LevelDB Filters

【LevelDB Filters】

　　Because of the way leveldb data is organized on disk, a single Get() call may involve multiple reads from disk. The optional FilterPolicy mechanism can be used to reduce the number of disk reads substantially.

   leveldb::Options options;
   options.filter_policy = NewBloomFilterPolicy(10);
   leveldb::DB* db;
   leveldb::DB::Open(options, "/tmp/testdb", &db);
   ... use the database ...
   delete db;
   delete options.filter_policy;

　　The preceding code associates a Bloom filter based filtering policy with the database. Bloom filter based filtering relies on keeping some number of bits of data in memory per key (in this case 10 bits per key since that is the argument we passed to NewBloomFilterPolicy). This filter will reduce the number of unnecessary disk reads needed for Get() calls by a factor of approximately a 100. Increasing the bits per key will lead to a larger reduction at the cost of more memory usage.

　　We recommend that applications whose working set does not fit in memory and that do a lot of random reads set a filter policy.