On Adapting the Cache Block Size in SSD Caches

SSD-based block-level caches can notably increase the performance of HDD-based storage systems. However, this demands a sensible choice of the cache block size, which depends strongly on the workload characteristics. Many workloads will most likely favor either small or large cache blocks. Unfortunately, choosing the appropriate cache block size is difficult due to the diversity and dynamics of storage workloads. Thus, adapting the cache block size to the workload characteristics at run time has the potential to substantially improve the cache performance compared to using a fixed cache block size. However, changing the used cache block size for all cached data is very costly and neglects that distinct parts of the data may exhibit different access patterns, which favor distinct cache block sizes.In this paper, we experimentally study the performance impact of the cache block size and fine-grained adaptation, i.e., for individual parts of the data, between small and large cache blocks in write-back SSD caches. Based on our results, we make two major observations on the performance impact of the cache block size and its adaptation. First, using an inappropriate cache block size can reduce the overall throughput by up to 84% compared to using the most suitable cache block size. Second, fine-grained adaptation between small and large cache blocks is highly beneficial as it avoids such a performance deterioration, whereas it can increase the overall throughput by up to 126% in comparison to using the more suitable fixed cache block size.