He Xiaoqiao created HDFS-13183:

             Summary: Standby NameNode process getBlocks request to reduce 
Active load
                 Key: HDFS-13183
                 URL: https://issues.apache.org/jira/browse/HDFS-13183
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: balancer & mover, namenode
    Affects Versions: 2.7.5, 3.1.0, 2.9.1, 2.8.4, 3.0.2
            Reporter: He Xiaoqiao
            Assignee: He Xiaoqiao

The performance of Active NameNode could be impact when {{Balancer}} requests 
#getBlocks, since query blocks of overly full DNs performance is extremely 
inefficient currently. The main reason is {{NameNodeRpcServer#getBlocks}} hold 
read lock for long time. In extreme case, all handlers of Active NameNode RPC 
server are occupied by one reader {{NameNodeRpcServer#getBlocks}} and other 
write operation calls, thus Active NameNode enter a state of false death for 
number of seconds even for minutes.
The similar performance concerns of Balancer have reported by HDFS-9412, 
HDFS-7967, etc.

If Standby NameNode can shoulder #getBlocks heavy burden, it could speed up the 
progress of balancing and reduce performance impact to Active NameNode.

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