Package org.apache.hadoop.hdfs.server.balancer

Class Balancer

java.lang.Object
org.apache.hadoop.hdfs.server.balancer.Balancer
All Implemented Interfaces:
BalancerMXBean
@Private public class Balancer extends Object implements BalancerMXBean

The balancer is a tool that balances disk space usage on an HDFS cluster when some datanodes become full or when new empty nodes join the cluster. The tool is deployed as an application program that can be run by the cluster administrator on a live HDFS cluster while applications adding and deleting files.

SYNOPSIS 

 To start:
      bin/start-balancer.sh [-threshold <threshold>]
      Example: bin/ start-balancer.sh 
                     start the balancer with a default threshold of 10%
               bin/ start-balancer.sh -threshold 5
                     start the balancer with a threshold of 5%
               bin/ start-balancer.sh -idleiterations 20
                     start the balancer with maximum 20 consecutive idle iterations
               bin/ start-balancer.sh -idleiterations -1
                     run the balancer with default threshold infinitely
 To stop:
      bin/ stop-balancer.sh

DESCRIPTION

The threshold parameter is a fraction in the range of (1%, 100%) with a default value of 10%. The threshold sets a target for whether the cluster is balanced. A cluster is balanced if for each datanode, the utilization of the node (ratio of used space at the node to total capacity of the node) differs from the utilization of the (ratio of used space in the cluster to total capacity of the cluster) by no more than the threshold value. The smaller the threshold, the more balanced a cluster will become. It takes more time to run the balancer for small threshold values. Also for a very small threshold the cluster may not be able to reach the balanced state when applications write and delete files concurrently.

The tool moves blocks from highly utilized datanodes to poorly utilized datanodes iteratively. In each iteration a datanode moves or receives no more than the lesser of 10G bytes or the threshold fraction of its capacity. Each iteration runs no more than 20 minutes. At the end of each iteration, the balancer obtains updated datanodes information from the namenode.

A system property that limits the balancer's use of bandwidth is defined in the default configuration file: 

 <property>
   <name>dfs.datanode.balance.bandwidthPerSec</name>
   <value>1048576</value>
 <description>  Specifies the maximum bandwidth that each datanode
 can utilize for the balancing purpose in term of the number of bytes 
 per second.
 </description>
 </property>

This property determines the maximum speed at which a block will be moved from one datanode to another. The default value is 1MB/s. The higher the bandwidth, the faster a cluster can reach the balanced state, but with greater competition with application processes. If an administrator changes the value of this property in the configuration file, the change is observed when HDFS is next restarted.

MONITERING BALANCER PROGRESS

After the balancer is started, an output file name where the balancer progress will be recorded is printed on the screen. The administrator can monitor the running of the balancer by reading the output file. The output shows the balancer's status iteration by iteration. In each iteration it prints the starting time, the iteration number, the total number of bytes that have been moved in the previous iterations, the total number of bytes that are left to move in order for the cluster to be balanced, and the number of bytes that are being moved in this iteration. Normally "Bytes Already Moved" is increasing while "Bytes Left To Move" is decreasing.

Running multiple instances of the balancer in an HDFS cluster is prohibited by the tool.

The balancer automatically exits when any of the following five conditions is satisfied:

  1. The cluster is balanced;
  2. No block can be moved;
  3. No block has been moved for specified consecutive iterations (5 by default);
  4. An IOException occurs while communicating with the namenode;
  5. Another balancer is running.

Upon exit, a balancer returns an exit code and prints one of the following messages to the output file in corresponding to the above exit reasons:

  1. The cluster is balanced. Exiting
  2. No block can be moved. Exiting...
  3. No block has been moved for specified iterations (5 by default). Exiting...
  4. Received an IO exception: failure reason. Exiting...
  5. Another balancer is running. Exiting...

The administrator can interrupt the execution of the balancer at any time by running the command "stop-balancer.sh" on the machine where the balancer is running.

  • Method Details

    • getVersion

      public String getVersion()
      Description copied from interface: BalancerMXBean
      Gets the version of Hadoop.
      Specified by:
      getVersion in interface BalancerMXBean
      Returns:
      the version of Hadoop

    • getSoftwareVersion

      public String getSoftwareVersion()
      Description copied from interface: BalancerMXBean
      Get the version of software running on the Balancer.
      Specified by:
      getSoftwareVersion in interface BalancerMXBean
      Returns:
      a string representing the version.

    • getCompileInfo

      public String getCompileInfo()
      Description copied from interface: BalancerMXBean
      Get the compilation information which contains date, user and branch.
      Specified by:
      getCompileInfo in interface BalancerMXBean
      Returns:
      the compilation information, as a JSON string.

    • main

      public static void main(String[] args)
      Run a balancer
      Parameters:
      args - Command line arguments