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Dataset was 20x times larger than available RAM in that case, so it makes sense that OS cashing was useless and only induced overhead..

Another potential issue was that they compared their mmap code to fio O_DIRECT code, which kinda not clean experiment, fio could be just much more optimized itself..

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Besides this point, I mostly mentioned it as an example of probably the simplest in-memory yet disk-backed database imaginable. For example, if you want to implement any sort of transactions, you don't want to actually do anything like fsync until you start trying to commit the data.

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