以hadoop0.21爲例。
NameNode.java: 主要維護文件系統的名字空間和文件的元數據，以下是代碼中的說明。
/**********************************************************
* 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: 主要維護幾個表的信息：維護了文件名與block列表的映射關係；有效的block的集合；block與節點列表的映射關係；節點與block列表的映射關係；更新的heatbeat節點的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
***************************************************//**
* 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 handles checkpointing and logging of the namespace edits.
*
*/FSEditLog.java:寫Edits文件
/**
* FSEditLog maintains a log of the namespace modifications.
*
*/BlockInfo.java:INode主要是所文件和目錄信息的，而對於文件的內容來說，這是用block描述的。我們假設一個文件的長度大小爲Size,那麼從文件的0偏移開始，按照固定大小，順序對文件劃分並編號，劃分好的每一塊爲一個block
/**
* Internal class for block metadata.
*/DatanodeDescriptor.java:代表的具體的存儲對象。
/**************************************************
* 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中的所有目錄和結構屬性
/*************************************************
* 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:所有的日誌記錄都是通過EditLogOutputStream輸出，在具體實例化的時候，這一組EditLogOutputStream包含多個EditLogFIleOutputStream和一個EditLogBackupOutputStream
/**
* A generic abstract class to support journaling of edits logs into
* a persistent storage.
*/EditLogFileOutputStream.java:將日誌記錄寫到edits或edits.new中。
/**
* An implementation of the abstract class {@link EditLogOutputStream}, which
* stores edits in a local file.
*/EditLogBackupOutputStream.java：將日誌通過網絡發送到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：升級階段：Secondary Name Node -》Checkpoint Node(定期保存元數據，定期checkpoint) -》Backup
Node(在內存中保持一份和Name Node完全一致的鏡像，當元數據發生變化時，其元數據進行更新，可以利用自身的鏡像來checkpoint，無需從nameNode下載)-》Standby Node（可以進行熱備）
/**
* 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備份目錄下創建jspool，並創建edits.new,將輸出流指向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:負責從name Node去文件。
/**
* This class provides fetching a specified file from the NameNode.
*/GetImageServlet.java：是httpServlet的子類，處理doGet請求。
/**
* 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.
*/