Caching Hybrid
Caching-hybrid systems have an architecture in which flash is used as a cache for a pool of disks, either as a read and write cache or solely as a read cache. While caching-hybrid systems do deliver better performance than post-process-tiering systems, they come with significant drawbacks. Because flash is being used as a cache, rather than as storage media, the cache does not contribute to the overall capacity of the system, making these caching-hybrid systems more expensive than tiering systems in terms of cost per gigabyte of raw capacity. In addition, systems that use flash only to cache reads have to employ DRAM to serialize writes that are made directly to spinning media. This strategy works only with very small working sets. If the working set is large, write performance slows to the pace of disk drives, lowering overall system performance.
Moreover, as innovations in both flash and other solid-state storage media continue, hybrid systems that use multiple (two or more) solid-state media types will soon become possible. Systems that employ a caching-hybrid architecture can't benefit from these innovations, since when two types of flash are used there is very little performance difference between the cache and the pooled media. Therefore caching-hybrid systems using multiple solid state media types only secure marginal performance benefits from caching, despite a significant increase in cost. Also many caching hybrids do not support dedupe, and when dedupe is supported it must be disabled for volumes that are performance sensitive.