set to a list of directories, separated by <codeph>,</codeph>, followed by a
      <codeph>:</codeph>, and a capacity <codeph><varname>quota</varname></codeph> per
      directory.
    </p>

    <p>
      If set to an empty string, data caching is disabled.
    </p>

    <p>
      Cached data is stored in the specified directories.
    </p>

    <p>
      The specified directories must exist in the local filesystem of each Impala Daemon, or
      Impala will fail to start.
    </p>

    <p>
      In addition, the filesystem which the directory resides in must support hole punching.
    </p>

    <p>
      The cache can consume up to the <codeph>quota</codeph> bytes for each of the directories
      specified.
    </p>

    <p>
      The default setting for <codeph>--data_cache</codeph> is an empty string.
    </p>

    <p>
      For example, with the following setting, the data cache may use up to 1 TB, with 500 GB
      max in <codeph>/data/0</codeph> and <codeph>/data/1</codeph> respectively.
    </p>

<codeblock>--data_cache=/data/0,/data/1:500GB</codeblock>

  </conbody>

</concept>