How quickly can I increase the number of replicas?

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My cluser version hdfs 2.2 stable ( 2 ha namenodes, 10 datanodes)
 
I was command bin/hdfs dfs -setrep -R 2 / ( replication 1 to 2 )
 
I found that HDFS is actually replicating the under replicated blocks but it 
works very slowly. 
HDFS performs the replication about 1 block per second.
I have about 400000 under replicated blocks so it will take about 4 more days.
Is there any way to speed it up?