Posted by: Randy Kerns
storage tiering: caching
We hear a lot these days about tiering and caching in storage systems. These are not the same thing. Some systems implement tiering across types of media, while others cache data into a solid-state device as transient storage. Other storage systems have both capabilities.
IT professionals may wonder what the differences between tiering and caching are, and whether they need to tier or cache data. There are clear differences, but the performance implications between the approaches vary primarily based on the specific storage system implementation.
Tiering storage systems use different devices such as solid-state devices, high-performance disks, and high-capacity disks. Each of these device types make up a tier. The systems intelligently move data between the tiers based on patterns of access — a process known as automated tiering.
Tiering greatly increases the overall system performance, with access to the most active data coming from the highest performance devices. The higher performance allows the systems to support more demanding applications. Tiering also lets an organization get by with smaller amounts of the most expensive types of storage by moving less frequently accessed data to cheaper drives.
Caching systems use memory or solid state to store highly active data as transient data that may be accessed from the higher performing technology. The caching data resides in a permanent location in the storage system in addition to the cache.
Caching may be done in RAM or in solid-state devices used specifically for caching. RAM cache can be protected by a battery or capacitor.
Caching has been used effectively to speed storage performance for many years. In the mainframe world, the caching is controlled with information communicated from the operating system. In open systems, the storage systems contain the intelligence to stage or leave copies of active data in the cache. Storage systems can cache read data only, or they can also accelerate writes.
If a storage system features tiering and caching, the features need to work in concert to avoid wasted or conflicting data movement. There can be improved performance if the two capabilities work together.
IT professionals need to consider the cost/benefit tradeoffs of tiering and caching. What performance is gained versus the cost? The overall performance benefit needs to be considered in the context of the workload from the applications that use the stored information. Most of the vendors of tiered storage systems have effective tools that analyze the environment and report on the effectiveness of tiering. This is necessary to optimize performance.
There is no easy answer to the choice of tiering, caching, or doing both in a storage system. It becomes a matter of maximizing the performance capabilities of the storage system and what value it brings in consolidation, reduced costs, and overall efficiency gains. An analysis of the value gained versus the cost must be done for any individual system.
(Randy Kerns is Senior Strategist at Evaluator Group, an IT analyst firm).