Tiered storage is the assignment of different categories of data to various types of storage media to reduce total storage cost. Tiers are determined by performance and cost of the media, and data is ranked by how often it is accessed. Tiered storage policies place the most frequently accessed data on the highest performing storage. Rarely accessed data goes on low-performance, cheaper storage.
Storage tiers can also have different levels of data protection. Storage tiering began as a manual process, but storage array vendors and software vendors developed applications to automatically manage the process of moving data to the appropriate tier based on a company-defined policy.
In traditional tiered storage, Tier 1 data -- such as mission-critical, recently accessed or top-secret files -- might be stored on expensive, high-quality media such as double-parity RAID. Tier 2 data -- such as financial, seldom-used or classified files -- might be stored on less-expensive media in a conventional storage-area network. As the tiered storage number increased, cheaper media could be used. Thus, Tier 3 in a three-tier system might contain event-driven, rarely used or unclassified files on slow-spinning hard disk drives (HDDs), recordable compact discs or tapes.
Tier 0 storage
The rise of solid-state storage and flash storage has brought about what many observers refer to as Tier 0 storage. Tier 0 is faster than traditional Tier 1 storage, and much of the data formerly considered Tier 1 is now stored on Tier 0.
Cloud storage has also changed the way users look at tiers, as the public cloud can replace higher tiers for rarely accessed data. Many storage experts predict the use of fewer storage tiers, possibly even only two, with primary data going on a flash tier and archived and backup data in the cloud.
Technologies such as hierarchical storage management are used to automatically move data to less-expensive storage as data ages. Because of the cost of solid-state drives (SSDs), the advent of hybrid storage arrays mixing flash and HDDs brought about a need for automated storage tiering software to ensure that only the most important data stayed on expensive SSDs.
Tiering vs. caching
Storage tiering and caching technologies are often used interchangeably -- especially when dealing with flash media -- but they are different processes. Tiered data resides on one media type at any time, but moves between media as data access patterns change. Caching temporarily places a copy of the data on a high-performance medium such as dynamic RAM or solid-state memory to improve performance. But the cached data also resides on a lower storage tier, usually an HDD.
Read about the Fujitsu Eternus DX8900 S3, which supports automated storage tiering through the Eternus SF software.