spark DiskBlockManager

RDD本身presist可以是本地存储，本地存储级别的持久化实现方式如下：

DiskBlockManager负责管理和维护block和磁盘存储的映射关系，通过blockId作为文件名称，然后如果是多个目录通过blcokId的hash值进行分发。

包括创建目录，删除，读取文件，以及一些退出删除文件的机制。

/**
  * Creates and maintains the logical mapping between logical blocks and physical on-disk
  * locations. One block is mapped to one file with a name given by its BlockId.
  * 创建和维护blocks和磁盘存储位置的映射关系。每个block对应一个文件。文件名字是bclockId。
  * Block files are hashed among the directories listed in spark.local.dir (or in
  * SPARK_LOCAL_DIRS, if it's set).
  *
  * spark.local.dir目录存储 block 的文件。是通过文件名的hash到各个spark.local.dirs目录里面
  */
private[spark] class DiskBlockManager(conf: SparkConf, deleteFilesOnStop: Boolean) extends Logging {

  private[spark] val subDirsPerLocalDir = conf.getInt("spark.diskStore.subDirectories", 64)

  /* Create one local directory for each path mentioned in spark.local.dir; then, inside this
   * directory, create multiple subdirectories that we will hash files into, in order to avoid
   * having really large inodes at the top level. */
  private[spark] val localDirs: Array[File] = createLocalDirs(conf)
  if (localDirs.isEmpty) {
    logError("Failed to create any local dir.")
    System.exit(ExecutorExitCode.DISK_STORE_FAILED_TO_CREATE_DIR)
  }
  // The content of subDirs is immutable but the content of subDirs(i) is mutable. And the content
  // of subDirs(i) is protected by the lock of subDirs(i)
  private val subDirs = Array.fill(localDirs.length)(new Array[File](subDirsPerLocalDir))

  private val shutdownHook = addShutdownHook()

  /** Looks up a file by hashing it into one of our local subdirectories. */
  // This method should be kept in sync with
  // org.apache.spark.network.shuffle.ExternalShuffleBlockResolver#getFile().

  // 通过文件名的hash在目录中查找文件
  def getFile(filename: String): File = {
    // Figure out which local directory it hashes to, and which subdirectory in that
    val hash = Utils.nonNegativeHash(filename)
    val dirId = hash % localDirs.length
    val subDirId = (hash / localDirs.length) % subDirsPerLocalDir

    // Create the subdirectory if it doesn't already exist
    val subDir = subDirs(dirId).synchronized {
      val old = subDirs(dirId)(subDirId)
      if (old != null) {
        old
      } else {
        val newDir = new File(localDirs(dirId), "%02x".format(subDirId))
        if (!newDir.exists() && !newDir.mkdir()) {
          throw new IOException(s"Failed to create local dir in $newDir.")
        }
        subDirs(dirId)(subDirId) = newDir
        newDir
      }
    }

    new File(subDir, filename)
  }

  def getFile(blockId: BlockId): File = getFile(blockId.name)

  /** Check if disk block manager has a block. */
  def containsBlock(blockId: BlockId): Boolean = {
    getFile(blockId.name).exists()
  }

  /** List all the files currently stored on disk by the disk manager. */
  def getAllFiles(): Seq[File] = {
    // Get all the files inside the array of array of directories
    subDirs.flatMap { dir =>
      dir.synchronized {
        // Copy the content of dir because it may be modified in other threads
        dir.clone()
      }
    }.filter(_ != null).flatMap { dir =>
      val files = dir.listFiles()
      if (files != null) files else Seq.empty
    }
  }

  /** List all the blocks currently stored on disk by the disk manager. */
  def getAllBlocks(): Seq[BlockId] = {
    getAllFiles().map(f => BlockId(f.getName))
  }

  /** Produces a unique block id and File suitable for storing local intermediate results. */
  def createTempLocalBlock(): (TempLocalBlockId, File) = {
    var blockId = new TempLocalBlockId(UUID.randomUUID())
    while (getFile(blockId).exists()) {
      blockId = new TempLocalBlockId(UUID.randomUUID())
    }
    (blockId, getFile(blockId))
  }

  /** Produces a unique block id and File suitable for storing shuffled intermediate results. */
  def createTempShuffleBlock(): (TempShuffleBlockId, File) = {
    var blockId = new TempShuffleBlockId(UUID.randomUUID())
    while (getFile(blockId).exists()) {
      blockId = new TempShuffleBlockId(UUID.randomUUID())
    }
    (blockId, getFile(blockId))
  }

  /**
    * Create local directories for storing block data. These directories are
    * located inside configured local directories and won't
    * be deleted on JVM exit when using the external shuffle service.
    *
    *  在rootDir中创建blockmgr目录，用来存储block数据
    *
    */
  private def createLocalDirs(conf: SparkConf): Array[File] = {
    Utils.getConfiguredLocalDirs(conf).flatMap { rootDir =>
      try {
        val localDir = Utils.createDirectory(rootDir, "blockmgr")
        logInfo(s"Created local directory at $localDir")
        Some(localDir)
      } catch {
        case e: IOException =>
          logError(s"Failed to create local dir in $rootDir. Ignoring this directory.", e)
          None
      }
    }
  }

  private def addShutdownHook(): AnyRef = {
    logDebug("Adding shutdown hook") // force eager creation of logger
    ShutdownHookManager.addShutdownHook(ShutdownHookManager.TEMP_DIR_SHUTDOWN_PRIORITY + 1) { () =>
      logInfo("Shutdown hook called")
      DiskBlockManager.this.doStop()
    }
  }

  /** Cleanup local dirs and stop shuffle sender. */
  private[spark] def stop() {
    // Remove the shutdown hook.  It causes memory leaks if we leave it around.
    try {
      ShutdownHookManager.removeShutdownHook(shutdownHook)
    } catch {
      case e: Exception =>
        logError(s"Exception while removing shutdown hook.", e)
    }
    doStop()
  }

  //删除目录
  private def doStop(): Unit = {
    if (deleteFilesOnStop) {
      localDirs.foreach { localDir =>
        if (localDir.isDirectory() && localDir.exists()) {
          try {
            if (!ShutdownHookManager.hasRootAsShutdownDeleteDir(localDir)) {
              Utils.deleteRecursively(localDir)
            }
          } catch {
            case e: Exception =>
              logError(s"Exception while deleting local spark dir: $localDir", e)
          }
        }
      }
    }
  }
}

具体调用句柄在DiskStore中，调用put方法，将指定的block写到本地。

private[spark] class DiskStore(conf: SparkConf, diskManager: DiskBlockManager) extends Logging {

  private val minMemoryMapBytes = conf.getSizeAsBytes("spark.storage.memoryMapThreshold", "2m")

  def getSize(blockId: BlockId): Long = {
    diskManager.getFile(blockId.name).length
  }

  /**
    * Invokes the provided callback function to write the specific block.
    * 调用提供的回掉方法把指定的block写到磁盘
    *
    * @throws IllegalStateException if the block already exists in the disk store.
    */
  def put(blockId: BlockId)(writeFunc: FileOutputStream => Unit): Unit = {
    if (contains(blockId)) {
      throw new IllegalStateException(s"Block $blockId is already present in the disk store")
    }
    logDebug(s"Attempting to put block $blockId")
    val startTime = System.currentTimeMillis
    //生成block文件，blockid作为文件名，包含一些创建文件夹的操作
    val file = diskManager.getFile(blockId)
    val fileOutputStream = new FileOutputStream(file)
    var threwException: Boolean = true
    try {
      writeFunc(fileOutputStream)
      threwException = false
    } finally {
      try {
        Closeables.close(fileOutputStream, threwException)
      } finally {
        if (threwException) {
          remove(blockId)
        }
      }
    }
    val finishTime = System.currentTimeMillis
    logDebug("Block %s stored as %s file on disk in %d ms".format(
      file.getName,
      Utils.bytesToString(file.length()),
      finishTime - startTime))
  }

  def putBytes(blockId: BlockId, bytes: ChunkedByteBuffer): Unit = {
    put(blockId) { fileOutputStream =>
      val channel = fileOutputStream.getChannel
      Utils.tryWithSafeFinally {
        bytes.writeFully(channel)
      } {
        channel.close()
      }
    }
  }

  //读取出指定的block数据放到内存中
  def getBytes(blockId: BlockId): ChunkedByteBuffer = {
    val file = diskManager.getFile(blockId.name)
    val channel = new RandomAccessFile(file, "r").getChannel
    Utils.tryWithSafeFinally {
      // For small files, directly read rather than memory map
      if (file.length < minMemoryMapBytes) {
        val buf = ByteBuffer.allocate(file.length.toInt)
        channel.position(0)
        while (buf.remaining() != 0) {
          if (channel.read(buf) == -1) {
            throw new IOException("Reached EOF before filling buffer\n" +
              s"offset=0\nfile=${file.getAbsolutePath}\nbuf.remaining=${buf.remaining}")
          }
        }
        buf.flip()
        new ChunkedByteBuffer(buf)
      } else {
        new ChunkedByteBuffer(channel.map(MapMode.READ_ONLY, 0, file.length))
      }
    } {
      channel.close()
    }
  }

  //删除block数据
  def remove(blockId: BlockId): Boolean = {
    val file = diskManager.getFile(blockId.name)
    if (file.exists()) {
      val ret = file.delete()
      if (!ret) {
        logWarning(s"Error deleting ${file.getPath()}")
      }
      ret
    } else {
      false
    }
  }

  def contains(blockId: BlockId): Boolean = {
    val file = diskManager.getFile(blockId.name)
    file.exists()
  }
}