一、现象：
下节点特别慢。10台节点，每台大约需要退役60w个块。但是3个小时才退役了3000多个块。
NN侧如下日志，可以看到30秒只退役了512-494 = 20个块，这要是退役600w个块，得猴年马月？
2024-03-19 14:44:42,952 INFO org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor: There are 10000 blocks pending replication and the limit is 10000. A further 6174331 blocks are waiting to be processed. The replication queue currently has 112518 blocks
2024-03-19 14:45:12,972 INFO org.apache.hadoop.hdfs.server.blockmanagement.DatanodeAdminBackoffMonitor: There are 10000 blocks pending replication and the limit is 10000. A further 6174331 blocks are waiting to be processed. The replication queue currently has 112494 blocks
NN监控里，也能发现不少块复制超时了。
DN侧监控，发现只有一台节点在处理复制任务，它的XmitsInProgress线程数不为0，其它的DN都为0ÿ
