The notion of droppedMemorySize is confusing to me, but it seems to be originally computed from here: https://github.com/RongGu/spark-1/blob/master/core/src/main/scala/org/apache/spark/storage/BlockManager.scala#L837

It seems like this should be just
`val inTachyonSize = status.tachyonSize`
therefore, but I could easily just be misunderstanding.