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The implementation seems to use Log-Structured Merge Trees.

The only paper on this data structure seems to be: http://goo.gl/CVF1l

This paper is poorly written and quite honestly not useful to implement an LSM tree. Does anyone know of a better paper than this one?

But it doesn't really describe the LSM data structure!

It describes the approach used here (cf. all of the discussion of the tablet system). The few differences are documented:

Read "Cache-Oblivious Streaming B-trees" http://supertech.csail.mit.edu/cacheObliviousBTree.html It' LSM with faster searches.

Thanks for the pointer. Are there any known open-source implementations of the same?

Not that I know of. Also, they have a patent, but that didn't stop Acunu from reimplementing and improving the algorithm.