SecondaryCache is the interface for caching blocks on a secondary tier, which can be a non-volatile media or alternate forms of caching such as compressed data. The purpose of the secondary cache is to support other ways of caching the object, such as persistent or compressed data. It can be viewed as an extension of RocksDB’s current volatile block cache. The RocksDB team has created two concrete implementations of SecondaryCache, a Cachelib based implementation on non-volatile media and a LRUCache based implementation (CompressedSecondaryCache) on DRAM.

To build a concrete SecondaryCache, you need to prepare the implementations of SecondaryCacheResultHandle, SecondaryCache, and some callbacks. Their details can be found in secondary_cache.h and cache.h.

• SecondaryCacheResultHandle refers to an item in the secondary cache.
• The callbacks in CacheItemHelper must be implemented for saving items to and restoring them from the secondary cache.
• SecondaryCache includes the essential abstract functions of the secondary cache.

To integrate the secondary cache with the primary cache, a SecondaryCache can be configured by adding a pointer to it in LRUCacheOptions.

CompressedSecondaryCache leverages compression libraries (e.g. LZ4) to compress items and stores the compressed blocks into an LRUCache. User can create a secondary cache instance by calling NewCompressedSecondaryCache. The options can be passed in respectively or together in CompressedSecondaryCacheOptions. The options extend LRUCacheOptions by adding the following two options for compression.

• compression_type: The compression method (if any) that is used to compress data.
• compress_format_version: The format version needed by some compression methods.

CompressedSecondaryCacheOptions secondary_cache_opts;
secondary_cache_opts.capacity = sec_cache_capacity;
LRUCacheOptions lru_cache_opts(cache_capacity);
lru_cache_opts.secondary_cache = NewCompressedSecondaryCache(secondary_cache_opts);
std::shared_ptr<Cache> cache = NewLRUCache(lru_cache_opts);

LRUCacheOptions lru_cache_opts(cache_capacity);
lru_cache_opts.secondary_cache = NewCompressedSecondaryCache(secondary_cache_capacity);
std::shared_ptr<Cache> cache = NewLRUCache(lru_cache_opts);

This blog includes the initial introduction to SecondaryCache.