今天用虚拟机搭了小型hadoop集群，但namenode始终起不来，报错信息如下：
2015-03-27 16:46:08,915 WARN org.apache.hadoop.hdfs.server.namenode.NameNodeResourceChecker: Space available on volume '/dev/mapper/vg_justin-lv_root' is 0, which is below the configured reserved amount 104857600
2015-03-27 16:46:08,915 WARN org.apache.hadoop.hdfs.server.namenode.FSNamesystem: NameNode low on available disk space. Already in safe mode.
2015-03-27 16:46:08,915 WARN org.apache.hadoop.hdfs.server.namenode.FSNamesystem: NameNode low on available disk space. Already in safe mode.
2015-03-27 16:46:08,916 INFO org.apache.hadoop.hdfs.StateChange: STATE* Safe mode is ONResources are low on NN. Please add or free up more resources then turn off safe mode manually. NOTE: If you turn off safe mode before adding resources, the NN will immediately return to safe mode. Use "hdfs dfsadmin -safemode leave" to turn safe mode off.
实际的/dev/mapper/vg_justin-lv_root空间仍然有很大，但log竟然显示为0
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_justin-lv_root 37G 3.0G 33G 9% /
tmpfs 504M 0 504M 0% /dev/shm
/dev/sda1 485M 32M 428M 7% /boot

实在没办法，删除hadoop.tmp.dir下的文件、删除/tmp下的文件（slave、master），然后重启执行格式化、start-dfs.sh，启动正常了

安全模式可以强退，不过不建议，治标不治本，不用管，10分钟左右恢复，这是因为你元数据丢失达到一个比率造成的，通过log日志找出问题所在

看看权限

完全分布式目测是防火墙问题，遇到过坑，其他就不知道啦