spark源码系列（9）BlockManager的原理

上一篇说到CacheManager和checkpoint来管理缓存和数据相关的东西。但实际上，他们底层都是通过BlockManger来管理数据的。

找到RDD#getOrCompute中的

SparkEnv.get.blockManager.getOrElseUpdate(blockId, storageLevel, elementClassTag,我们就可以看到最终是通过BlockManager来管理数据。在分析源码前老规矩，先画一张整体的图

首先Driver上有一个BlockManagerMaster，它的功能就是负责各个节点上的BlockManager元数据管理，进行维护。比如block的增删改查等操作，都会在这里维护变更。

每个节点上都会有blockManager，BlockManager有几个关键组件：

DiskStore：负责磁盘上的数据进行读写

MemoryStore：负责对内存上的数据进行读写

BlockTransferService：负责和远程节点建立数据传输的组件

每个BlockManager创建之后，向Driver中的BlockManagerMaster进行注册，此时BlockManager会为创建对应的BlockManagerInfo。

使用BlockManager进行读写操作时，比如RDD运行过程中间数据，或者手动指定了persisit，优先将数据写入内存，如果内存不够，会使用算法将内存中的数据写入磁盘。如果persisit指定了replica，那么会使用BlockTransferService将数据replicate一份到其他的节点的blockManager上去。

只要使用了blockManager执行了数据的增删改操作，。那么必须将Block的BlockStatus上报到BlockManagerMaster上去，在BlockManagerMaster上，会对指定的BlockManager的BlockManagerInfo内部的blockStatus增删改查操作。从而达到维护元数据的功能。。

从blockManager上读数据，优先从本地读取，DisStore或者MemoryStore。如果本地没有数据的话，那么会用BlockTransferService从远程节点上读取数据。

-----------------------------------

先看下BlockManager的注册过程：

在Driver和Executor端都会初始化BlockManager。

在创建SparkContext对象的时候就会调用_env.blockManager.initialize(_applicationId)创建BlockManager对象

创建Executor的时候，Executor内部会调用_env.blockManager.initialize(conf.getAppId)方法创建BlockManager

点击进入Blockmanager#initialize方法

/**
 * BlockManager 运行在每个节点上，包括driver和executors都会有一份主要提供了本地或者远程存储的功能
 * 支持内存 磁盘 堆外存储（Tychyon）
 */
private[spark] class BlockManager(
    executorId: String,  //BlockManager运行在哪个Executor之上
    rpcEnv: RpcEnv,   //远程通信体
    val master: BlockManagerMaster,  //BlockManagerMaster,管理整个集群的BlockManger
    val serializerManager: SerializerManager,//默认序列化器
    val conf: SparkConf,
    memoryManager: MemoryManager,//内存管理器
    mapOutputTracker: MapOutputTracker,//shuffle输出
    shuffleManager: ShuffleManager,//shuffle管理器
    val blockTransferService: BlockTransferService,//用于Block间的网络通信(进行备份时)
    securityManager: SecurityManager,
    numUsableCores: Int)
  extends BlockDataManager with BlockEvictionHandler with Logging {

......

  //负责内存存储  
  private[spark] val memoryStore =
    new MemoryStore(conf, blockInfoManager, serializerManager, memoryManager, this)
  //负责磁盘存储
  private[spark] val diskStore = new DiskStore(conf, diskBlockManager, securityManager)
  memoryManager.setMemoryStore(memoryStore)

......


  def initialize(appId: String): Unit = {
    //初始化BlockTransferService，其实是它的子类NettyBlockTransferService是下了init方法
    //该方法的作用就是初始化传输服务，通过传输服务可以从不同的节点上拉取Block数据
    blockTransferService.init(this)
    shuffleClient.init(appId)

    //设置block的复制分片策略，由spark.storage.replication.policy指定
    blockReplicationPolicy = {
      val priorityClass = conf.get(
        "spark.storage.replication.policy", classOf[RandomBlockReplicationPolicy].getName)
      val clazz = Utils.classForName(priorityClass)
      val ret = clazz.newInstance.asInstanceOf[BlockReplicationPolicy]
      logInfo(s"Using $priorityClass for block replication policy")
      ret
    }

    /**
     * 根据给定参数为对对应的Executor封装一个BlockManagerId对象（块存储的唯一标识）
     * executorId:executor的Id，
     * blockTransferService.hostName:传输Block数据的服务的主机名
     * blockTransferService.port:传输Block数据的服务的主机名
     */
    val id =
      BlockManagerId(executorId, blockTransferService.hostName, blockTransferService.port, None)

    //调用BlockManagerMaster的registerBlockManager方法向Driver上的BlockManagerMaster注册
    val idFromMaster = master.registerBlockManager(
      id,
      maxOnHeapMemory,
      maxOffHeapMemory,
      slaveEndpoint)

    //更新BlockManagerId
    blockManagerId = if (idFromMaster != null) idFromMaster else id

    //判断是否开了外部shuffle服务
    shuffleServerId = if (externalShuffleServiceEnabled) {
      logInfo(s"external shuffle service port = $externalShuffleServicePort")
      BlockManagerId(executorId, blockTransferService.hostName, externalShuffleServicePort)
    } else {
      blockManagerId
    }

    // Register Executors' configuration with the local shuffle service, if one should exist.
    //如果开启了外部shuffle服务，并且该节点是Driver的话就调用registerWithExternalShuffleServer方法
    //将BlockManager注册在本地
    if (externalShuffleServiceEnabled && !blockManagerId.isDriver) {
      registerWithExternalShuffleServer()
    }

    logInfo(s"Initialized BlockManager: $blockManagerId")
  }

BlockManagerMaster类里的registerBlockManager方法，向Driver发送RegisterBlockManager消息进行注册

  /**
   * 向Driver发送RegisterBlockManager消息进行注册
   */
  def registerBlockManager(
      blockManagerId: BlockManagerId,
      maxOnHeapMemSize: Long,
      maxOffHeapMemSize: Long,
      slaveEndpoint: RpcEndpointRef): BlockManagerId = {
    logInfo(s"Registering BlockManager $blockManagerId")
    //向Driver发送注册BlockManager请求
    //blockManagerId：块存储的唯一标识，里边封装了该BlockManager所在的executorId，提供Netty服务的主机名和端口
    //maxMemSize最大的内存
    val updatedId = driverEndpoint.askSync[BlockManagerId](
      RegisterBlockManager(blockManagerId, maxOnHeapMemSize, maxOffHeapMemSize, slaveEndpoint))
    logInfo(s"Registered BlockManager $updatedId")
    updatedId
  }

BlockManagerMasterEndpoint类里的receiveAndReply方法，这个方法就是接受请求的消息，然后并处理请求

 override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
    //接收到来自Executor上的BlockManager注册请求的时候,调用register方法开始注册BlockManager，
    case RegisterBlockManager(blockManagerId, maxOnHeapMemSize, maxOffHeapMemSize, slaveEndpoint) =>
      context.reply(register(blockManagerId, maxOnHeapMemSize, maxOffHeapMemSize, slaveEndpoint))

......

}

BlockManagerMasterEndpoint类里的register方法，该方法的作用就是开始注册executor上的BlockManager

/**
 * 负责维护各个executor和BlockManager的元数据 BlockManagerInfo、BlockStatus
 */
private[spark]
class BlockManagerMasterEndpoint(
    override val rpcEnv: RpcEnv,
    val isLocal: Boolean,
    conf: SparkConf,
    listenerBus: LiveListenerBus)
  extends ThreadSafeRpcEndpoint with Logging {
  
  // Mapping from block manager id to the block manager's information.
  // BlockManagerId->BlockManagerInfo的映射
  private val blockManagerInfo = new mutable.HashMap[BlockManagerId, BlockManagerInfo]

  // Mapping from executor ID to block manager ID.
  //executorId->BlockManagerId的映射
  private val blockManagerIdByExecutor = new mutable.HashMap[String, BlockManagerId]

......

  /**
   * Returns the BlockManagerId with topology information populated, if available.
   * 
   * 开始注册executor上的BlockManager
   * 
   */
  private def register(
      idWithoutTopologyInfo: BlockManagerId,
      maxOnHeapMemSize: Long,
      maxOffHeapMemSize: Long,
      slaveEndpoint: RpcEndpointRef): BlockManagerId = {
    // the dummy id is not expected to contain the topology information.
    // we get that info here and respond back with a more fleshed out block manager id
    //利用从Executor上传过来的BlockManagerId信息重新封装BlockManagerId，并且
    //之前传过来没有拓扑信息，这次直接将拓扑信息也封装进去，得到一个更完整的BlockManagerId
    val id = BlockManagerId(
      idWithoutTopologyInfo.executorId,
      idWithoutTopologyInfo.host,
      idWithoutTopologyInfo.port,
      topologyMapper.getTopologyForHost(idWithoutTopologyInfo.host))

    val time = System.currentTimeMillis()
    //判断当前这个BlockManagerId是否注册过，注册结构为：HashMap[BlockManagerId, BlockManagerInfo]
    //如果没注册过就向下执行开始注册
    if (!blockManagerInfo.contains(id)) {
      /**
       * 首先会根据executorId查找内存缓存结构中是否有对应的BlockManagerId,如果为存在那么就将调用removeExecutor方法,将executor从BlockManagerMaster中移除
       * 首先会移除executorId对应的BlockManagerId,然后在移除该旧的BlockManager。
       * 其实就是移除以前的注册过的旧数据
       */
      blockManagerIdByExecutor.get(id.executorId) match {
        case Some(oldId) =>
          // A block manager of the same executor already exists, so remove it (assumed dead)
          logError("Got two different block manager registrations on same executor - "
              + s" will replace old one $oldId with new one $id")
          removeExecutor(id.executorId)
        case None =>
      }
      logInfo("Registering block manager %s with %s RAM, %s".format(
        id.hostPort, Utils.bytesToString(maxOnHeapMemSize + maxOffHeapMemSize), id))

      //将executorId与BlockManagerId映射起来，放到内存缓存中
      blockManagerIdByExecutor(id.executorId) = id

      //将BlockManagerId与BlockManagerInfo映射起来，放入内存缓存中
     //BlockManagerInfo封住了BlockMangerId，当前注册的事件，最大的内存
      blockManagerInfo(id) = new BlockManagerInfo(
        id, System.currentTimeMillis(), maxOnHeapMemSize, maxOffHeapMemSize, slaveEndpoint)
    }
    listenerBus.post(SparkListenerBlockManagerAdded(time, id, maxOnHeapMemSize + maxOffHeapMemSize,
        Some(maxOnHeapMemSize), Some(maxOffHeapMemSize)))
    id
  }

..... 
 
}

找到BlockManagerInfo.scala 

/**
 * 每个BlockManager的BlockManagerInfo,相当于是BlockManager的元数据
 */
private[spark] class BlockManagerInfo(
    val blockManagerId: BlockManagerId,
    timeMs: Long,
    val maxOnHeapMem: Long,
    val maxOffHeapMem: Long,
    val slaveEndpoint: RpcEndpointRef)
  extends Logging {

  val maxMem = maxOnHeapMem + maxOffHeapMem

  private var _lastSeenMs: Long = timeMs
  private var _remainingMem: Long = maxMem

  // Mapping from block id to its status.
  //BlockManagerInfo 管理了每个BlockManager内部的BlockId->BlockStatus映射
  private val _blocks = new JHashMap[BlockId, BlockStatus]

......

}
----------------------------------------------

上面简述了一下blockManager的注册机制，接下去说一下数据的读取

先用一张图简要说明下整体流程：

 

找到RDD#getOrCompute方法

 

SparkEnv.get.blockManager.getOrElseUpdate(blockId, storageLevel, elementClassTag, () => {

 /**
   * 如果指定的block存在，则直接获取，否则调用makeIterator方法去计算block，然后持久化最后返回值
   */
  def getOrElseUpdate[T](
      blockId: BlockId,
      level: StorageLevel,
      classTag: ClassTag[T],
      makeIterator: () => Iterator[T]): Either[BlockResult, Iterator[T]] = {
    // Attempt to read the block from local or remote storage. If it's present, then we don't need
    // to go through the local-get-or-put path.
     // 尝试从本地获取数据，如果获取不到则从远端获取
    get[T](blockId)(classTag) match {
      case Some(block) =>
        return Left(block)
      case _ =>
        // Need to compute the block.
    }
    // Initially we hold no locks on this block.
    // 如果本地化和远端都没有获取到数据，则调用makeIterator计算，最后将结果写入block
    doPutIterator(blockId, makeIterator, level, classTag, keepReadLock = true) match {
      // 表示写入成功
      case None =>
        // doPut() didn't hand work back to us, so the block already existed or was successfully
        // stored. Therefore, we now hold a read lock on the block.
        // 从本地获取数据块
        val blockResult = getLocalValues(blockId).getOrElse {
          // Since we held a read lock between the doPut() and get() calls, the block should not
          // have been evicted, so get() not returning the block indicates some internal error.
          releaseLock(blockId)
          throw new SparkException(s"get() failed for block $blockId even though we held a lock")
        }
        // We already hold a read lock on the block from the doPut() call and getLocalValues()
        // acquires the lock again, so we need to call releaseLock() here so that the net number
        // of lock acquisitions is 1 (since the caller will only call release() once).
        releaseLock(blockId)
        Left(blockResult)
      case Some(iter) => // 如果写入失败
        // The put failed, likely because the data was too large to fit in memory and could not be
        // dropped to disk. Therefore, we need to pass the input iterator back to the caller so
        // that they can decide what to do with the values (e.g. process them without caching).
        // 如果put操作失败，表示可能是因为数据太大，无法写入内存，又无法被磁盘drop，因此我们需要返回这个iterator给调用者
       Right(iter)
    }
  }

BlockManager的get方法

  /**
   * 读取数据的入口点，会判断数据是否在本地而选择是直接从本地读取还是通过BlockTransferService读取远程数据：
   */
  def get[T: ClassTag](blockId: BlockId): Option[BlockResult] = {
   // 从本地获取block
    val local = getLocalValues(blockId)
      // 如果本地获取到了则返回
    if (local.isDefined) {
      logInfo(s"Found block $blockId locally")
      return local
    }
    // 如果本地没有获取到则从远端获取
    val remote = getRemoteValues[T](blockId)
    // 如果远端获取到了则返回，没有返回None
    if (remote.isDefined) {
      logInfo(s"Found block $blockId remotely")
      return remote
    }
    None
  }

BlockManager的getLocalValues方法

  /**
   * 从本地获取block,如果存在返回BlockResult，不存在返回None；
   * 如果storage level是磁盘，则还需将得到的block缓存到内存存储，方便下次读取
   */
  def getLocalValues(blockId: BlockId): Option[BlockResult] = {
    logDebug(s"Getting local block $blockId")
    // 调用block info manager，锁定该block，然后读取block,返回该block 元数据block info
    blockInfoManager.lockForReading(blockId) match {
      case None =>  // 没有读取到则返回None
        logDebug(s"Block $blockId was not found")
        None
      case Some(info) =>  // 读取到block元数据
        // 获取存储级别storage level
        val level = info.level
        logDebug(s"Level for block $blockId is $level")
        val taskAttemptId = Option(TaskContext.get()).map(_.taskAttemptId())
        // 如果使用内存，且内存memory store包含这个block id
        if (level.useMemory && memoryStore.contains(blockId)) {
          // 如果序列化了，那么说明是对象数据，使用getValues
          val iter: Iterator[Any] = if (level.deserialized) {
            memoryStore.getValues(blockId).get
          } else {
            // 没序列化，那么是数据流，使用getBytes()
            serializerManager.dataDeserializeStream(
              blockId, memoryStore.getBytes(blockId).get.toInputStream())(info.classTag)
          }
          // We need to capture the current taskId in case the iterator completion is triggered
          // from a different thread which does not have TaskContext set; see SPARK-18406 for
          // discussion.
          val ci = CompletionIterator[Any, Iterator[Any]](iter, {
            releaseLock(blockId, taskAttemptId)
          })
          // 构建一个BlockResult对象返回,这个对象包括数据，读取方式以及字节大小
          Some(new BlockResult(ci, DataReadMethod.Memory, info.size))
        } 
        // 如果使用磁盘存储，且disk store包含这个block则从磁盘获取，并且把结果放入内存
        else if (level.useDisk && diskStore.contains(blockId)) {
           // 先读取数据
          val diskData = diskStore.getBytes(blockId)
          val iterToReturn: Iterator[Any] = {
            //如果需要反序列化，则进行反序列
            if (level.deserialized) {
              val diskValues = serializerManager.dataDeserializeStream(
                blockId,
                diskData.toInputStream())(info.classTag)
              // 先序列化，后将数据放入内存
              maybeCacheDiskValuesInMemory(info, blockId, level, diskValues)
            } else {
              // 先将数据放入内存
              val stream = maybeCacheDiskBytesInMemory(info, blockId, level, diskData)
                .map { _.toInputStream(dispose = false) }
                .getOrElse { diskData.toInputStream() }
              //序列化返回的值
              serializerManager.dataDeserializeStream(blockId, stream)(info.classTag)
            }
          }
          val ci = CompletionIterator[Any, Iterator[Any]](iterToReturn, {
            releaseLockAndDispose(blockId, diskData, taskAttemptId)
          })
          // 构建BlockResult返回
          Some(new BlockResult(ci, DataReadMethod.Disk, info.size))
        } else {
          // 处理本地读取block失败，报告driver这是一个无效的block,将会删除这个block
          handleLocalReadFailure(blockId)
        }
    }
  }

memoryStore.getValues(blockId).get方法

  //存放每个block数据
  private val entries = new LinkedHashMap[BlockId, MemoryEntry[_]](32, 0.75f, true)

......

  def getValues(blockId: BlockId): Option[Iterator[_]] = {
   //多线程并发访问同步
    val entry = entries.synchronized { entries.get(blockId) }
    entry match {
      case null => None
      case e: SerializedMemoryEntry[_] =>
        throw new IllegalArgumentException("should only call getValues on deserialized blocks")
      case DeserializedMemoryEntry(values, _, _) =>
        val x = Some(values)
        x.map(_.iterator)
    }
  }

  def getBytes(blockId: BlockId): Option[ChunkedByteBuffer] = {
    val entry = entries.synchronized { entries.get(blockId) }
    entry match {
      case null => None
      case e: DeserializedMemoryEntry[_] =>
        throw new IllegalArgumentException("should only call getBytes on serialized blocks")
      case SerializedMemoryEntry(bytes, _, _) => Some(bytes)
    }
  }

磁盘读取diskStore.getBytes(blockId)方法

  def getBytes(blockId: BlockId): BlockData = {
    val file = diskManager.getFile(blockId.name)
    val blockSize = getSize(blockId)

    securityManager.getIOEncryptionKey() match {
      case Some(key) =>
        // Encrypted blocks cannot be memory mapped; return a special object that does decryption
        // and provides InputStream / FileRegion implementations for reading the data.
        new EncryptedBlockData(file, blockSize, conf, key)

      case _ =>
        new DiskBlockData(minMemoryMapBytes, maxMemoryMapBytes, file, blockSize)
    }
  }

回到BlockManager的get方法

远程读取val remote = getRemoteValues[T](blockId)

 /**
   * Get block from remote block managers.
   *
   * This does not acquire a lock on this block in this JVM.
   */
  private def getRemoteValues[T: ClassTag](blockId: BlockId): Option[BlockResult] = {
    val ct = implicitly[ClassTag[T]]
   // 将远程fetch的结果进行反序列化，然后构建BlockResult返回
    getRemoteBytes(blockId).map { data =>
      val values =
        serializerManager.dataDeserializeStream(blockId, data.toInputStream(dispose = true))(ct)
      new BlockResult(values, DataReadMethod.Network, data.size)
    }
  }

点击进入getRemoteBytes

  def getRemoteBytes(blockId: BlockId): Option[ChunkedByteBuffer] = {
    // TODO if we change this method to return the ManagedBuffer, then getRemoteValues
    // could just use the inputStream on the temp file, rather than reading the file into memory.
    // Until then, replication can cause the process to use too much memory and get killed
    // even though we've read the data to disk.
    logDebug(s"Getting remote block $blockId")
    require(blockId != null, "BlockId is null")
    var runningFailureCount = 0
    var totalFailureCount = 0

    // Because all the remote blocks are registered in driver, it is not necessary to ask
    // all the slave executors to get block status.
    //查询这个数据的具体位置,获取到了所有持有这个block 的block manager id
    val locationsAndStatus = master.getLocationsAndStatus(blockId)
    val blockSize = locationsAndStatus.map { b =>
      b.status.diskSize.max(b.status.memSize)
    }.getOrElse(0L)
    val blockLocations = locationsAndStatus.map(_.locations).getOrElse(Seq.empty)

    // If the block size is above the threshold, we should pass our FileManger to
    // BlockTransferService, which will leverage it to spill the block; if not, then passed-in
    // null value means the block will be persisted in memory.
    val tempFileManager = if (blockSize > maxRemoteBlockToMem) {
      remoteBlockTempFileManager
    } else {
      null
    }
    val locations = sortLocations(blockLocations)
    // 最大允许的获取block的失败次数为该block对应的block manager数量
    val maxFetchFailures = locations.size
    var locationIterator = locations.iterator
    // 开始遍历block manager
    while (locationIterator.hasNext) {
      val loc = locationIterator.next()
      logDebug(s"Getting remote block $blockId from $loc")
      // 通过调用BlockTransferSerivce的fetchBlockSync方法从远端获取block
      val data = try {
        blockTransferService.fetchBlockSync(
          loc.host, loc.port, loc.executorId, blockId.toString, tempFileManager)
      } catch {
        case NonFatal(e) =>
          runningFailureCount += 1
          totalFailureCount += 1
          // 如果总的失败数量大于了阀值则返回None
          if (totalFailureCount >= maxFetchFailures) {
            // Give up trying anymore locations. Either we've tried all of the original locations,
            // or we've refreshed the list of locations from the master, and have still
            // hit failures after trying locations from the refreshed list.
            logWarning(s"Failed to fetch block after $totalFailureCount fetch failures. " +
              s"Most recent failure cause:", e)
            return None
          }

          logWarning(s"Failed to fetch remote block $blockId " +
            s"from $loc (failed attempt $runningFailureCount)", e)

          // If there is a large number of executors then locations list can contain a
          // large number of stale entries causing a large number of retries that may
          // take a significant amount of time. To get rid of these stale entries
          // we refresh the block locations after a certain number of fetch failures
          if (runningFailureCount >= maxFailuresBeforeLocationRefresh) {
            locationIterator = sortLocations(master.getLocations(blockId)).iterator
            logDebug(s"Refreshed locations from the driver " +
              s"after ${runningFailureCount} fetch failures.")
            runningFailureCount = 0
          }

          // This location failed, so we retry fetch from a different one by returning null here
          null
      }
      // 返回ChunkedByteBuffer
      if (data != null) {
        // SPARK-24307 undocumented "escape-hatch" in case there are any issues in converting to
        // ChunkedByteBuffer, to go back to old code-path.  Can be removed post Spark 2.4 if
        // new path is stable.
        if (remoteReadNioBufferConversion) {
          return Some(new ChunkedByteBuffer(data.nioByteBuffer()))
        } else {
          return Some(ChunkedByteBuffer.fromManagedBuffer(data))
        }
      }
      logDebug(s"The value of block $blockId is null")
    }
    logDebug(s"Block $blockId not found")
    None
  }

blockTransferService.fetchBlockSync)方法负责根据BlockManagerId读取数据，需要说明的是BlockManagerId不是个字段，是个class,有host,port,executor id等字段

 def fetchBlockSync(
      host: String,
      port: Int,
      execId: String,
      blockId: String,
      tempFileManager: DownloadFileManager): ManagedBuffer = {
    // A monitor for the thread to wait on.
    // 监控等待的线程.
    val result = Promise[ManagedBuffer]()
    fetchBlocks(host, port, execId, Array(blockId),
      new BlockFetchingListener {
        override def onBlockFetchFailure(blockId: String, exception: Throwable): Unit = {
          result.failure(exception)
        }
        override def onBlockFetchSuccess(blockId: String, data: ManagedBuffer): Unit = {
          data match {
            case f: FileSegmentManagedBuffer =>
              result.success(f)
            case _ =>
              val ret = ByteBuffer.allocate(data.size.toInt)
              ret.put(data.nioByteBuffer())
              ret.flip()
              result.success(new NioManagedBuffer(ret))
          }
        }
      }, tempFileManager)
    ThreadUtils.awaitResult(result.future, Duration.Inf)
  }

fetchBlocks是抽象方法，实际上调用了NettyBlockTransferService里面的实现

  /**
   * 用于获取远程shuffle文件，实际上是利用NettyBlockTransferService中创建的netty服务。
    *
   */
  override def fetchBlocks(
      host: String,
      port: Int,
      execId: String,
      blockIds: Array[String],
      listener: BlockFetchingListener,
      tempFileManager: DownloadFileManager): Unit = {
    logTrace(s"Fetch blocks from $host:$port (executor id $execId)")
    try {
      val blockFetchStarter = new RetryingBlockFetcher.BlockFetchStarter {
        override def createAndStart(blockIds: Array[String], listener: BlockFetchingListener) {
           //通过C/S模式从远程进行通信，来拉去数据。
          val client = clientFactory.createClient(host, port)
           // 一对一读取数据
          new OneForOneBlockFetcher(client, appId, execId, blockIds, listener,
            transportConf, tempFileManager).start()
        }
      }

      val maxRetries = transportConf.maxIORetries()
      if (maxRetries > 0) {
        // Note this Fetcher will correctly handle maxRetries == 0; we avoid it just in case there's
        // a bug in this code. We should remove the if statement once we're sure of the stability.
        new RetryingBlockFetcher(transportConf, blockFetchStarter, blockIds, listener).start()
      } else {
        blockFetchStarter.createAndStart(blockIds, listener)
      }
    } catch {
      case e: Exception =>
        logError("Exception while beginning fetchBlocks", e)
        blockIds.foreach(listener.onBlockFetchFailure(_, e))
    }
  }

new OneForOneBlockFetcher(client, appId, execId, blockIds, listener,transportConf, tempFileManager).start()方法是用rpc向持有block的executor发送消息

  public void start() {
 

  public void start() {
    if (blockIds.length == 0) {
      throw new IllegalArgumentException("Zero-sized blockIds array");
    }

    client.sendRpc(openMessage.toByteBuffer(), new RpcResponseCallback() {
      @Override
      public void onSuccess(ByteBuffer response) {
        try {
          streamHandle = (StreamHandle) BlockTransferMessage.Decoder.fromByteBuffer(response);
          logger.trace("Successfully opened blocks {}, preparing to fetch chunks.", streamHandle);

          // Immediately request all chunks -- we expect that the total size of the request is
          // reasonable due to higher level chunking in [[ShuffleBlockFetcherIterator]].
          for (int i = 0; i < streamHandle.numChunks; i++) {
            if (downloadFileManager != null) {
              client.stream(OneForOneStreamManager.genStreamChunkId(streamHandle.streamId, i),
                new DownloadCallback(i));
            } else {
              client.fetchChunk(streamHandle.streamId, i, chunkCallback);
            }
          }
        } catch (Exception e) {
          logger.error("Failed while starting block fetches after success", e);
          failRemainingBlocks(blockIds, e);
        }
      }

      @Override
      public void onFailure(Throwable e) {
        logger.error("Failed while starting block fetches", e);
        failRemainingBlocks(blockIds, e);
      }
    });
  }

消息将由对应Executor的NettyBlockRpcServer中的receive收到，并调用getBlockData方法来读取数据

override def receive(
      client: TransportClient,
      rpcMessage: ByteBuffer,
      responseContext: RpcResponseCallback): Unit = {
    val message = BlockTransferMessage.Decoder.fromByteBuffer(rpcMessage)
    logTrace(s"Received request: $message")

    message match {
      case openBlocks: OpenBlocks =>
        val blocksNum = openBlocks.blockIds.length
        val blocks = for (i <- (0 until blocksNum).view)
          yield blockManager.getBlockData(BlockId.apply(openBlocks.blockIds(i)))
        val streamId = streamManager.registerStream(appId, blocks.iterator.asJava)
        logTrace(s"Registered streamId $streamId with $blocksNum buffers")
        responseContext.onSuccess(new StreamHandle(streamId, blocksNum).toByteBuffer)

      case uploadBlock: UploadBlock =>
        // StorageLevel and ClassTag are serialized as bytes using our JavaSerializer.
        val (level: StorageLevel, classTag: ClassTag[_]) = {
          serializer
            .newInstance()
            .deserialize(ByteBuffer.wrap(uploadBlock.metadata))
            .asInstanceOf[(StorageLevel, ClassTag[_])]
        }
        val data = new NioManagedBuffer(ByteBuffer.wrap(uploadBlock.blockData))
        val blockId = BlockId(uploadBlock.blockId)
        logDebug(s"Receiving replicated block $blockId with level ${level} " +
          s"from ${client.getSocketAddress}")
        blockManager.putBlockData(blockId, data, level, classTag)
        responseContext.onSuccess(ByteBuffer.allocate(0))
    }
  }

getBlockData方法来读取数据

 override def getBlockData(blockId: BlockId): ManagedBuffer = {
    if (blockId.isShuffle) {
      //先调用的是ShuffleManager的shuffleBlockResolver方法，得到ShuffleBlockResolver
      //调用其IndexShuffleBlockResolver.getBlockData方法
      shuffleManager.shuffleBlockResolver.getBlockData(blockId.asInstanceOf[ShuffleBlockId])
    } else {
      getLocalBytes(blockId) match {
        case Some(blockData) =>
          new BlockManagerManagedBuffer(blockInfoManager, blockId, blockData, true)
        case None =>
          // If this block manager receives a request for a block that it doesn't have then it's
          // likely that the master has outdated block statuses for this block. Therefore, we send
          // an RPC so that this block is marked as being unavailable from this block manager.
          reportBlockStatus(blockId, BlockStatus.empty)
          throw new BlockNotFoundException(blockId.toString)
      }
    }
  }

----------------------------------------------------------

上面说了BlockManager管理数据读取的方法，接下去说一下BlockManager管理数据写入

老规矩，用一张图来说明整体流程

入口在BlockManager#doPutIteratord()方法

  /**
   * 根据不同的配置，选择了不同的方法写入数据，然后更新了数据块的状态，然后做了备份的更新
   */
  private def doPutIterator[T](
      blockId: BlockId,
      iterator: () => Iterator[T],
      level: StorageLevel,
      classTag: ClassTag[T],
      tellMaster: Boolean = true,
      keepReadLock: Boolean = false): Option[PartiallyUnrolledIterator[T]] = {
    doPut(blockId, level, classTag, tellMaster = tellMaster, keepReadLock = keepReadLock) { info =>
      val startTimeMs = System.currentTimeMillis
      var iteratorFromFailedMemoryStorePut: Option[PartiallyUnrolledIterator[T]] = None
      // Size of the block in bytes
      var size = 0L
      //存储级别是内存
      if (level.useMemory) {
        // Put it in memory first, even if it also has useDisk set to true;
        // We will drop it to disk later if the memory store can't hold it.
        //需要序列化
        if (level.deserialized) {
          memoryStore.putIteratorAsValues(blockId, iterator(), classTag) match {
            case Right(s) =>
              size = s
            case Left(iter) =>
              // Not enough space to unroll this block; drop to disk if applicable
              //空间不够展开块且允许写入磁盘
              if (level.useDisk) {
                logWarning(s"Persisting block $blockId to disk instead.")
                diskStore.put(blockId) { channel =>
                  val out = Channels.newOutputStream(channel)
                  serializerManager.dataSerializeStream(blockId, out, iter)(classTag)
                }
                size = diskStore.getSize(blockId)
              } else {
                //返回错误提示
                iteratorFromFailedMemoryStorePut = Some(iter)
              }
          }
        } else { // !level.deserialized
          //不用序列化
          memoryStore.putIteratorAsBytes(blockId, iterator(), classTag, level.memoryMode) match {
            case Right(s) =>
              size = s
            case Left(partiallySerializedValues) =>
              // Not enough space to unroll this block; drop to disk if applicable
              //空间不够展开块且允许写入磁盘
              if (level.useDisk) {
                logWarning(s"Persisting block $blockId to disk instead.")
                diskStore.put(blockId) { channel =>
                  val out = Channels.newOutputStream(channel)
                  partiallySerializedValues.finishWritingToStream(out)
                }
                size = diskStore.getSize(blockId)
              } else {
                iteratorFromFailedMemoryStorePut = Some(partiallySerializedValues.valuesIterator)
              }
          }
        }
      //存储级别是磁盘
      } else if (level.useDisk) {
        diskStore.put(blockId) { channel =>
          val out = Channels.newOutputStream(channel)
          serializerManager.dataSerializeStream(blockId, out, iterator())(classTag)
        }
        size = diskStore.getSize(blockId)
      }
      val putBlockStatus = getCurrentBlockStatus(blockId, info)
      val blockWasSuccessfullyStored = putBlockStatus.storageLevel.isValid
      if (blockWasSuccessfullyStored) {
        // Now that the block is in either the memory or disk store, tell the master about it.
        info.size = size
        if (tellMaster && info.tellMaster) {
          reportBlockStatus(blockId, putBlockStatus)
        }
        //更新数据块信息
        addUpdatedBlockStatusToTaskMetrics(blockId, putBlockStatus)
        logDebug("Put block %s locally took %s".format(blockId, Utils.getUsedTimeMs(startTimeMs)))
        //备份数目>1
        if (level.replication > 1) {
          val remoteStartTime = System.currentTimeMillis
          val bytesToReplicate = doGetLocalBytes(blockId, info)
          // [SPARK-16550] Erase the typed classTag when using default serialization, since
          // NettyBlockRpcServer crashes when deserializing repl-defined classes.
          // TODO(ekl) remove this once the classloader issue on the remote end is fixed.
          val remoteClassTag = if (!serializerManager.canUseKryo(classTag)) {
            scala.reflect.classTag[Any]
          } else {
            classTag
          }
          try {
            replicate(blockId, bytesToReplicate, level, remoteClassTag)
          } finally {
            bytesToReplicate.dispose()
          }
          logDebug("Put block %s remotely took %s"
            .format(blockId, Utils.getUsedTimeMs(remoteStartTime)))
        }
      }
      assert(blockWasSuccessfullyStored == iteratorFromFailedMemoryStorePut.isEmpty)
      iteratorFromFailedMemoryStorePut
    }
  }

memoryStore.putIteratorAsValues 尝试将当前块作为value保存在内存中

/**
   * Attempt to put the given block in memory store as values.
   * 尝试将当前块作为value保存在内存中
   *  
   *  
   *  有可能iterator太大以至于不能保存到内存中，为了避免OOM,这个方法会逐渐展开iterator并间歇性检查是否有足够的空余内存
   *  如果这个块成功地保存到了内存中，那么这些在保存过程中暂时展开的的内存就成了存储内存，因此我们不会获取多余的内存。
   *  
   * @return in case of success, the estimated size of the stored data. In case of failure, return
   *         an iterator containing the values of the block. The returned iterator will be backed
   *         by the combination of the partially-unrolled block and the remaining elements of the
   *         original input iterator. The caller must either fully consume this iterator or call
   *         `close()` on it in order to free the storage memory consumed by the partially-unrolled
   *         block.
   */
  private[storage] def putIteratorAsValues[T](
      blockId: BlockId,
      values: Iterator[T],
      classTag: ClassTag[T]): Either[PartiallyUnrolledIterator[T], Long] = {

    val valuesHolder = new DeserializedValuesHolder[T](classTag)

    putIterator(blockId, values, classTag, MemoryMode.ON_HEAP, valuesHolder) match {
      case Right(storedSize) => Right(storedSize)
      case Left(unrollMemoryUsedByThisBlock) =>
        val unrolledIterator = if (valuesHolder.vector != null) {
          valuesHolder.vector.iterator
        } else {
          valuesHolder.arrayValues.toIterator
        }

        Left(new PartiallyUnrolledIterator(
          this,
          MemoryMode.ON_HEAP,
          unrollMemoryUsedByThisBlock,
          unrolled = unrolledIterator,
          rest = values))
    }
  }

  private def putIterator[T](
      blockId: BlockId,
      values: Iterator[T],
      classTag: ClassTag[T],
      memoryMode: MemoryMode,
      valuesHolder: ValuesHolder[T]): Either[Long, Long] = {
    require(!contains(blockId), s"Block $blockId is already present in the MemoryStore")

    // Number of elements unrolled so far
     // 当前已经展开的数据块
    var elementsUnrolled = 0
    // Whether there is still enough memory for us to continue unrolling this block
    // 是否仍有足够的内存来展开数据块
    var keepUnrolling = true
    // Initial per-task memory to request for unrolling blocks (bytes).
    // 每个展开线程初始的内存大小
    val initialMemoryThreshold = unrollMemoryThreshold
    // How often to check whether we need to request more memory
    // 每隔几次检查是否有足够的空余空间
    val memoryCheckPeriod = conf.get(UNROLL_MEMORY_CHECK_PERIOD)
    // Memory currently reserved by this task for this particular unrolling operation
    // 当前线程保留用来做展开块工作的内存大小
    var memoryThreshold = initialMemoryThreshold
    // Memory to request as a multiple of current vector size
   // 内存增长因子，每次请求的内存大小为(memoryGrowthFactor * vector .size())-memoryThreshold
    val memoryGrowthFactor = conf.get(UNROLL_MEMORY_GROWTH_FACTOR)
    // Keep track of unroll memory used by this particular block / putIterator() operation
    // 展开这个块使用的内存大小
    var unrollMemoryUsedByThisBlock = 0L

    // Request enough memory to begin unrolling
    // 请求足够的内存做unrolling
    keepUnrolling =
      reserveUnrollMemoryForThisTask(blockId, initialMemoryThreshold, memoryMode)

    if (!keepUnrolling) {
      logWarning(s"Failed to reserve initial memory threshold of " +
        s"${Utils.bytesToString(initialMemoryThreshold)} for computing block $blockId in memory.")
    } else {
      unrollMemoryUsedByThisBlock += initialMemoryThreshold
    }

    // Unroll this block safely, checking whether we have exceeded our threshold periodically
    // 安全地展开这个数据库，定期检查剩余内存是否足够
    while (values.hasNext && keepUnrolling) {
      valuesHolder.storeValue(values.next())
       // 每16次检查一次是否超出了分配的内存的大小
      if (elementsUnrolled % memoryCheckPeriod == 0) {
        val currentSize = valuesHolder.estimatedSize()
        // If our vector's size has exceeded the threshold, request more memory
        // 如果不够
        if (currentSize >= memoryThreshold) {
          val amountToRequest = (currentSize * memoryGrowthFactor - memoryThreshold).toLong
          // 申请内存
          keepUnrolling =
            reserveUnrollMemoryForThisTask(blockId, amountToRequest, memoryMode)
          if (keepUnrolling) {
            unrollMemoryUsedByThisBlock += amountToRequest
          }
          // New threshold is currentSize * memoryGrowthFactor
          memoryThreshold += amountToRequest
        }
      }
      elementsUnrolled += 1
    }

    // Make sure that we have enough memory to store the block. By this point, it is possible that
    // the block's actual memory usage has exceeded the unroll memory by a small amount, so we
    // perform one final call to attempt to allocate additional memory if necessary.
     
    if (keepUnrolling) {
      val entryBuilder = valuesHolder.getBuilder()
      val size = entryBuilder.preciseSize
      // 展开所用的内存小于数据块的大小
      if (size > unrollMemoryUsedByThisBlock) {
        val amountToRequest = size - unrollMemoryUsedByThisBlock
        // 获取额外的空间
        keepUnrolling = reserveUnrollMemoryForThisTask(blockId, amountToRequest, memoryMode)
        if (keepUnrolling) {
          unrollMemoryUsedByThisBlock += amountToRequest
        }
      }
      // 如果成功展开了这个块，估计该块在内存中占的空间的大小
      if (keepUnrolling) {
        val entry = entryBuilder.build()
        // Synchronize so that transfer is atomic
        // 将展开所用的内存转为存储的内存，释放掉展开的空间，然后获取内存用于存放block
        memoryManager.synchronized {
          releaseUnrollMemoryForThisTask(memoryMode, unrollMemoryUsedByThisBlock)
          val success = memoryManager.acquireStorageMemory(blockId, entry.size, memoryMode)
          assert(success, "transferring unroll memory to storage memory failed")
        }
        // 如果内存足够，那么写入数据到entry
        entries.synchronized {
          entries.put(blockId, entry)
        }

        logInfo("Block %s stored as values in memory (estimated size %s, free %s)".format(blockId,
          Utils.bytesToString(entry.size), Utils.bytesToString(maxMemory - blocksMemoryUsed)))
        Right(entry.size)
      } else {
        // We ran out of space while unrolling the values for this block
        logUnrollFailureMessage(blockId, entryBuilder.preciseSize)
        Left(unrollMemoryUsedByThisBlock)
      }
    } else {
      // We ran out of space while unrolling the values for this block
      logUnrollFailureMessage(blockId, valuesHolder.estimatedSize())
      Left(unrollMemoryUsedByThisBlock)
    }
  }

diskStore.put(blockId)写入磁盘

  def put(blockId: BlockId)(writeFunc: WritableByteChannel => 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
    val file = diskManager.getFile(blockId)
    val out = new CountingWritableChannel(openForWrite(file))
    var threwException: Boolean = true
    try {
      writeFunc(out)
      blockSizes.put(blockId, out.getCount)
      threwException = false
    } finally {
      try {
        out.close()
      } catch {
        case ioe: IOException =>
          if (!threwException) {
            threwException = true
            throw ioe
          }
      } 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))
  }

回调了参数中的writerFunc,将数据写入到文件

  def finishWritingToStream(os: OutputStream): Unit = {
    verifyNotConsumedAndNotDiscarded()
    consumed = true
    // `unrolled`'s underlying buffers will be freed once this input stream is fully read:
    ByteStreams.copy(unrolledBuffer.toInputStream(dispose = true), os)
    memoryStore.releaseUnrollMemoryForThisTask(memoryMode, unrollMemory)
    redirectableOutputStream.setOutputStream(os)
    while (rest.hasNext) {
     //writeObject方法来将数据写入到流中
      serializationStream.writeObject(rest.next())(classTag)
    }
    serializationStream.close()
  }

--------------------------------------