Choose three reasons why should you run the HDFS balancer periodically?

Question

Options

• ATo improve data locality for MapReduce tasks.
• BTo ensure that there is consistent disk utilization across the DataNodes.
• CTo ensure that there is capacity in HDFS tor additional data.
• DTo ensure that all blocks in the cluster are 128MB in size.
• ETo help HDFS deliver consistent performance under heavy loads.

• A
  73% (19)
• C
  12% (3)
• D
  15% (4)

Explanation

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. The threshold parameter is a fraction in the range of (0%, 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.

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