Tier 1 storage is a reference to the higher performing systems in a tiered storage environment. In such an environment, important data is stored on more expensive, higher performing storage systems while less critical data is stored on less expensive, lower performing storage.
Tiers are referred to as Tier 0, Tier 1, Tier 2, et cetera, with the lower numbers representing a higher performing storage system. Tier 1 commonly refers to high-performance hard disks that store an organization's more critical or frequently accessed data, such as transactional data.
See also: Hierarchical Storage Management