Bloom Filters

Bloom filters are used to reduce read amplification. Bloom filters can be set on a per-column family basis (see myrocks-column-families).

Bloom Filter Parameters

• How many bits to use

• whole_key_filtering=true/false

• Whether the bloom filter is for the entire key or for the prefix. In case of a prefix, you need to look at the index definition and compute the desired prefix length.

Computing Prefix Length

• It's 4 bytes for index_nr

• Then, for fixed-size columns (integer, date[time], decimal) it is key_length as shown by EXPLAIN. For VARCHAR columns, determining the length is tricky (It depends on the values stored in the table. Note that MyRocks encodes VARCHARs with "Variable-Length Space-Padded Encoding" format).

Configuring Bloom Filter

To enable 10-bit bloom filter for 8-byte prefix length for column family "cf1", put this into my.cnf:

rocksdb_override_cf_options='cf1={block_based_table_factory={filter_policy=bloomfilter:10:false;whole_key_filtering=0;};prefix_extractor=capped:8};'

and restart the server.

Check if the column family actually uses the bloom filter:

select * 
from information_schema.rocksdb_cf_options 
where 
  cf_name='cf1' and
  option_type IN ('TABLE_FACTORY::FILTER_POLICY','PREFIX_EXTRACTOR');

+---------+------------------------------+----------------------------+
| CF_NAME | OPTION_TYPE                  | VALUE                      |
+---------+------------------------------+----------------------------+
| cf1     | PREFIX_EXTRACTOR             | rocksdb.CappedPrefix.8     |
| cf1     | TABLE_FACTORY::FILTER_POLICY | rocksdb.BuiltinBloomFilter |
+---------+------------------------------+----------------------------+

Checking if Bloom Filter is Useful

Watch these status variables:

show status like '%bloom%';
+-------------------------------------+-------+
| Variable_name                       | Value |
+-------------------------------------+-------+
| Rocksdb_bloom_filter_prefix_checked | 1     |
| Rocksdb_bloom_filter_prefix_useful  | 0     |
| Rocksdb_bloom_filter_useful         | 0     |
+-------------------------------------+-------+

Other useful variables are:

• rocksdb_force_flush_memtable_now - bloom filter is only used when reading data from disk. If you are doing testing, flush the data to disk first.

• rocksdb_skip_bloom_filter_on_read - skip using the bloom filter (default is FALSE).

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