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NameNode is busy, Network IO is high when a datanode is down

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One datanode was down due to temporary network disconnection, and it was back online about 30~40mins later. We observed that NameNode was busy and unresponsive, and a lot of nodes reported the incoming and outgoing traffic more than 800Mbps during this down time. 


We didn't have jobs running at time. I understood HDFS was busy to copying the blocks under the replication number. But this made the whole cluster significantly downgraded. Is it normal?


We have replication factor = 3. 16 nodes each has about 8TB data. 


We are running CDH 5.3.2. 

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