@Madhura Mhatre
HDFS balancer doesn't run at background, has to run manually. A threshold parameter is a float number between 0 and 100. From the average cluster utilization, the balancer process will try to converge all data node's usage in the range [average - threshold, average + threshold].
The default threshold is 10%
For example, if the cluster current utilization is 50% full, then higher usage data nodes will start to move data to lower usage nodes.
- Higher (average + threshold): 60%
- Lower (average - threshold): 40%
TIP:
If you haven't balance your cluster for a long time, you should start by balancing with a higher threshold like 25, and then converging to a smaller target threshold like 10.
The smaller your threshold, the more balanced your data nodes will be. For very small threshold ie 2, the cluster may not be able to reach the balanced state if other clients concurrently write and delete data in the cluster. So in your scenario, I would advise a threshold of 25
$ hdfs balancer -threshold 25
First, the balancer will pick data nodes with current usage above the higher threshold, and try to find blocks from these data nodes that could be copied into nodes with current usage below the lower threshold
Second, the balancer will select over-utilized nodes to move blocks to nodes with utilization below average. Third, the balancer will pick nodes with utilization above average to move data to under-utilized nodes.
In addition to that selection process, the balancer can also pick a proxy node if the source and the destination are not located in the same rack (i.e. a data node storing a replica of the block and located in the same rack than the destination). Yes, the balancer is rack aware and will generate very little rack-to-rack noise.
HTH
