Alluxio with YARN

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This document discusses how to run Alluxio in a YARN environment. There are two approaches: running Alluxio alongside YARN, and running Alluxio as an application on top of YARN. We support both approaches, but recommend running Alluxio alongside YARN for a few reasons:

  • Alluxio servers are long-running daemons like HDFS. HDFS is not supposed to run on YARN.
  • Alluxio’s architecture is designed to have one worker per host, but this is not well supported by YARN.
  • Running Alluxio on top of YARN adds the additional complexity of managing Alluxio’s ApplicationMaster.
  • Multi-master high availability mode is not currently supported by our YARN integration.
  • The YARN integration does not support restarting workers in case they crash.

Running Alluxio Alongside YARN

Alluxio should be run alongside YARN so that all YARN nodes have access to a local Alluxio worker. For YARN and Alluxio to play nicely together on the same nodes, you must partition the node’s memory and compute resources between YARN and Alluxio. To do this, first determine how much memory and cpu will be needed by Alluxio, then subtract those resources from what YARN would otherwise use.

Memory

In yarn-site.xml, set

  1. yarn.nodemanager.resource.memory-mb = Total RAM - Other services RAM - Alluxio RAM

On Alluxio worker nodes, Alluxio RAM usage is 1GB plus the ramdisk size configured by alluxio.worker.memory.size. On Alluxio master nodes, Alluxio RAM usage is proportional to the number of files. Allocate at least 1GB. 32GB is recommended for a production deployment.

CPU vcores

In yarn-site.xml, set

  1. yarn.nodemanager.resource.cpu-vcores = Total cores - Other services vcores - Alluxio vcores

On Alluxio worker nodes, Alluxio needs only one or two vcores. On Alluxio master nodes, we recommend at least 4 vcores.

Restart YARN

After updating the YARN configuration, restart YARN so that it picks up the changes:

  1. ${HADOOP_HOME}/sbin/stop-yarn.sh
  2. ${HADOOP_HOME}/sbin/start-yarn.sh

Running Alluxio as a YARN Application

This section goes over how to run Alluxio servers (master and workers) on a Yarn cluster. In this mode, you run an Alluxio ApplicationMaster which communicates with YARN to request containers for launching the master and workers.

Prerequisites

A running YARN cluster

Alluxio downloaded locally

Launch Alluxio

Use the script integration/yarn/bin/alluxio-yarn.sh to start Alluxio. This script takes three arguments:

  1. The total number of Alluxio workers to start. (required)
  2. An HDFS path to distribute the binaries for Alluxio ApplicationMaster. (required)
  3. The Yarn name for the node on which to run the Alluxio Master (optional, defaults to ${ALLUXIO_MASTER_HOSTNAME})

For example, to launch an Alluxio cluster with 3 worker nodes, where an HDFS temp directory is hdfs://${HDFS_MASTER}:9000/tmp/ and the master hostname is ${ALLUXIO_MASTER}, you would run

  1. # If Yarn does not reside in `HADOOP_HOME`, set the environment variable `YARN_HOME` to the base path of Yarn.
  2. export HADOOP_HOME=<path to hadoop home>
  3. ${HADOOP_HOME}/bin/hadoop fs -mkdir hdfs://${HDFS_MASTER}:9000/tmp
  4. ${ALLUXIO_HOME}/integration/yarn/bin/alluxio-yarn.sh 3 hdfs://${HDFS_MASTER}:9000/tmp/ ${ALLUXIO_MASTER}

You may also start the Alluxio Master node separately from Yarn in which case the above startup will automatically detect the Master at the address provided and skip initialization of a new instance. This is useful if you have a particular host you’d like to run the Master on, which isn’t part of your Yarn cluster, like an AWS EMR Master Instance.

The script will launch an Alluxio Application Master on Yarn, which will then request containers for the Alluxio master and workers. You can check the YARN UI in the browser to watch the status of the Alluxio job.

Running the script will produce output containing something like

  1. INFO impl.YarnClientImpl: Submitted application application_1445469376652_0002

This application ID can be used to destroy the application by running

  1. ${HADOOP_HOME}/bin/yarn application -kill application_1445469376652_0002

The ID can also be found in the YARN web UI.

Verify the Cluster

Once you have the Alluxio application running, you can access the web UI at master_hostname:19999. To test IO functionality, configure alluxio.master.hostname=master_hostname in conf/alluxio-site.properties and run

  1. ${ALLUXIO_HOME}/bin/alluxio runTests

Configuration

To customize Alluxio master and worker with specific properties (e.g., tiered storage setup on each worker), see Configuration settings. To ensure your configuration can be read by both the ApplicationMaster and Alluxio master/workers, put alluxio-site.properties in /etc/alluxio/alluxio-site.properties.

Troubleshooting

If Alluxio doesn’t come up after running alluxio-yarn.sh, check the YARN application logs to see what happened. Some common issues to look out for:

  • Not enough memory/cpu: Make sure you have enough YARN capacity to run the Alluxio master and all workers. The CPU requirements are configured by alluxio.integration.master.resource.cpu and alluxio.integration.worker.resource.cpu (both 1 by default). Memory requirements are configured by alluxio.integration.master.resource.mem and alluxio.integration.worker.resource.mem, both 1024MB by default. Alluxio will avoid placing multiple workers on the same node.
  • Hostname mismatch: The hostname you specify for the Alluxio master must exactly match the hostname used by YARN for one of the hosts in the cluster. If you pass an IP address but YARN is using hostnames, the master cannot be started.

The Alluxio user mailing list is a good resource for getting help if the log messages are insufficient.

Configure yarn.application.classpath

To enable your YARN applications to communicate with Alluxio servers, add the Alluxio client jar to each YARN node, either by adding it to a directory on your yarn.application.classpath, or updating the classpath to include the client jar. For example,

  1. <property>
  2.  <name>yarn.application.classpath</name>
  3.  <value>$HADOOP_CONF_DIR,
  4.    $HADOOP_COMMON_HOME/share/hadoop/common/*,
  5.    $HADOOP_COMMON_HOME/share/hadoop/common/lib/*,
  6.    $HADOOP_HDFS_HOME/share/hadoop/hdfs/*,
  7.    $HADOOP_HDFS_HOME/share/hadoop/hdfs/lib/*,
  8.    $HADOOP_YARN_HOME/share/hadoop/yarn/*,
  9.    $HADOOP_YARN_HOME/share/hadoop/yarn/lib/*,
  10.    $ALLUXIO_HOME/client/*
  11.  </value>
  12. </property>

Make sure to replace $ALLUXIO_HOME with the path of your Alluxio installation.