Github user dorx commented on the pull request:
https://github.com/apache/spark/pull/1581#issuecomment-50101155
The issue is what other things we can reasonably serialize into 8 bytes.
Not sure how other types of doubles are relevant here since the size would be
different and cause problems right away. Longs are also 8 bytes, so would some
scheme of serializing an array of 2 shorts/chars. It's a tradeoff between
efficiency and safety. We can remove the magic byte assuming no one's ever
going to serialize an RDD of Doubles into 8-byte arrays and then use a Long
deser on the 8-byte array. A compromise would be embedding the type metadata in
the RDD of bytearray itself so we don't incur the cost of per point blow-up.
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