private File addBlock(Block b, File src, boolean createOk, boolean resetIdx)
        throws IOException {
      if (numBlocks < maxBlocksPerDir) {
        File dest = new File(dir, b.getBlockName());
        File metaData = getMetaFile(src, b);
        File newmeta = getMetaFile(dest, b);
        if (!metaData.renameTo(newmeta) || !src.renameTo(dest)) {
          throw new IOException(
              "could not move files for " + b + " from tmp to " + dest.getAbsolutePath());
        }
        if (DataNode.LOG.isDebugEnabled()) {
          DataNode.LOG.debug("addBlock: Moved " + metaData + " to " + newmeta);
          DataNode.LOG.debug("addBlock: Moved " + src + " to " + dest);
        }

        numBlocks += 1;
        return dest;
      }

      if (lastChildIdx < 0 && resetIdx) {
        // reset so that all children will be checked
        lastChildIdx = random.nextInt(children.length);
      }

      if (lastChildIdx >= 0 && children != null) {
        // Check if any child-tree has room for a block.
        for (int i = 0; i < children.length; i++) {
          int idx = (lastChildIdx + i) % children.length;
          File file = children[idx].addBlock(b, src, false, resetIdx);
          if (file != null) {
            lastChildIdx = idx;
            return file;
          }
        }
        lastChildIdx = -1;
      }

      if (!createOk) {
        return null;
      }

      if (children == null || children.length == 0) {
        children = new FSDir[maxBlocksPerDir];
        for (int idx = 0; idx < maxBlocksPerDir; idx++) {
          children[idx] = new FSDir(new File(dir, DataStorage.BLOCK_SUBDIR_PREFIX + idx));
        }
      }

      // now pick a child randomly for creating a new set of subdirs.
      lastChildIdx = random.nextInt(children.length);
      return children[lastChildIdx].addBlock(b, src, true, false);
    }
  /**
   * Start writing to a block file If isRecovery is true and the block pre-exists, then we kill all
   * volumeMap.put(b, v); volumeMap.put(b, v); other threads that might be writing to this block,
   * and then reopen the file.
   */
  public BlockWriteStreams writeToBlock(Block b, boolean isRecovery) throws IOException {
    //
    // Make sure the block isn't a valid one - we're still creating it!
    //
    if (isValidBlock(b)) {
      if (!isRecovery) {
        throw new BlockAlreadyExistsException(
            "Block " + b + " is valid, and cannot be written to.");
      }
      // If the block was successfully finalized because all packets
      // were successfully processed at the Datanode but the ack for
      // some of the packets were not received by the client. The client
      // re-opens the connection and retries sending those packets.
      // The other reason is that an "append" is occurring to this block.
      detachBlock(b, 1);
    }
    long blockSize = b.getNumBytes();

    //
    // Serialize access to /tmp, and check if file already there.
    //
    File f = null;
    List<Thread> threads = null;
    synchronized (this) {
      //
      // Is it already in the create process?
      //
      ActiveFile activeFile = ongoingCreates.get(b);
      if (activeFile != null) {
        f = activeFile.file;
        threads = activeFile.threads;

        if (!isRecovery) {
          throw new BlockAlreadyExistsException(
              "Block "
                  + b
                  + " has already been started (though not completed), and thus cannot be created.");
        } else {
          for (Thread thread : threads) {
            thread.interrupt();
          }
        }
        ongoingCreates.remove(b);
      }
      FSVolume v = null;
      if (!isRecovery) {
        v = volumes.getNextVolume(blockSize);
        // create temporary file to hold block in the designated volume
        f = createTmpFile(v, b);
        volumeMap.put(b, new DatanodeBlockInfo(v));
      } else if (f != null) {
        DataNode.LOG.info("Reopen already-open Block for append " + b);
        // create or reuse temporary file to hold block in the
        // designated volume
        v = volumeMap.get(b).getVolume();
        volumeMap.put(b, new DatanodeBlockInfo(v));
      } else {
        // reopening block for appending to it.
        DataNode.LOG.info("Reopen Block for append " + b);
        v = volumeMap.get(b).getVolume();
        f = createTmpFile(v, b);
        File blkfile = getBlockFile(b);
        File oldmeta = getMetaFile(b);
        File newmeta = getMetaFile(f, b);

        // rename meta file to tmp directory
        DataNode.LOG.debug("Renaming " + oldmeta + " to " + newmeta);
        if (!oldmeta.renameTo(newmeta)) {
          throw new IOException(
              "Block "
                  + b
                  + " reopen failed. "
                  + " Unable to move meta file  "
                  + oldmeta
                  + " to tmp dir "
                  + newmeta);
        }

        // rename block file to tmp directory
        DataNode.LOG.debug("Renaming " + blkfile + " to " + f);
        if (!blkfile.renameTo(f)) {
          if (!f.delete()) {
            throw new IOException(
                "Block " + b + " reopen failed. " + " Unable to remove file " + f);
          }
          if (!blkfile.renameTo(f)) {
            throw new IOException(
                "Block "
                    + b
                    + " reopen failed. "
                    + " Unable to move block file "
                    + blkfile
                    + " to tmp dir "
                    + f);
          }
        }
        volumeMap.put(b, new DatanodeBlockInfo(v));
      }
      if (f == null) {
        DataNode.LOG.warn("Block " + b + " reopen failed " + " Unable to locate tmp file.");
        throw new IOException("Block " + b + " reopen failed " + " Unable to locate tmp file.");
      }
      ongoingCreates.put(b, new ActiveFile(f, threads));
    }

    try {
      if (threads != null) {
        for (Thread thread : threads) {
          thread.join();
        }
      }
    } catch (InterruptedException e) {
      throw new IOException("Recovery waiting for thread interrupted.");
    }

    //
    // Finally, allow a writer to the block file
    // REMIND - mjc - make this a filter stream that enforces a max
    // block size, so clients can't go crazy
    //
    File metafile = getMetaFile(f, b);
    DataNode.LOG.debug("writeTo blockfile is " + f + " of size " + f.length());
    DataNode.LOG.debug("writeTo metafile is " + metafile + " of size " + metafile.length());
    return createBlockWriteStreams(f, metafile);
  }
  /**
   * Try to update an old block to a new block. If there are ongoing create threads running for the
   * old block, the threads will be returned without updating the block.
   *
   * @return ongoing create threads if there is any. Otherwise, return null.
   */
  private synchronized List<Thread> tryUpdateBlock(Block oldblock, Block newblock)
      throws IOException {
    // check ongoing create threads
    final ActiveFile activefile = ongoingCreates.get(oldblock);
    if (activefile != null && !activefile.threads.isEmpty()) {
      // remove dead threads
      for (Iterator<Thread> i = activefile.threads.iterator(); i.hasNext(); ) {
        final Thread t = i.next();
        if (!t.isAlive()) {
          i.remove();
        }
      }

      // return living threads
      if (!activefile.threads.isEmpty()) {
        return new ArrayList<Thread>(activefile.threads);
      }
    }

    // No ongoing create threads is alive. Update block.
    File blockFile = findBlockFile(oldblock.getBlockId());
    if (blockFile == null) {
      throw new IOException("Block " + oldblock + " does not exist.");
    }

    File oldMetaFile = findMetaFile(blockFile);
    long oldgs = parseGenerationStamp(blockFile, oldMetaFile);

    // rename meta file to a tmp file
    File tmpMetaFile =
        new File(
            oldMetaFile.getParent(),
            oldMetaFile.getName() + "_tmp" + newblock.getGenerationStamp());
    if (!oldMetaFile.renameTo(tmpMetaFile)) {
      throw new IOException("Cannot rename block meta file to " + tmpMetaFile);
    }

    // update generation stamp
    if (oldgs > newblock.getGenerationStamp()) {
      throw new IOException(
          "Cannot update block (id="
              + newblock.getBlockId()
              + ") generation stamp from "
              + oldgs
              + " to "
              + newblock.getGenerationStamp());
    }

    // update length
    if (newblock.getNumBytes() > oldblock.getNumBytes()) {
      throw new IOException(
          "Cannot update block file (="
              + blockFile
              + ") length from "
              + oldblock.getNumBytes()
              + " to "
              + newblock.getNumBytes());
    }
    if (newblock.getNumBytes() < oldblock.getNumBytes()) {
      truncateBlock(blockFile, tmpMetaFile, oldblock.getNumBytes(), newblock.getNumBytes());
    }

    // rename the tmp file to the new meta file (with new generation stamp)
    File newMetaFile = getMetaFile(blockFile, newblock);
    if (!tmpMetaFile.renameTo(newMetaFile)) {
      throw new IOException("Cannot rename tmp meta file to " + newMetaFile);
    }

    updateBlockMap(ongoingCreates, oldblock, newblock);
    updateBlockMap(volumeMap, oldblock, newblock);

    // paranoia! verify that the contents of the stored block
    // matches the block file on disk.
    validateBlockMetadata(newblock);
    return null;
  }
  /**
   * Method to move files from HDFS to local filesystem
   *
   * <p>localPath: Path on the machines filesystem fs:FileSystem object from HDFS pathList:List of
   * paths for files that might need to be backed up size:max size in bytes to be backed up
   *
   * <p>ReturnsDate of the last files backed up if reached size limit, else, zero
   */
  public long backupFiles(
      String localPath, String preservePath, FileSystem fs, ArrayList<Path> pathList, long size) {
    Path fsPath;
    long tmpSize = 0;
    long tmpDate = 0;

    // Start iterating over all paths
    for (Path hdfsPath : pathList) {
      try {
        long nFileSize = fs.getContentSummary(hdfsPath).getLength();
        tmpSize = tmpSize + nFileSize;

        if ((tmpSize <= size) || (size == 0)) {
          FileStatus stat = fs.getFileStatus(hdfsPath);

          System.err.println(
              "File "
                  + hdfsPath.toUri().getPath()
                  + " "
                  + nFileSize
                  + " bytes, "
                  + "perms: "
                  + stat.getOwner()
                  + "/"
                  + stat.getGroup()
                  + ", "
                  + stat.getPermission().toString());

          tmpDate = stat.getModificationTime() / 1000;

          String sFsPath = localPath + hdfsPath.toUri().getPath();
          fsPath = new Path(sFsPath);

          File f = new File(sFsPath);

          // COMMENTED OUT: until a few backup cycles run
          // and the mtime gets in fact set on all copied
          // files.
          //
          // ignore it if the file exists and has the same mtime
          // if (f.exists() && f.isFile() && f.lastModified() == stat.getModificationTime())
          // {
          // System.out.println("no need to backup " + f.toString() + ", mtime matches hdfs");
          // continue;
          // }

          if (false == m_bDryRun) {
            // check if we need to back up the local file
            // (not directory), if it already exists.
            if (f.exists() && f.isFile()) {
              // ignore files with substrings in the
              // no-preserve file
              if (true == doPreserveFile(sFsPath)) {
                // move it to the backup path
                String sNewPath = preservePath + hdfsPath.toUri().getPath();
                File newFile = new File(sNewPath);

                // create directory structure for new file?
                if (false == newFile.getParentFile().exists()) {
                  if (false == newFile.getParentFile().mkdirs()) {
                    System.err.println("Failed to mkdirs " + newFile.getParentFile().toString());
                    System.exit(1);
                  }
                }

                // rename existing file to new location
                if (false == f.renameTo(newFile)) {
                  System.err.println(
                      "Failed to renameTo " + f.toString() + " to " + newFile.toString());
                  System.exit(1);
                }

                System.out.println("preserved " + f.toString() + " into " + newFile.toString());
              } else {
                System.out.println("skipped preservation of " + f.toString());
              }
            }

            // copy from hdfs to local filesystem
            fs.copyToLocalFile(hdfsPath, fsPath);

            // set the mtime to match hdfs file
            f.setLastModified(stat.getModificationTime());

            // compare checksums on both files
            compareChecksums(fs, hdfsPath, sFsPath);
          }

          // don't print the progress after every file -- go
          // by at least 1% increments
          long nPercentDone = (long) (100 * tmpSize / m_nTotalBytes);
          if (nPercentDone > m_nLastPercentBytesDone) {
            System.out.println(
                "progress: copied "
                    + prettyPrintBytes(tmpSize)
                    + ", "
                    + nPercentDone
                    + "% done"
                    + ", tstamp="
                    + tmpDate);

            m_nLastPercentBytesDone = nPercentDone;
          }

          if (m_nSleepSeconds > 0) {
            try {
              Thread.sleep(1000 * m_nSleepSeconds);
            } catch (Exception e2) {
              // ignore
            }
          }
        } else {
          return tmpDate;
        }
      } catch (IOException e) {
        System.err.println("FATAL ERROR: Something wrong with the file");
        System.err.println(e);
        System.out.println(tmpDate);
        System.exit(1);

        return 0;
      }
    }

    return 0;
  }