Riak LevelDB

The original Google README, AUTHORS, LICENSE, NEWS, and TODO files have been renamed with the extension .google.

Introduction

This repository contains the Google source code as modified to benefit the Riak environment. The typical Riak environment has two attributes that necessitate leveldb adjustments, both in options and code:

Production Servers

Riak often runs in heavy Internet environments on servers with many CPU cores, lots of memory, and 24x7 disk activity. Riak LevelDB takes advantage of the environment by adding hardware CRC calculation, increasing Bloom filter accuracy, and defaulting to integrity checking enabled.

Multiple Databases:

Riak often opens up to 128 databases simultaneously. Google's leveldb supports this, but its background compaction thread can fall behind. LevelDB will "stall" new user writes whenever the compaction thread gets too far behind. Basho's modifications include multiple thread blocks that each contain prioritized threads for specific compaction activities.

Wiki

Details of Riak-specific customizations are found in the wiki.

Basic Options Needed

Those wishing to truly savor the benefits of Basho's modifications need to initialize a new leveldb::Options structure similar to the following before each call to leveldb::DB::Open:

    leveldb::Options * options;

    options=new Leveldb::Options;

    options.filter_policy=leveldb::NewBloomFilterPolicy2(16);
    options.write_buffer_size=62914560;     // 60MB
    options.total_leveldb_mem=2684354560;   // 2.5GB (details below)
    options.env=leveldb::Env::Default();

Memory Plan

Riak LevelDB dramatically departed from Google's original internal memory allotment plan with Riak 2.0, using a methodology called flexcache. The technical details are here

The key points are:

• options.total_leveldb_mem is an allocation for the entire process, not a single database

• Giving different values to options.total_leveldb_mem on subsequent Open calls causes memory to rearrange to current value across all databases

• Recommended minimum for Riak LevelDB is 340MB per database.

• Performance improves rapidly from 340MB to 2.5GB per database (3.0GB if using Riak's Active Anti-Entropy). Even more is nice, but not as helpful.

• Never assign more than 75% of available RAM to total_leveldb_mem. There is too much unaccounted memory overhead (worse if you use tcmalloc library).

• options.max_open_files and options.block_cache should not be used.

To Do

Since the leveldb_ee code is now open-source, it should be incorporated into this repository and the build (here and in eleveldb) adjusted accordingly.