我在cloudera manager中不时收到以下错误：
This DataNode is not connected to one or more of its NameNode(s).
and
The Cloudera Manager agent got an unexpected response from this role's web server.
（通常是一起，有时只有其中之一）
在 SO 和 Google 中大多数对这些错误的引用中，问题是配置问题（并且数据节点永远不会连接到名称节点）
就我而言，数据节点通常在启动时连接，但在一段时间后断开连接 - 所以它看起来并不是一个糟糕的配置。
- 还有其他选择吗？
- 是否可以强制数据节点重新连接到名称节点？
- 是否可以从数据节点“ping”名称节点（模拟数据节点的连接尝试）
- 可能是某种资源问题（对于许多打开的文件\连接）？
示例日志（错误随时变化）
2014-02-25 06:39:49,179 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: exception:
java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be ready for write. ch : java.nio.channels.SocketChannel[connected local=/10.56.144.18:50010 remote=/10.56.144.28:48089]
at org.apache.hadoop.net.SocketIOWithTimeout.doIO(SocketIOWithTimeout.java:165)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:153)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:114)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendPacket(BlockSender.java:504)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendBlock(BlockSender.java:673)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.readBlock(DataXceiver.java:338)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opReadBlock(Receiver.java:92)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:64)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:221)
at java.lang.Thread.run(Thread.java:662)
2014-02-25 06:39:49,180 INFO org.apache.hadoop.hdfs.server.datanode.DataNode.clienttrace: src: /10.56.144.18:50010, dest: /10.56.144.28:48089, bytes: 132096, op: HDFS_READ, cliID: DFSClient_NONMAPREDUCE_1315770947_27, offset: 0, srvID: DS-990970275-10.56.144.18-50010-1384349167420, blockid: BP-1381780028-10.56.144.16-1384349161741:blk_-8718668700255896235_5121440, duration: 480291679056
2014-02-25 06:39:49,180 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(10.56.144.18, storageID=DS-990970275-10.56.144.18-50010-1384349167420, infoPort=50075, ipcPort=50020, storageInfo=lv=-40;cid=cluster16;nsid=7043943;c=0):Got exception while serving BP-1381780028-10.56.144.16-1384349161741:blk_-8718668700255896235_5121440 to /10.56.144.28:48089
java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be ready for write. ch : java.nio.channels.SocketChannel[connected local=/10.56.144.18:50010 remote=/10.56.144.28:48089]
at org.apache.hadoop.net.SocketIOWithTimeout.doIO(SocketIOWithTimeout.java:165)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:153)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:114)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendPacket(BlockSender.java:504)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendBlock(BlockSender.java:673)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.readBlock(DataXceiver.java:338)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opReadBlock(Receiver.java:92)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:64)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:221)
at java.lang.Thread.run(Thread.java:662)
2014-02-25 06:39:49,181 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: host.com:50010:DataXceiver error processing READ_BLOCK operation src: /10.56.144.28:48089 dest: /10.56.144.18:50010
java.net.SocketTimeoutException: 480000 millis timeout while waiting for channel to be ready for write. ch : java.nio.channels.SocketChannel[connected local=/10.56.144.18:50010 remote=/10.56.144.28:48089]
at org.apache.hadoop.net.SocketIOWithTimeout.doIO(SocketIOWithTimeout.java:165)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:153)
at org.apache.hadoop.net.SocketOutputStream.write(SocketOutputStream.java:114)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendPacket(BlockSender.java:504)
at org.apache.hadoop.hdfs.server.datanode.BlockSender.sendBlock(BlockSender.java:673)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.readBlock(DataXceiver.java:338)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opReadBlock(Receiver.java:92)
at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:64)
at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:221)
at java.lang.Thread.run(Thread.java:662)
