namenode上对Datanode的表征和管理--Datanode*系列
DatanodeID是继承自WritableComparable，封装了DN的一些基本信息：

namenode在处理心跳FSNamesystem.handleHeartbeat时会调用DatanodeDescriptor.getXXXCommand方法获得BlockCommand，BlockCommand整合了要处理的块和对应的DatanodeInfo信息(XXX可以是Replication、LeaseRecovery、Invalidate)。
 void readFieldsFromFSEditLog(DataInput in) throws IOException {    this.name = UTF8.readString(in);    this.storageID = UTF8.readString(in);    this.infoPort = in.readShort() & 0x0000ffff;    this.capacity = in.readLong();    this.dfsUsed = in.readLong();    this.remaining = in.readLong();    this.lastUpdate = in.readLong();    this.xceiverCount = in.readInt();    this.location = Text.readString(in);    this.hostName = Text.readString(in);    setAdminState(WritableUtils.readEnum(in, AdminStates.class));  }