configs to tune leveldb

leveldb-tuning.txt

These are tunings from https://github.com/basho/leveldb
Video: https://www.youtube.com/watch?v=vo88IdglU_8

https://github.com/google/leveldb/blob/master/include/leveldb/options.h#L83
- write buffer size - default 4MB (increase this for higher performance) (try 10x or 100x will get 72% performance) 

https://github.com/google/leveldb/blob/master/db/skiplist.h#L98
- if increasing write buffer size, also increase skip list max height. Since skip list is probabilistic, 
expected number of keys in write buffer has to specified ahead. (change it from 2^12 to 2^17, affects read and write performance) 

- enable bloom filter (stored at top of sstables)

- look for write amplifications (same keys gets written to multiple levels during compaction to maintain sortedness at each level)
Bigger files will reduce write amplification? (google write amplication level 11, basho level 5.7 and new release level 3?)

- only one compaction thread and only one queue where compaction works are added. 
This can lead to stalls caused by compactions on other DBs

- Thread Group of size 5 for optimal compaction performance. RW tiered locks across following (IMM - Immutable Memory Buffer or old write buffer) 
IMM to Level 0 highest priority
Level 0 to Level 1 next highest priority
Level 1+ after that

- Write throttle (backpressure) instead of default write blocking

- check file handles config