以hadoop0.21为例。
NameNode.java: 主要维护文件系统的名字空间和文件的元数据，以下是代码中的说明。
/**********************************************************
* NameNode serves as both directory namespace manager and
* "inode table" for the Hadoop DFS. There is a single NameNode
* running in any DFS deployment. (Well, except when there
* is a second backup/failover NameNode.)
*
* The NameNode controls two critical tables:
* 1) filename ->blocksequence (namespace)
* 2) block ->machinelist ("inodes")
*
* The first table is stored on disk and is very precious.
* The second table is rebuilt every time the NameNode comes
* up.
*
* 'NameNode' refers to both this class as well as the 'NameNode server'.
* The 'FSNamesystem' class actually performs most of the filesystem
* management. The majority of the 'NameNode' class itself is concerned
* with exposing the IPC interface and the http server to the outside world,
* plus some configuration management.
*
* NameNode implements the ClientProtocol interface, which allows
* clients to ask for DFS services. ClientProtocol is not
* designed for direct use by authors of DFS client code. End -users
* should instead use the org.apache.nutch.hadoop.fs.FileSystem class.
*
* NameNode also implements the DatanodeProtocol interface, used by
* DataNode programs that actually store DFS data blocks. These
* methods are invoked repeatedly and automatically by all the
* DataNodes in a DFS deployment.
*
* NameNode also implements the NamenodeProtocol interface, used by
* secondary namenodes or rebalancing processes to get partial namenode's
* state, for example partial blocksMap etc.
**********************************************************/
FSNamesystem.java: 主要维护几个表的信息：维护了文件名与block列表的映射关系；有效的block的集合；block与节点列表的映射关系；节点与block列表的映射关系；更新的heatbeat节点的LRU cache
/***************************************************
* FSNamesystem does the actual bookkeeping work for the
* DataNode.
*
* It tracks several important tables.
*
* 1) valid fsname --> blocklist (kept on disk, logged)
* 2) Set of all valid blocks (inverted #1)
* 3) block --> machinelist (kept in memory, rebuilt dynamically from reports)
* 4) machine --> blocklist (inverted #2)
* 5) LRU cache of updated -heartbeat machines
***************************************************/
INode.java：HDFS将文件和文件目录抽象成INode。
/**
* We keep an in-memory representation of the file/block hierarchy.
* This is a base INode class containing common fields for file and
* directory inodes.
*/
FSImage.java：需要将INode信息持久化到磁盘上FSImage上。
/**
* FSImage handles checkpointing and logging of the namespace edits.
*
*/
FSEditLog.java:写Edits文件
/**
* FSEditLog maintains a log of the namespace modifications.
*
*/
BlockInfo.java:INode主要是所文件和目录信息的，而对于文件的内容来说，这是用block描述的。我们假设一个文件的长度大小为Size,那么从文件的0偏移开始，按照固定大小，顺序对文件划分并编号，划分好的每一块为一个block
/**
* Internal class for block metadata.
*/
DatanodeDescriptor.java:代表的具体的存储对象。
/**************************************************
* DatanodeDescriptor tracks stats on a given DataNode,
* such as available storage capacity, last update time, etc.,
* and maintains a set of blocks stored on the datanode.
*
* This data structure is a data structure that is internal
* to the namenode. It is *not* sent over- the- wire to the Client
* or the Datnodes. Neither is it stored persistently in the
* fsImage.
**************************************************/
FSDirectory.java: 代表了HDFS中的所有目录和结构属性
/*************************************************
* FSDirectory stores the filesystem directory state.
* It handles writing/loading values to disk, and logging
* changes as we go.
*
* It keeps the filename->blockset mapping always- current
* and logged to disk.
*
*************************************************/
EditLogOutputStream.java:所有的日志记录都是通过EditLogOutputStream输出，在具体实例化的时候，这一组EditLogOutputStream包含多个EditLogFIleOutputStream和一个EditLogBackupOutputStream
/**
* A generic abstract class to support journaling of edits logs into
* a persistent storage.
*/
EditLogFileOutputStream.java:将日志记录写到edits或edits.new中。
/**
* An implementation of the abstract class {@link EditLogOutputStream}, which
* stores edits in a local file.
*/
EditLogBackupOutputStream.java：将日志通过网络发送到backupnode上。
/**
* An implementation of the abstract class {@link EditLogOutputStream},
* which streams edits to a backup node.
*
* @see org.apache.hadoop.hdfs.server.protocol.NamenodeProtocol#journal
* (org.apache.hadoop.hdfs.server.protocol.NamenodeRegistration,
* int, int, byte[])
*/
BackupNode.java：name Node的backup：升级阶段：Secondary Name Node -》Checkpoint Node(定期保存元数据，定期checkpoint) -》Backup Node(在内存中保持一份和Name Node完全一致的镜像，当元数据发生变化时，其元数据进行更新，可以利用自身的镜像来checkpoint，无需从nameNode下载)-》Standby Node（可以进行热备）
/**
* BackupNode.
* <p>
* Backup node can play two roles.
* <ol>
* <li>{@link NamenodeRole#CHECKPOINT} node periodically creates checkpoints,
* that is downloads image and edits from the active node, merges them, and
* uploads the new image back to the active. </li>
* <li>{@link NamenodeRole#BACKUP} node keeps its namespace in sync with the
* active node, and periodically creates checkpoints by simply saving the
* namespace image to local disk(s).</li>
* </ol>
*/
BackupStorage.java：在Backup Node备份目录下创建jspool，并创建edits.new,将输出流指向edits.new
/**
* Load checkpoint from local files only if the memory state is empty.<br>
* Set new checkpoint time received from the name -node. <br>
* Move <code>lastcheckpoint.tmp </code> to <code>previous.checkpoint</code> .
* @throws IOException
*/
TransferFsImage.java:负责从name Node去文件。
/**
* This class provides fetching a specified file from the NameNode.
*/
GetImageServlet.java：是httpServlet的子类，处理doGet请求。
/**
* This class is used in Namesystem's jetty to retrieve a file.
* Typically used by the Secondary NameNode to retrieve image and
* edit file for periodic checkpointing.
*/
