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Hadoop-2.4.1学习之创建fsimage和edits源码分析

       在Hadoop中fsimage保存最新的检查点信息,edits保存自最新检查点后的命名空间的变化。在分析hdfs namenode–format的源代码时,已经明确了该过程根据配置文件的信息创建fsimage和edits文件,这篇文章具体分析一下创建fsimage和edits文件的源代码。在NameNode的format方法中,有如下的代码:

FSImage fsImage = new FSImage(conf, nameDirsToFormat, editDirsToFormat);
    try {
      FSNamesystem fsn = new FSNamesystem(conf, fsImage);
      fsImage.getEditLog().initJournalsForWrite();

      if (!fsImage.confirmFormat(force, isInteractive)) {
        return true; // aborted
      }

      fsImage.format(fsn, clusterId);
    } catch (IOException ioe) {
      LOG.warn("Encountered exception during format: ", ioe);
      fsImage.close();
      throw ioe;
    }

      在这段代码中主要涉及到了三个类,分别为FSImage、FSNamesystem和FSEditLog,其中FSImage负责检查点,FSEditLog维护命名空间变动的日志,FSNamesystem为DataNode执行实际的簿记工作。创建fsImage对象的源代码为

/**
   * Construct the FSImage. Set the default checkpoint directories.
   *
   * Setup storage and initialize the edit log.
   *
   * @param conf Configuration
   * @param imageDirs Directories the image can be stored in.
   * @param editsDirs Directories the editlog can be stored in.
   * @throws IOException if directories are invalid.
   */
  protected FSImage(Configuration conf,Collection<URI> imageDirs,
                    List<URI> editsDirs)throws IOException {
this.conf = conf;
/* NNStorage负责管理NameNode使用的StorageDirectories*/
storage = new NNStorage(conf, imageDirs, editsDirs);
/*根据dfs.namenode.name.dir.restore的值决定是否尝试重新存储失败的存储目录
* 默认值为false
*/
    if(conf.getBoolean(DFSConfigKeys.DFS_NAMENODE_NAME_DIR_RESTORE_KEY,
                       DFSConfigKeys.DFS_NAMENODE_NAME_DIR_RESTORE_DEFAULT)) {
      storage.setRestoreFailedStorage(true);
    }
this.editLog = new FSEditLog (conf, storage, editsDirs);
/*NNStorageRetentionManager负责检查NameNode的存储目录,
    *并在fsimage和edits文件上执行保留策略。
*/
    archivalManager = new NNStorageRetentionManager(conf, storage, editLog);
  }

     FSImage的构造方法中,创建了NNStorage、FSEditLog和NNStorageRetentionManager对象,NNStorage的构造方法源代码如下:

public NNStorage(Configuration conf, Collection<URI> imageDirs, Collection<URI> editsDirs) throws IOException {
    super(NodeType.NAME_NODE);
    storageDirs = new CopyOnWriteArrayList<StorageDirectory>();
    
    // this may modify the editsDirs, so copy before passing in
    setStorageDirectories(imageDirs, 
                          Lists.newArrayList(editsDirs),
                          FSNamesystem.getSharedEditsDirs(conf));
  }

      NNStorage中的setStorageDirectories方法用于初始化存储fsimage和edits文件的目录。这里不分析该方法的全部源代码,主要分析初始化fsimage的部分,如下:

// Add all name dirs with appropriate NameNodeDirType
    for (URI dirName : fsNameDirs) {
      checkSchemeConsistency(dirName);
      boolean isAlsoEdits = false;
      for (URI editsDirName : fsEditsDirs) {
        if (editsDirName.compareTo(dirName) == 0) {
          isAlsoEdits = true;
          fsEditsDirs.remove(editsDirName);
          break;
        }
      }
      NameNodeDirType dirType = (isAlsoEdits) ?
                          NameNodeDirType.IMAGE_AND_EDITS :
                          NameNodeDirType.IMAGE;
      // Add to the list of storage directories, only if the
      // URI is of type file://
      if(dirName.getScheme().compareTo("file") == 0) {
        this.addStorageDir(new StorageDirectory(new File(dirName.getPath()),
            dirType,
            sharedEditsDirs.contains(dirName))); // Don't lock the dir if it's shared.
      }
    }

      在该段代码中fsNameDirs和fsEditsDirs分别为参数dfs.namenode.name.dir和dfs.namenode.edits.dir设置的目录,默认情况下二者指向相同的目录。如果二者的值相同那么fsimage的类型就为NameNodeDirType.IMAGE_AND_EDITS,否则为NameNodeDirType.IMAGE。需要注意的是,到目前为止虽然根据配置文件创建了表示相应目录的File对象,但还没有在本地文件系统上创建实际的目录。现在回到FSIamge的构造方法中,在创建了NNStorage对象后,接下来创建FSEditLog对象,该类的构造方法为:

/**
   * Constructor for FSEditLog. Underlying journals are constructed, but 
   * no streams are opened until open() is called.
   * 
   * @param conf The namenode configuration
   * @param storage Storage object used by namenode
   * @param editsDirs List of journals to use
   */
  FSEditLog(Configuration conf, NNStorage storage, List<URI> editsDirs) {
    isSyncRunning = false;
    this.conf = conf;
this.storage = storage;
//在format阶段,metrics的值为null
    metrics = NameNode.getNameNodeMetrics();
    lastPrintTime = now();
    // If this list is empty, an error will be thrown on first use
    // of the editlog, as no journals will exist
    this.editsDirs = Lists.newArrayList(editsDirs);
    this.sharedEditsDirs = FSNamesystem.getSharedEditsDirs(conf);
  }

      在创建完了FSEditLog和NNStorageRetentionManager对象后,FSIamge的构造方法执行完毕,回到NameNode的format方法中。接下来就是要根据FSImage对象创建FSNamesystem,前面曾提到FSNamesystem为DataNode执行实际的簿记工作,实际执行的该类的构造方法如下,其中ignoreRetryCache的值为false:

/**
   * Create an FSNamesystem associated with the specified image.
   * 
   * Note that this does not load any data off of disk -- if you would
   * like that behavior, use {@link #loadFromDisk(Configuration)}
   *
   * @param conf configuration
   * @param fsImage The FSImage to associate with
   * @param ignoreRetryCache Whether or not should ignore the retry cache setup
   *                         step. For Secondary NN this should be set to true.
   * @throws IOException on bad configuration
   */
  FSNamesystem(Configuration conf, FSImage fsImage, boolean ignoreRetryCache)
      throws IOException

      由于该构造方法代码比较多,就不贴出来了。可以概括为在该构造方法中,实例化了诸如BlockManager、DatanodeStatistics、FSDirectory、CacheManager等对象,读取了配置文件中的一些信息(后面计划专门研究该类)。在创建完毕FSNamesystem对象后,执行的是FSEditLog的initJournalsForWrite()方法,该方法的代码如下:

private State state = State.UNINITIALIZED;
public synchronized void initJournalsForWrite() {
    Preconditions.checkState(state == State.UNINITIALIZED ||
        state == State.CLOSED, "Unexpected state: %s", state);
    initJournals(this.editsDirs);
    state = State.BETWEEN_LOG_SEGMENTS;
  }

      在该方法中先检查日志的状态,FSEditLog刚创建时的状态为UNINITIALIZED,执行完initJournals()方法后日志的状态为BETWEEN_LOG_SEGMENTS,意味日志还未打开。日志的初始化具体由initJournals()完成,该方法的代码如下:

private synchronized void initJournals(List<URI> dirs) {
    int minimumRedundantJournals = conf.getInt(
        DFSConfigKeys.DFS_NAMENODE_EDITS_DIR_MINIMUM_KEY,
        DFSConfigKeys.DFS_NAMENODE_EDITS_DIR_MINIMUM_DEFAULT);
//管理日志集合的对象
    journalSet = new JournalSet(minimumRedundantJournals);

    for (URI u : dirs) {
      boolean required = FSNamesystem.getRequiredNamespaceEditsDirs(conf)
          .contains(u);
      if (u.getScheme().equals(NNStorage.LOCAL_URI_SCHEME)) {
        StorageDirectory sd = storage.getStorageDirectory(u);
        if (sd != null) {
          journalSet.add(new FileJournalManager(conf, sd, storage),
              required, sharedEditsDirs.contains(u));
        }
      } else {
        journalSet.add(createJournal(u), required,
            sharedEditsDirs.contains(u));
      }
    }
 
    if (journalSet.isEmpty()) {
      LOG.error("No edits directories configured!");
    } 
  }

      该方法根据传递的目录集合创建FileJournalManager对象,并添加到JournalSet对象中,其中一个FileJournalManager对象管理一个保存edits文件的目录。初始化edits文件后,接下来就是对fsimage进行格式化了,具体代码为:

void format(FSNamesystem fsn, String clusterId) throws IOException {
    long fileCount = fsn.getTotalFiles();
    // Expect 1 file, which is the root inode
    Preconditions.checkState(fileCount == 1,
        "FSImage.format should be called with an uninitialized namesystem, has " +
        fileCount + " files");
    NamespaceInfo ns = NNStorage.newNamespaceInfo();
    LOG.info("Allocated new BlockPoolId: " + ns.getBlockPoolID());
    ns.clusterID = clusterId;
    
    storage.format(ns);
    editLog.formatNonFileJournals(ns);
    saveFSImageInAllDirs(fsn, 0);
  }

      在该代码中实际执行创建fsimage文件的代码为最后一行的saveFSImageInAllDirs(fsn,0),该方法将实际的工作委托给下面的方法:

saveFSImageInAllDirs(source, NameNodeFile.IMAGE, txid, null);

      在该方法中会根据保存fsimage的目录的个数创建与之相等的线程,这些线程完成实际的创建fsimage的任务。最后会清理旧的edits和检查点文件。

      通过上面的分析可以发现,在执行hdfs namenode –format时仅创建了fsimage文件,并没有创建edits文件,但已经创建了相关对象。这一点也可以在执行完format后在本地文件系统中确认,目录中只有fsimage文件。

Hadoop-2.4.1学习之创建fsimage和edits源码分析