[hyc_symas]

Stated another way - assume you want to sustain a user workload writing 20MB/sec, and you don't do any throttling. Level 0 consists of 4 1MB files - it will fill in 1/5th of a second, and then compaction will reduce it by 1MB. After that it will be compacting continuously every 1/20th of a second. To sustain this workload for the 1st second will thus require 17 compactions to Level 1. Assuming an already populated Level 1 and worst-case key distribution that means in 1 second it will trigger compactions that read 238MB and write 238MB to store the incoming 20MB.

Level 1 is only 10MB, so if it was empty it would fill in the first 1/2 second. For the remaining 1/2 second it would trigger 5 more compactions to Level 2, reading 130MB and writing 130MB. If it started out full then this would be 260MB/260MB respectively.

So for a 20MB/sec input workload you would need a disk subsystem capable of sustaining 498MB/sec of reads concurrent with 498MB/sec of writes. And that's only for a small DB, only Level 0-2 present (smaller than 110MB), and excluding the actual cost of filesystem operations (create/delete/etc).

That's only for the 1st second of load. For every second after that, you're dumping from Level 0 to Level 1 at 280MB read and 280MB write/sec. And dumping from Level 1 to Level 2 at 260/260 as before. 540/540 - so a disk capable of 1080MB/sec I/O is needed to sustain a 20MB/sec workload. And this is supposed to be HDD-optimized? Write-optimized? O(N logN) - what a laugh.

Maybe LSMs in general can be more efficient than this. LevelDB is pretty horrible though.

[Bogdanovich]

It would only trigger compaction if sst tables have overlapping keys. And if you only write new items, goleveldb implementation would just create 3.7Mb sst tables by default without trying to merge them into bigger chunks (what's the point? they are all sorted and non-overlapping). When you have queue consumption workload it would start merging tombstones with sst tables and since tombstones are also in sorted order it would not pick up multiple sst tables at a time, and just either completely or partially remove stale sst files. I added some more benchmarks including queue packing with 200M messages of 64 byte size, and benchmarks of consumption of 200M messages. The speed is sustainable. https://github.com/bogdanovich/siberite/blob/master/docs/ben...