1.每次迭代一个datanode会移动不超过如下两者中较小的一个的数据量
1）10G
2）能力阈值 dfs.datanode.balance.bandwidthPerSec
每次迭代时间不会超过20分钟
<p>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.
2.balancer自动停止的情景
* <p>The balancer automatically exits when any of the following five
* conditions is satisfied:
* <ol>
* <li>The cluster is balanced;
* <li>No block can be moved;
* <li>No block has been moved for five consecutive iterations;
* <li>An IOException occurs while communicating with the namenode;
* <li>Another balancer is running.
* </ol>
<p>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:
* <ol>
* <li>The cluster is balanced. Exiting
* <li>No block can be moved. Exiting...
* <li>No block has been moved for 5 iterations. Exiting...
* <li>Received an IO exception: failure reason. Exiting...
* <li>Another balancer is running. Exiting...
* </ol>
3.一个datanode同时可用于balance的移动最大数
/** The maximum number of concurrent blocks moves for
* balancing purpose at a datanode
*/
public static final int MAX_NUM_CONCURRENT_MOVES = 5;
private static final int MAX_NO_PENDING_BLOCK_ITERATIONS = 5;
4.