Apache Spark-1.0.0浅析（十）：数据存储——读写操作

“RDD是由不同的partition组成的，transformation和action是在partition上面进行的；而在storage模块内部，RDD又被视为由不同的block组成，对于RDD的存取是以block为单位进行的，本质上partition和block是等价的，只是看待的角度不同。在Spark storage模块中中存取数据的最小单位是block，所有的操作都是以block为单位进行的。”

BlockManager中定义了三种主要的存储类型（tackyonStore暂且不做分析）

private[storage] val memoryStore = new MemoryStore(this, maxMemory)
private[storage] val diskStore = new DiskStore(this, diskBlockManager)
private[storage] lazy val tachyonStore: TachyonStore

一、DiskStore

首先看diskStore，实例化DiskStore时带入diskBlockManager参数

val diskBlockManager = new DiskBlockManager(shuffleBlockManager,
    conf.get("spark.local.dir",  System.getProperty("java.io.tmpdir")))

DiskBlockManager初始化时，类中为spark.local.dir中的每个路径创建一个本地目录，在这些目录中，创建多个子目录hash存放文件，避免在顶层目录存在大的索引节点

// 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 val localDirs: Array[File] = createLocalDirs()
  private val subDirs = Array.fill(localDirs.length)(new Array[File](subDirsPerLocalDir))

createLocalDir创建本地目录，每个本地目录其实是一个文件夹，文件夹以“spark-local“+”日期”+“随机整数“形式命名，block以文件的形式存放在localDir中

private def createLocalDirs(): Array[File] = {
    logDebug("Creating local directories at root dirs ‘" + rootDirs + "‘")
    val dateFormat = new SimpleDateFormat("yyyyMMddHHmmss")
    rootDirs.split(",").map { rootDir =>
      var foundLocalDir = false
      var localDir: File = null
      var localDirId: String = null
      var tries = 0
      val rand = new Random()
      while (!foundLocalDir && tries < MAX_DIR_CREATION_ATTEMPTS) {
        tries += 1
        try {
          localDirId = "%s-%04x".format(dateFormat.format(new Date), rand.nextInt(65536))
          localDir = new File(rootDir, "spark-local-" + localDirId)
          if (!localDir.exists) {
            foundLocalDir = localDir.mkdirs()
          }
        } catch {
          case e: Exception =>
            logWarning("Attempt " + tries + " to create local dir " + localDir + " failed", e)
        }
      }
      if (!foundLocalDir) {
        logError("Failed " + MAX_DIR_CREATION_ATTEMPTS +
          " attempts to create local dir in " + rootDir)
        System.exit(ExecutorExitCode.DISK_STORE_FAILED_TO_CREATE_DIR)
      }
      logInfo("Created local directory at " + localDir)
      localDir
    }
  }

所以，DiskStoreManager中对于block的所有操作，都归结为对于File的存取操作，即维护和创建逻辑Block和物理地址的映射。默认的，一个block映射为该blockID命名的文件。

getFile方法创建这种映射，根据filename，hash得到子文件夹，将filename映射到其中

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
    var subDir = subDirs(dirId)(subDirId)
    if (subDir == null) {
      subDir = subDirs(dirId).synchronized {
        val old = subDirs(dirId)(subDirId)
        if (old != null) {
          old
        } else {
          val newDir = new File(localDirs(dirId), "%02x".format(subDirId))
          newDir.mkdir()
          subDirs(dirId)(subDirId) = newDir
          newDir
        }
      }
    }

    new File(subDir, filename)
  }

存入数据时：

BlockManager对于diskStore，根据不同的值类型，有两种操作putValues和putBytes

putValues通过blockId得到对应file，根据file创建FileOutputStream，然后序列化写入values

override def putValues(
      blockId: BlockId,
      values: Iterator[Any],
      level: StorageLevel,
      returnValues: Boolean)
    : PutResult = {

    logDebug("Attempting to write values for block " + blockId)
    val startTime = System.currentTimeMillis
    val file = diskManager.getFile(blockId)
    val outputStream = new FileOutputStream(file)
    blockManager.dataSerializeStream(blockId, outputStream, values)
    val length = file.length

    val timeTaken = System.currentTimeMillis - startTime
    logDebug("Block %s stored as %s file on disk in %d ms".format(
      file.getName, Utils.bytesToString(length), timeTaken))

    if (returnValues) {
      // Return a byte buffer for the contents of the file
      val buffer = getBytes(blockId).get
      PutResult(length, Right(buffer))
    } else {
      PutResult(length, null)
    }
  }

putBytes则通过blockId得到file, 根据file获得通道channel，然后写入bytes

override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel) : PutResult = {
    // So that we do not modify the input offsets !
    // duplicate does not copy buffer, so inexpensive
    val bytes = _bytes.duplicate()
    logDebug("Attempting to put block " + blockId)
    val startTime = System.currentTimeMillis
    val file = diskManager.getFile(blockId)
    val channel = new FileOutputStream(file).getChannel()
    while (bytes.remaining > 0) {
      channel.write(bytes)
    }
    channel.close()
    val finishTime = System.currentTimeMillis
    logDebug("Block %s stored as %s file on disk in %d ms".format(
      file.getName, Utils.bytesToString(bytes.limit), (finishTime - startTime)))
    return PutResult(bytes.limit(), Right(bytes.duplicate()))
  }

读取数据时：

执行getValues和getBytes，getValues也调用getBytes

override def getValues(blockId: BlockId): Option[Iterator[Any]] = {
    getBytes(blockId).map(buffer => blockManager.dataDeserialize(blockId, buffer))
  }

getBytes通过blockId找到文件位置，然后建立channel，对于小文件，直接读入栈中，大文件则映射到内存中

override def getBytes(blockId: BlockId): Option[ByteBuffer] = {
    val segment = diskManager.getBlockLocation(blockId)
    val channel = new RandomAccessFile(segment.file, "r").getChannel()

    try {
      // For small files, directly read rather than memory map
      if (segment.length < minMemoryMapBytes) {
        val buf = ByteBuffer.allocate(segment.length.toInt)
        channel.read(buf, segment.offset)
        buf.flip()
        Some(buf)
      } else {
        Some(channel.map(MapMode.READ_ONLY, segment.offset, segment.length))
      }
    } finally {
      channel.close()
    }
  }

二、MemoryStore

MemoryStore在内存中存放blocks，不是作为反序列化的Java对象的ArrayBuffer，就是作为序列化的ByteBuffer。与diskStore需要创建本地目录相比，memoryStore实例化时，创建一个LinkedHashMap，以此维护BlockId和block entry的映射

case class Entry(value: Any, size: Long, deserialized: Boolean)

private val entries = new LinkedHashMap[BlockId, Entry](32, 0.75f, true)

存入数据时：

类似的，BlockManager对于memoryStore，也有两种操作putValues和putBytes

putValues首先估计存入实例的大小，然后调用tryToPut尝试放入内存中内存

override def putValues(
      blockId: BlockId,
      values: ArrayBuffer[Any],
      level: StorageLevel,
      returnValues: Boolean): PutResult = {
    if (level.deserialized) {
      val sizeEstimate = SizeEstimator.estimate(values.asInstanceOf[AnyRef])
      val putAttempt = tryToPut(blockId, values, sizeEstimate, deserialized = true)
      PutResult(sizeEstimate, Left(values.iterator), putAttempt.droppedBlocks)
    } else {
      val bytes = blockManager.dataSerialize(blockId, values.iterator)
      val putAttempt = tryToPut(blockId, bytes, bytes.limit, deserialized = false)
      PutResult(bytes.limit(), Right(bytes.duplicate()), putAttempt.droppedBlocks)
    }
  }

putBytes同样估计存入实例的大小，然后调用tryToPut尝试存入内存

override def putBytes(blockId: BlockId, _bytes: ByteBuffer, level: StorageLevel): PutResult = {
    // Work on a duplicate - since the original input might be used elsewhere.
    val bytes = _bytes.duplicate()
    bytes.rewind()
    if (level.deserialized) {
      val values = blockManager.dataDeserialize(blockId, bytes)
      val elements = new ArrayBuffer[Any]
      elements ++= values
      val sizeEstimate = SizeEstimator.estimate(elements.asInstanceOf[AnyRef])
      tryToPut(blockId, elements, sizeEstimate, true)
      PutResult(sizeEstimate, Left(values.toIterator))
    } else {
      tryToPut(blockId, bytes, bytes.limit, false)
      PutResult(bytes.limit(), Right(bytes.duplicate()))
    }
  }

两种操作最终都调用tryToPut，putLock用来确保所有的存放请求和相关block存入只被唯一的线程完成，否则，当一个线程正在向空闲内存存放block时，另一个线程可能也在用同一块空闲内存存放不同的block。调用ensureFreeSpace确保内存中有足够的空间存放block。如果memory空间充足，新建entry，加入LinkedHashMap；否则，调用dropFromMemory将block落入磁盘

/**
   * Try to put in a set of values, if we can free up enough space. The value should either be
   * an ArrayBuffer if deserialized is true or a ByteBuffer otherwise. Its (possibly estimated)
   * size must also be passed by the caller.
   *
   * Lock on the object putLock to ensure that all the put requests and its associated block
   * dropping is done by only on thread at a time. Otherwise while one thread is dropping
   * blocks to free memory for one block, another thread may use up the freed space for
   * another block.
   *
   * Return whether put was successful, along with the blocks dropped in the process.
   */
  private def tryToPut(
      blockId: BlockId,
      value: Any,
      size: Long,
      deserialized: Boolean): ResultWithDroppedBlocks = {

    /* TODO: Its possible to optimize the locking by locking entries only when selecting blocks
     * to be dropped. Once the to-be-dropped blocks have been selected, and lock on entries has
     * been released, it must be ensured that those to-be-dropped blocks are not double counted
     * for freeing up more space for another block that needs to be put. Only then the actually
     * dropping of blocks (and writing to disk if necessary) can proceed in parallel. */

    var putSuccess = false
    val droppedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]

    putLock.synchronized {
      val freeSpaceResult = ensureFreeSpace(blockId, size)
      val enoughFreeSpace = freeSpaceResult.success
      droppedBlocks ++= freeSpaceResult.droppedBlocks

      if (enoughFreeSpace) {
        val entry = new Entry(value, size, deserialized)
        entries.synchronized {
          entries.put(blockId, entry)
          currentMemory += size
        }
        if (deserialized) {
          logInfo("Block %s stored as values to memory (estimated size %s, free %s)".format(
            blockId, Utils.bytesToString(size), Utils.bytesToString(freeMemory)))
        } else {
          logInfo("Block %s stored as bytes to memory (size %s, free %s)".format(
            blockId, Utils.bytesToString(size), Utils.bytesToString(freeMemory)))
        }
        putSuccess = true
      } else {
        // Tell the block manager that we couldn‘t put it in memory so that it can drop it to
        // disk if the block allows disk storage.
        val data = if (deserialized) {
          Left(value.asInstanceOf[ArrayBuffer[Any]])
        } else {
          Right(value.asInstanceOf[ByteBuffer].duplicate())
        }
        val droppedBlockStatus = blockManager.dropFromMemory(blockId, data)
        droppedBlockStatus.foreach { status => droppedBlocks += ((blockId, status)) }
      }
    }
    ResultWithDroppedBlocks(putSuccess, droppedBlocks)
  }

dropFromMemory同样调用diskstore.putValue或diskstore.putBytes将blocks存入硬盘，当然首先需要判断存储级别是否使用硬盘，最后从memoryStore中删除blockId信息。如果存储级别不使用硬盘，则直接移除blockId

/**
   * Drop a block from memory, possibly putting it on disk if applicable. Called when the memory
   * store reaches its limit and needs to free up space.
   *
   * Return the block status if the given block has been updated, else None.
   */
  def dropFromMemory(
      blockId: BlockId,
      data: Either[ArrayBuffer[Any], ByteBuffer]): Option[BlockStatus] = {

    logInfo("Dropping block " + blockId + " from memory")
    val info = blockInfo.get(blockId).orNull

    // If the block has not already been dropped
    if (info != null)  {
      info.synchronized {
        // required ? As of now, this will be invoked only for blocks which are ready
        // But in case this changes in future, adding for consistency sake.
        if (!info.waitForReady()) {
          // If we get here, the block write failed.
          logWarning("Block " + blockId + " was marked as failure. Nothing to drop")
          return None
        }

        var blockIsUpdated = false
        val level = info.level

        // Drop to disk, if storage level requires
        if (level.useDisk && !diskStore.contains(blockId)) {
          logInfo("Writing block " + blockId + " to disk")
          data match {
            case Left(elements) =>
              diskStore.putValues(blockId, elements, level, false)
            case Right(bytes) =>
              diskStore.putBytes(blockId, bytes, level)
          }
          blockIsUpdated = true
        }

        // Actually drop from memory store
        val droppedMemorySize =
          if (memoryStore.contains(blockId)) memoryStore.getSize(blockId) else 0L
        val blockIsRemoved = memoryStore.remove(blockId)
        if (blockIsRemoved) {
          blockIsUpdated = true
        } else {
          logWarning("Block " + blockId + " could not be dropped from memory as it does not exist")
        }

        val status = getCurrentBlockStatus(blockId, info)
        if (info.tellMaster) {
          reportBlockStatus(blockId, info, status, droppedMemorySize)
        }
        if (!level.useDisk) {
          // The block is completely gone from this node; forget it so we can put() it again later.
          blockInfo.remove(blockId)
        }
        if (blockIsUpdated) {
          return Some(status)
        }
      }
    }
    None
  }

读取数据时：

相应的两个方法getValues和getBytes类似，都是先从entry中获得blockId，然后从LinkedHashMap中根据BlockId得到对应的数据。getValues如下

override def getValues(blockId: BlockId): Option[Iterator[Any]] = {
    val entry = entries.synchronized {
      entries.get(blockId)
    }
    if (entry == null) {
      None
    } else if (entry.deserialized) {
      Some(entry.value.asInstanceOf[ArrayBuffer[Any]].iterator)
    } else {
      val buffer = entry.value.asInstanceOf[ByteBuffer].duplicate() // Doesn‘t actually copy data
      Some(blockManager.dataDeserialize(blockId, buffer))
    }
  }

getBytes如下

override def getBytes(blockId: BlockId): Option[ByteBuffer] = {
    val entry = entries.synchronized {
      entries.get(blockId)
    }
    if (entry == null) {
      None
    } else if (entry.deserialized) {
      Some(blockManager.dataSerialize(blockId, entry.value.asInstanceOf[ArrayBuffer[Any]].iterator))
    } else {
      Some(entry.value.asInstanceOf[ByteBuffer].duplicate())   // Doesn‘t actually copy the data
    }
  }

三、BlockManager封装

BlockManager为我们提供了doPut和doGet方法，使用这两个方法对block进行存取操作，无需关心底层实现

存入操作：

对于三种不同的数据类型：Iterator, ArrayBuffer和ByteBuffer，有三种不同的put操作相对应，但是统一调用doPut方法

def put(
      blockId: BlockId,
      values: Iterator[Any],
      level: StorageLevel,
      tellMaster: Boolean): Seq[(BlockId, BlockStatus)] = {
    doPut(blockId, IteratorValues(values), level, tellMaster)
  }

def put(    blockId: BlockId,    values: ArrayBuffer[Any],    level: StorageLevel,    tellMaster: Boolean = true): Seq[(BlockId, BlockStatus)] = {  require(values != null, "Values is null")  doPut(blockId, ArrayBufferValues(values), level, tellMaster)}

def putBytes(    blockId: BlockId,    bytes: ByteBuffer,    level: StorageLevel,    tellMaster: Boolean = true): Seq[(BlockId, BlockStatus)] = {  require(bytes != null, "Bytes is null")  doPut(blockId, ByteBufferValues(bytes), level, tellMaster)}

doPut方法，要求blockId和storageLevel不能为空，为block创建BlockInfo实例，同时在blockInfo中将其加锁，使其他线程不能get访问此block。然后根据storageLevel将数据存储到memory或者disk上，然后markReady使其可以被其他线程读取。最后，如果level.replication大于1，调用replicate将该block复制到其他节点

private def doPut(
      blockId: BlockId,
      data: Values,
      level: StorageLevel,
      tellMaster: Boolean = true): Seq[(BlockId, BlockStatus)] = {

    require(blockId != null, "BlockId is null")
    require(level != null && level.isValid, "StorageLevel is null or invalid")

    // Return value
    val updatedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]

    // Remember the block‘s storage level so that we can correctly drop it to disk if it needs
    // to be dropped right after it got put into memory. Note, however, that other threads will
    // not be able to get() this block until we call markReady on its BlockInfo.
    val putBlockInfo = {
      val tinfo = new BlockInfo(level, tellMaster)
      // Do atomically !
      val oldBlockOpt = blockInfo.putIfAbsent(blockId, tinfo)

      if (oldBlockOpt.isDefined) {
        if (oldBlockOpt.get.waitForReady()) {
          logWarning("Block " + blockId + " already exists on this machine; not re-adding it")
          return updatedBlocks
        }

        // TODO: So the block info exists - but previous attempt to load it (?) failed.
        // What do we do now ? Retry on it ?
        oldBlockOpt.get
      } else {
        tinfo
      }
    }

    val startTimeMs = System.currentTimeMillis

    // If we‘re storing values and we need to replicate the data, we‘ll want access to the values,
    // but because our put will read the whole iterator, there will be no values left. For the
    // case where the put serializes data, we‘ll remember the bytes, above; but for the case where
    // it doesn‘t, such as deserialized storage, let‘s rely on the put returning an Iterator.
    var valuesAfterPut: Iterator[Any] = null

    // Ditto for the bytes after the put
    var bytesAfterPut: ByteBuffer = null

    // Size of the block in bytes
    var size = 0L

    // If we‘re storing bytes, then initiate the replication before storing them locally.
    // This is faster as data is already serialized and ready to send.
    val replicationFuture = if (data.isInstanceOf[ByteBufferValues] && level.replication > 1) {
      // Duplicate doesn‘t copy the bytes, just creates a wrapper
      val bufferView = data.asInstanceOf[ByteBufferValues].buffer.duplicate()
      Future {
        replicate(blockId, bufferView, level)
      }
    } else {
      null
    }

    putBlockInfo.synchronized {
      logTrace("Put for block " + blockId + " took " + Utils.getUsedTimeMs(startTimeMs)
        + " to get into synchronized block")

      var marked = false
      try {
        if (level.useMemory) {
          // Save it just to 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.
          val res = data match {
            case IteratorValues(iterator) =>
              memoryStore.putValues(blockId, iterator, level, true)
            case ArrayBufferValues(array) =>
              memoryStore.putValues(blockId, array, level, true)
            case ByteBufferValues(bytes) =>
              bytes.rewind()
              memoryStore.putBytes(blockId, bytes, level)
          }
          size = res.size
          res.data match {
            case Right(newBytes) => bytesAfterPut = newBytes
            case Left(newIterator) => valuesAfterPut = newIterator
          }
          // Keep track of which blocks are dropped from memory
          res.droppedBlocks.foreach { block => updatedBlocks += block }
        } else if (level.useOffHeap) {
          // Save to Tachyon.
          val res = data match {
            case IteratorValues(iterator) =>
              tachyonStore.putValues(blockId, iterator, level, false)
            case ArrayBufferValues(array) =>
              tachyonStore.putValues(blockId, array, level, false)
            case ByteBufferValues(bytes) =>
              bytes.rewind()
              tachyonStore.putBytes(blockId, bytes, level)
          }
          size = res.size
          res.data match {
            case Right(newBytes) => bytesAfterPut = newBytes
            case _ =>
          }
        } else {
          // Save directly to disk.
          // Don‘t get back the bytes unless we replicate them.
          val askForBytes = level.replication > 1

          val res = data match {
            case IteratorValues(iterator) =>
              diskStore.putValues(blockId, iterator, level, askForBytes)
            case ArrayBufferValues(array) =>
              diskStore.putValues(blockId, array, level, askForBytes)
            case ByteBufferValues(bytes) =>
              bytes.rewind()
              diskStore.putBytes(blockId, bytes, level)
          }
          size = res.size
          res.data match {
            case Right(newBytes) => bytesAfterPut = newBytes
            case _ =>
          }
        }

        val putBlockStatus = getCurrentBlockStatus(blockId, putBlockInfo)
        if (putBlockStatus.storageLevel != StorageLevel.NONE) {
          // Now that the block is in either the memory, tachyon, or disk store,
          // let other threads read it, and tell the master about it.
          marked = true
          putBlockInfo.markReady(size)
          if (tellMaster) {
            reportBlockStatus(blockId, putBlockInfo, putBlockStatus)
          }
          updatedBlocks += ((blockId, putBlockStatus))
        }
      } finally {
        // If we failed in putting the block to memory/disk, notify other possible readers
        // that it has failed, and then remove it from the block info map.
        if (!marked) {
          // Note that the remove must happen before markFailure otherwise another thread
          // could‘ve inserted a new BlockInfo before we remove it.
          blockInfo.remove(blockId)
          putBlockInfo.markFailure()
          logWarning("Putting block " + blockId + " failed")
        }
      }
    }
    logDebug("Put block " + blockId + " locally took " + Utils.getUsedTimeMs(startTimeMs))

    // Either we‘re storing bytes and we asynchronously started replication, or we‘re storing
    // values and need to serialize and replicate them now:
    if (level.replication > 1) {
      data match {
        case ByteBufferValues(bytes) => Await.ready(replicationFuture, Duration.Inf)
        case _ => {
          val remoteStartTime = System.currentTimeMillis
          // Serialize the block if not already done
          if (bytesAfterPut == null) {
            if (valuesAfterPut == null) {
              throw new SparkException(
                "Underlying put returned neither an Iterator nor bytes! This shouldn‘t happen.")
            }
            bytesAfterPut = dataSerialize(blockId, valuesAfterPut)
          }
          replicate(blockId, bytesAfterPut, level)
          logDebug("Put block " + blockId + " remotely took " +
            Utils.getUsedTimeMs(remoteStartTime))
        }
      }
    }

    BlockManager.dispose(bytesAfterPut)

    if (level.replication > 1) {
      logDebug("Put for block " + blockId + " with replication took " +
        Utils.getUsedTimeMs(startTimeMs))
    } else {
      logDebug("Put for block " + blockId + " without replication took " +
        Utils.getUsedTimeMs(startTimeMs))
    }

    updatedBlocks
  }

读取操作：

get首先根据blockId调用getLocal从本地获取block，如果不能得到，则调用getRemote从其他节点BlockManger获取block。“在通常情况下Spark任务的分配是根据block的分布决定的，任务往往会被分配到拥有block的节点上，因此getLocal()就能找到所需的block；但是在资源有限的情况下，Spark会将任务调度到与block不同的节点上，这样就必须通过getRemote()来获得block。”

def get(blockId: BlockId): Option[Iterator[Any]] = {
    val local = getLocal(blockId)
    if (local.isDefined) {
      logInfo("Found block %s locally".format(blockId))
      return local
    }
    val remote = getRemote(blockId)
    if (remote.isDefined) {
      logInfo("Found block %s remotely".format(blockId))
      return remote
    }
    None
  }

看一下getLocal，调用doGetLocal

/**
   * Get block from local block manager.
   */
  def getLocal(blockId: BlockId): Option[Iterator[Any]] = {
    logDebug("Getting local block " + blockId)
    doGetLocal(blockId, asValues = true).asInstanceOf[Option[Iterator[Any]]]
  }

doGetLocal首先判断storageLevel如果使用内存，则读入block；然后判断是否使用硬盘，如果使用硬盘且使用内存，则将数据读入内存中，如果只使用硬盘不使用内存，则读取block并返回；如果不使用硬盘，block未在本地找到

private def doGetLocal(blockId: BlockId, asValues: Boolean): Option[Any] = {
    val info = blockInfo.get(blockId).orNull
    if (info != null) {
      info.synchronized {

        // If another thread is writing the block, wait for it to become ready.
        if (!info.waitForReady()) {
          // If we get here, the block write failed.
          logWarning("Block " + blockId + " was marked as failure.")
          return None
        }

        val level = info.level
        logDebug("Level for block " + blockId + " is " + level)

        // Look for the block in memory
        if (level.useMemory) {
          logDebug("Getting block " + blockId + " from memory")
          val result = if (asValues) {
            memoryStore.getValues(blockId)
          } else {
            memoryStore.getBytes(blockId)
          }
          result match {
            case Some(values) =>
              return Some(values)
            case None =>
              logDebug("Block " + blockId + " not found in memory")
          }
        }

        // Look for the block in Tachyon
        if (level.useOffHeap) {
          logDebug("Getting block " + blockId + " from tachyon")
          if (tachyonStore.contains(blockId)) {
            tachyonStore.getBytes(blockId) match {
              case Some(bytes) => {
                if (!asValues) {
                  return Some(bytes)
                } else {
                  return Some(dataDeserialize(blockId, bytes))
                }
              }
              case None =>
                logDebug("Block " + blockId + " not found in tachyon")
            }
          }
        }

        // Look for block on disk, potentially storing it back into memory if required:
        if (level.useDisk) {
          logDebug("Getting block " + blockId + " from disk")
          val bytes: ByteBuffer = diskStore.getBytes(blockId) match {
            case Some(bytes) => bytes
            case None =>
              throw new Exception("Block " + blockId + " not found on disk, though it should be")
          }
          assert (0 == bytes.position())

          if (!level.useMemory) {
            // If the block shouldn‘t be stored in memory, we can just return it:
            if (asValues) {
              return Some(dataDeserialize(blockId, bytes))
            } else {
              return Some(bytes)
            }
          } else {
            // Otherwise, we also have to store something in the memory store:
            if (!level.deserialized || !asValues) {
              // We‘ll store the bytes in memory if the block‘s storage level includes
              // "memory serialized", or if it should be cached as objects in memory
              // but we only requested its serialized bytes:
              val copyForMemory = ByteBuffer.allocate(bytes.limit)
              copyForMemory.put(bytes)
              memoryStore.putBytes(blockId, copyForMemory, level)
              bytes.rewind()
            }
            if (!asValues) {
              return Some(bytes)
            } else {
              val values = dataDeserialize(blockId, bytes)
              if (level.deserialized) {
                // Cache the values before returning them:
                // TODO: Consider creating a putValues that also takes in a iterator?
                val valuesBuffer = new ArrayBuffer[Any]
                valuesBuffer ++= values
                memoryStore.putValues(blockId, valuesBuffer, level, true).data match {
                  case Left(values2) =>
                    return Some(values2)
                  case _ =>
                    throw new Exception("Memory store did not return back an iterator")
                }
              } else {
                return Some(values)
              }
            }
          }
        }
      }
    } else {
      logDebug("Block " + blockId + " not registered locally")
    }
    None
  }

再看一下getRemote，调用doGetRemote

/**
   * Get block from remote block managers.
   */
  def getRemote(blockId: BlockId): Option[Iterator[Any]] = {
    logDebug("Getting remote block " + blockId)
    doGetRemote(blockId, asValues = true).asInstanceOf[Option[Iterator[Any]]]
  }

doGetRemote首先取得该block的所有location信息，然后根据location向远端发送请求获取block，只要有一个远端返回block该函数就返回而不继续发送请求

private def doGetRemote(blockId: BlockId, asValues: Boolean): Option[Any] = {
    require(blockId != null, "BlockId is null")
    val locations = Random.shuffle(master.getLocations(blockId))
    for (loc <- locations) {
      logDebug("Getting remote block " + blockId + " from " + loc)
      val data = BlockManagerWorker.syncGetBlock(
        GetBlock(blockId), ConnectionManagerId(loc.host, loc.port))
      if (data != null) {
        if (asValues) {
          return Some(dataDeserialize(blockId, data))
        } else {
          return Some(data)
        }
      }
      logDebug("The value of block " + blockId + " is null")
    }
    logDebug("Block " + blockId + " not found")
    None
  }

四、Partition和Block

最后，说明Partition是如何转化为block存储的。资源调度——Task执行中曾经分析过，对于RDD的一系列transformation或action，将转化为对于partitions的tasks的执行，而最后是调用getOrCompute方法。getOrCompute首先根据RDD id和partition index构造出key（blockId），根据key从BlockManager中取出相应的block。如果该block存在，表示此RDD在之前已经被计算过并存储在BlockManager中，可以直接读取无需再重新计算。如果该block不存在，则需要调用RDD的computeOrReadCheckpoint方法，读取checkpoint或者计算得到新的block，并将其存储到BlockManager中。需要注意的是block的计算和存储是阻塞的，若另一线程也需要用到此block则需等到该线程block的loading结束。

END