Block Cache

Blocks of block-based table are cached in memory in a sharded LRU cache. By default the highest kNumShardBits (which is 4) of hash value is used as shard id, so there will be 2^kNumShardBits shards. Capacity is split evenly to each of the shards. Each shard maintains its own hash table for lookup and a linked-list of cached blocks for eviction. Operations on a shard need to lock the whole shard. As a result, shards with hot blocks (such as index blocks and filter blocks) can have lock contention.

Usage

To use block cache, call NewLRUCache() and set the result to BlockBasedTableOptions.block_cache. Example:

BlockBasedTableOptions table_options;
table_options.block_cache = NewLRUCache(capacity, num_shard_bits);
Options options;
options.table_factory.reset(new BlockBasedTableFactory(table_options));

If BlockBasedTableOptions.block_cache is null, a default 8MB cache will be used. To disable block cache, set BlockBasedTableOptions.no_block_cache to true.

Block cache stores uncompressed block contents. Optionally, you can have a second layer block cache which stores compressed block content. It can be enable via BlockBasedTableOptions.compressed_block_cache.

There are two more options to control whether to cache index blocks and filter blocks:

• BlockBasedTableOptions.cache_index_and_filter_blocks: Cache index blocks and filter blocks in block cache. Defaults to false, where index and filter blocks will be pre-loaded by BlockBasedTableReader and stored separately from block cache.

• BlockBasedTableOptions.pin_l0_filter_and_index_blocks_in_cache: For L0 block based tables, avoid index blocks and filter blocks being swap out of block cache. Instead, BlockBasedTableReader holds a reference of these blocks, and further access will not hit block cache. This option can help reduce lock contention on block cache.