"randomWrite" mode of PerformanceEvaluation (PE), with nclients > 1, does random writes only within a small range rather than across all rows.
./bin/hbase org.apache.hadoop.hbase.PerformanceEvaluation --nomapred --rows=1000 randomWrite 10
[Note: As per convention in all modes of PE, --rows is the number of rows per client]
So for the above, with # of clients set to 10, currently all clients generate writes to keys in the 0..999 range instead of the 0..9999 range.
[Fix appears to be simple. Will provide a patch.]