spark的cache和persist的区别

通过观察RDD.scala源代码即可知道cache和persist的区别：

def persist(newLevel: StorageLevel): this.type = {
　　if (storageLevel != StorageLevel.NONE && newLevel != storageLevel) {
　　　　throw new UnsupportedOperationException( "Cannot change storage level of an RDD after it was already assigned a level")
　　}
　　sc.persistRDD(this)
　　sc.cleaner.foreach(_.registerRDDForCleanup(this))
　　storageLevel = newLevel
　　this
}
/** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
def persist(): this.type = persist(StorageLevel.MEMORY_ONLY)

/** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
def cache(): this.type = persist()

可知：
1）RDD的cache()方法其实调用的就是persist方法，缓存策略均为MEMORY_ONLY；
2）可以通过persist方法手工设定StorageLevel来满足工程需要的存储级别；
3）cache或者persist并不是action；
附：cache和persist都可以用unpersist来取消