启动yarn时，报错信息如下：
org.apache.hadoop.ipc.RemoteException(org.apache.hadoop.ipc.RetriableException): org.apache.hadoop.hdfs.server.namenode.SafeModeException: Cannot create file/system/balancer.id. Name node is in safe mode.
The reported blocks 1293 needs additional 510 blocks to reach the threshold 0.9900 of total blocks 1821.
The number of live datanodes 3 has reached the minimum number 0. Safe mode will be turned off automatically once the thresholds have been reached.
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkNameNodeSafeMode(FSNamesystem.java:1331)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFileInt(FSNamesystem.java:2446)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFile(FSNamesystem.java:2335)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.create(NameNodeRpcServer.java:688)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.create(ClientNamenodeProtocolServerSideTranslatorPB.java:397)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:616)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:969)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2151)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2147)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1657)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2145)
Caused by: org.apache.hadoop.hdfs.server.namenode.SafeModeException: Cannot create file/system/balancer.id. Name node is in safe mode.
解决办法：
hdfs dfsadmin -safemode leave