StorageLevel

val NONE = new StorageLevel(false, false, false, false)
  val DISK_ONLY = new StorageLevel(true, false, false, false)
  val DISK_ONLY_2 = new StorageLevel(true, false, false, false, 2)
  val MEMORY_ONLY = new StorageLevel(false, true, false, true)
  val MEMORY_ONLY_2 = new StorageLevel(false, true, false, true, 2)
  val MEMORY_ONLY_SER = new StorageLevel(false, true, false, false)
  val MEMORY_ONLY_SER_2 = new StorageLevel(false, true, false, false, 2)
  val MEMORY_AND_DISK = new StorageLevel(true, true, false, true)
  val MEMORY_AND_DISK_2 = new StorageLevel(true, true, false, true, 2)
  val MEMORY_AND_DISK_SER = new StorageLevel(true, true, false, false)
  val MEMORY_AND_DISK_SER_2 = new StorageLevel(true, true, false, false, 2)
  val OFF_HEAP = new StorageLevel(true, true, true, false, 1)

加粗“_2”表示持久化存储份数为两份

StorageLevel有五个属性分别是：

private var _useDisk: Boolean, //useDisk_是否使用磁盘
    private var _useMemory: Boolean, //useMemory_是否使用内存
    private var _useOffHeap: Boolean, //useOffHeap_是否使用堆外内存如：Tachyon，
    private var _deserialized: Boolean,//deserialized_是否进行反序列化
    private var _replication: Int = 1) //replication_备份数目。

可以通过序列化减少内存占用量，但会降低效率