Advances in technologies have created a new layer or tier of storage between cache memory and traditional hard disk drives: the solid-state drive or SSD. SSDs use high-speed flash memory and offer capacities on a par with those of hard disk drives (HDDs). SSDs can be used either to create a very large cache (that substantially improves "hit" rates) or a very fast and full tier of disk storage. Either way, the performance gains an be significant for many applications.
