Github user andrewor14 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/50#discussion_r10245880
  
    --- Diff: core/src/main/scala/org/apache/spark/CacheManager.scala ---
    @@ -71,10 +71,21 @@ private[spark] class CacheManager(blockManager: 
BlockManager) extends Logging {
               val computedValues = rdd.computeOrReadCheckpoint(split, context)
               // Persist the result, so long as the task is not running locally
               if (context.runningLocally) { return computedValues }
    -          val elements = new ArrayBuffer[Any]
    -          elements ++= computedValues
    -          blockManager.put(key, elements, storageLevel, tellMaster = true)
    -          elements.iterator.asInstanceOf[Iterator[T]]
    +          if (storageLevel.useDisk && !storageLevel.useMemory) {
    +            blockManager.put(key, computedValues, storageLevel, tellMaster 
= true)
    +            return blockManager.get(key) match {
    +              case Some(values) =>
    +                return new InterruptibleIterator(context, 
values.asInstanceOf[Iterator[T]])
    +              case None =>
    +                logInfo("Failure to store %s".format(key))
    +                return null
    +            }
    +          } else {
    +            val elements = new ArrayBuffer[Any]
    +            elements ++= computedValues
    +            blockManager.put(key, elements, storageLevel, tellMaster = 
true)
    +            return elements.iterator.asInstanceOf[Iterator[T]]
    +          }
    --- End diff --
    
    Even if we use the same code path for say MEM_ONLY, we won't actually write 
the data to disk since we also pass the storage level to BlockManager along 
with the data.
    
    But I see your second point about possibly dropping a block before we read 
it. That does seem to prevent us from merging the two cases.


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