The BlockCache is written such that requests to cache a block (BlockCache.store call) can fail, making caching less effective. We should evaluate the impact of this storage failure and potentially reduce the number of storage failures.
The implementation reserves a single block of memory. In store, a block of memory is allocated, and then a pointer is inserted into the underling map. A block is only freed when the underlying map evicts the map entry.
This means that when two store() operations are called concurrently (even under low load), one can fail. This is made worse by the fact that concurrent maps typically tend to amortize the cost of eviction over many keys (i.e. the actual size of the map can grow beyond the configured maximum number of entries... both the older ConcurrentLinkedHashMap and newer Caffeine do this). When this is the case, store() won't be able to find a free block of memory, even if there aren't any other concurrently operating stores.