So Bad Random Read Performance

hi,all
 I set up a cluster on 4 machine.(1 HMaster,4RegionServer).
 Each Machine has a 16G mem, one 2T Sata disk,CentOS 5.3，XFS .
 I write 100M data in Hbase,only 1M key/value,so all keys are in only *one*
Store,and only 2 HFiles in that Store.(I used Large Data Set,but random
read
performance is bad,so i try 100M)
 Then I test random read with existing keys with only 1 client. But I got a
Bad Performance,About >100ms latency per read.

Why did I get so long latency?

 I dig into the metric log,and found that,
 in metrics_rpc.log:
      get_avg_time=146, get_num_ops=22
      get_avg_time=94, get_num_ops=127
      get_avg_time=113, get_num_ops=182
      .......

and in metrics_hbase.log:
      fsReadLatency_avg_time=140, fsReadLatency_num_ops=31
      fsReadLatency_avg_time=93, fsReadLatency_num_ops=137
      fsReadLatency_avg_time=109, fsReadLatency_num_ops=188
      .......

Is that say the fsReadLatency causes the random get latency so long？

But i didn't tuning hdfs almost,except some ips/ports and max.xcievers,
Should I tuning for hdfs?And How?