我已经设置了一个单节点集群（最初）并尝试从集群外的客户端编写文件。当write调用返回时，close调用会挂起很长时间，最终会返回，但HDFS中生成的文件长度为0个字节。日志说：
2016-10-03 22:01:41,367 INFO BlockStateChange: chooseUnderReplicatedBlocks selected 1 blocks at priority level 0; Total=1 Reset bookmarks? true
2016-10-03 22:01:41,367 INFO BlockStateChange: BLOCK* neededReplications = 1, pendingReplications = 0.
2016-10-03 22:01:41,367 INFO org.apache.hadoop.hdfs.server.blockmanagement.BlockManager: Blocks chosen but could not be replicated = 1; of which 1 have no target, 0 have no source, 0 are UC, 0 are abandoned, 0 already have enough replicas.
为什么块没有写入单个datanode（与namenode相同）？ “没有目标”是什么意思？复制计数为1，我认为该文件的单个副本将存储在单个群集节点上。