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leveldb: A key-value store Authors: Sanjay Ghemawat ([email protected]) and Jeff Dean ([email protected])

The original Google README is now README.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: servers with many CPU cores, lots of memory, and 24x7 disk activity. Basho's leveldb takes advantage of the environment by adding hardware CRC calculation, increasing Bloom filter accuracy, and defaulting to integrity checking enabled.

  • multiple databases open: Riak opens 8 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 leveldb modification include multiple thread blocks that each contain prioritized threads for specific compaction activities.

Details for Basho's customizations exist in the leveldb wiki:

** Branch pattern

This repository follows the Basho standard for branch management as of November 28, 2013. The standard is found here:

In summary, the "develop" branch contains the most recently reviewed engineering work. The "master" branch contains the most recently released work, i.e. distributed as part of a Riak release.

** 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; // 60Mbytes options.total_leveldb_mem=2684354560; // 2.5Gbytes (details below) options.env=leveldb::Env::Default();

** Memory plan

Basho's leveldb dramatically departed from Google's original internal memory allotment plan with Riak 2.0. Basho's leveldb uses a methodology called flexcache. The technical details are here:

The key points are:

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

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

  • recommended minimum for Basho's leveldb is 340Mbytes per database.

  • performance improves rapidly from 340Mbytes to 2.5Gbytes per database (3.0Gbytes if using Riak's active anti-entropy). Even more is nice, but not as helpful.

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

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

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