Chapter 4. Configuring Rack Awareness on HDP

Use the following instructions to configure rack awareness on a HDP cluster:

 1. Create a Rack Topology Script

Topology scripts are used by Hadoop to determine the rack location of nodes. This information is used by Hadoop to replicate block data to redundant racks.

  1. Create a topology script and data file. The topology script MUST be executable.

    Sample Topology Script

    File name:

    # Adjust/Add the property ""
    # to core-site.xml with the "absolute" path the this
    # file.  ENSURE the file is "executable".
    # Supply appropriate rack prefix
    # To test, supply a hostname as script input:
    if [ $# -gt 0 ]; then
    if [ ! -f ${HADOOP_CONF}/${CTL_FILE} ]; then
      echo -n "/$RACK_PREFIX/rack "
      exit 0
    while [ $# -gt 0 ] ; do
      exec< ${HADOOP_CONF}/${CTL_FILE}
      while read line ; do
        ar=( $line )
        if [ "${ar[0]}" = "$nodeArg" ] ; then
      if [ -z "$result" ] ; then
        echo -n "/$RACK_PREFIX/rack "
        echo -n "/$RACK_PREFIX/rack_$result "
      echo -n "/$RACK_PREFIX/rack "

    Sample Topology Data File

    File name:

    # This file should be:
    #  - Placed in the /etc/hadoop/conf directory
    #    - On the Namenode (and backups IE: HA, Failover, etc)
    #    - On the Job Tracker OR Resource Manager (and any Failover JT's/RM's) 
    # This file should be placed in the /etc/hadoop/conf directory.
    # Add Hostnames to this file. Format <host ip> <rack_location> 01 02 03
  2. Copy both of these files to the /etc/hadoop/conf directory on all cluster nodes.

  3. Run the script to ensure that it returns the correct rack information for each host.

 2. Add the Topology Script Property to core-site.xml

  1. Stop HDFS using the instructions on this page.

  2. Add the following property to core-site.xml:


    By default the topology script will process up to 100 requests per invocation. You can also specify a different number of requests with the net.topology.script.number.args property. For example:


 3. Restart HDFS and MapReduce

Restart HDFS and MapReduce using the instructions on this page.

 4. Verify Rack Awareness

After the services have started, you can use the following methods to verify that rack awareness has been activated:

  • Look in the NameNode logs located in /var/log/hadoop/hdfs/ (for example: hadoop-hdfs-namenode-sandbox.log). You should see an entry like this:

    014-01-13 15:58:08,495 INFO Adding a new node: /rack01/<ipaddress>

  • The Hadoop fsck command should return something like the following (if there are two racks):

    Status: HEALTHY 
    Total size: 123456789 B 
    Total dirs: 0 
    Total files: 1 
    Total blocks (validated): 1 (avg. block size 123456789 B) 
    Minimally replicated blocks: 1 (100.0 %) 
    Over-replicated blocks: 0 (0.0 %) 
    Under-replicated blocks: 0 (0.0 %) 
    Mis-replicated blocks: 0 (0.0 %) 
    Default replication factor: 3 
    Average block replication: 3.0 
    Corrupt blocks: 0 
    Missing replicas: 0 (0.0 %) 
    Number of data-nodes: 40 
    Number of racks: 2 
    FSCK ended at Mon Jan 13 17:10:51 UTC 2014 in 1 milliseconds

  • The Hadoop dfsadmin -report command will return a report that includes the rack name next to each machine. The report should look something like the following (partial) example:

    [bsmith@hadoop01 ~]$ sudo -u hdfs hadoop dfsadmin -report 
    Configured Capacity: 19010409390080 (17.29 TB)
    Present Capacity: 18228294160384 (16.58 TB)
    DFS Remaining: 5514620928000 (5.02 TB)
    DFS Used: 12713673232384 (11.56 TB) DFS Used%: 69.75%
    Under replicated blocks: 181
    Blocks with corrupt replicas: 0 
    Missing blocks: 0
    Datanodes available: 5 (5 total, 0 dead)
    Name: (h2d1.hdp.local)
    Hostname: h2d1.hdp.local
    Rack: /default/rack_02
    Decommission Status : Normal
    Configured Capacity: 15696052224 (14.62 GB)
    DFS Used: 314380288 (299.82 MB)
    Non DFS Used: 3238612992 (3.02 GB)
    DFS Remaining: 12143058944 (11.31 GB)
    DFS Used%: 2.00%
    DFS Remaining%: 77.36%
    Configured Cache Capacity: 0 (0 B)
    Cache Used: 0 (0 B)
    Cache Remaining: 0 (0 B)
    Cache Used%: 100.00%
    Cache Remaining%: 0.00%
    Last contact: Thu Jun 12 11:39:51 EDT 2014

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