Constant DEFAULT_BLOCK_CACHE_SIZE 

pub(crate) const DEFAULT_BLOCK_CACHE_SIZE: usize = _; // 2_097_152usize

Default block cache size: 2 MiB (RocksDB default is 8 MiB). Index/filter blocks are placed in this cache too (set_cache_index_and_filter_blocks), so everything is bounded. We only need correctness, not throughput, so we keep this minimal.