Heat-sink as add-on?

[Andrew2]

For the HDFS, I also was planning on getting some higher performance microsdxc modules, with specs like this or better:
http://www.amazon.com/PNY-Turbo-Performa...B00W77C1QU

Thoughts?

Forget about theoretical sequential transfer speeds since Pine64 won't exceed ~22MB/s anyway. Compare the random I/O speed ratings of your card with better ones (and keep in mind that cards of different size might perform different if they use a good controller -- the latter is the reason why I don't buy Kingston, PNY or any other vendor that does not produce own controllers and NAND dies, since you never know what you get, they exchange the controller between two batches and random I/O differs by 300% between batches).

And then please forget about the whole Hadoop idea or better check why it's important to use cluster nodes with ECC RAM for these sorts of storage clusters. If you still want to use cheap SBCs for this stuff seriously consider downclocking the DRAM and do extensive tests before (not possible with A64 ATM, when it's possible it will look like this)

Regarding heatsinks numbers are available in Linux Development forum, you can combine even cheap ones with somewhat controlled airflow but the whole approach is pretty useless when we're talking about Hadoop anyway (I/O bound so the A53 cores will be more or less idle). But simply get 160 of these and use 5 each for every Pine64 (SoC, 4 x DRAM, PMU doesn't need a heatsink in your 'mostly idle' scenario)