The LMDB test is a bit contrived. By choosing a key of 16 bytes and a value of 4000 bytes, it's nearly optimal. The underlying block device wants 4KiB aligned w

The LMDB test is a bit contrived. By choosing a key of 16 bytes and a value of 4000 bytes, it's nearly optimal. The underlying block device wants 4KiB aligned writes for best performance, and that's pretty much what this test is doing. RocksDB is best at performing writes into very small records, where it can coalesce them and reduce write amplification. LMDB can't do this. As the size of the records increases, the RocksDB performance advantage disappears, and then it eventually loses out. In my own tests, I've found that the crossover point is about 250 bytes. A better test comparing LMDB to RocksDB would show what this crossover is, in a more controlled fashion than what I have done.