Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/1581#issuecomment-50114441
I agree it is safer to put the magic byte in front of every record.
However, this is not a public API where users can throw in an arbitrary RDD and
ask the serializer to create an `RDD[Double]`. Either solution is fine but it
would be good if you can check the overhead in computation and in storage, for
example, call `RDD.sum` on a cached RDD.
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