NNBench not support run more than one map task on the same host

When benchmark NameNode performance with NNBench, I start with pseudo distributed deploy. Everything goes well with "-maps 1". BUT with -maps N (n>1) and -operation create_write, many exceptions meet during the benchmark.

Hostname is part of the file path, which can differentiate hosts. With more than two map tasks run on the same host, more than two map tasks may operate on the same file, leading to exceptions.

17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: AL Total #1: 84
17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: AL Total #2: 43
17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: TPS Total (ms): 2570
17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: Longest Map Time (ms): 814.0
17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: Late maps: 0
17/11/07 15:22:32 INFO hdfs.NNBench: RAW DATA: # of exceptions: 3000

2017-11-07 14:54:08,082 INFO org.apache.hadoop.hdfs.NNBench: Exception recorded in op: Create/Write/Close
2017-11-07 14:54:08,082 INFO org.apache.hadoop.hdfs.NNBench: Exception recorded in op: Create/Write/Close
2017-11-07 14:54:08,083 INFO org.apache.hadoop.hdfs.NNBench: Exception recorded in op: Create/Write/Close