IBM DS8000 manual What is the challenge?, Speed gap between server and disk storage

12.1 What is the challenge?

In recent years we have seen an increasing speed in developing new storage servers which can compete with the speed at which processor development introduces new processors. On the other side, investment protection as a goal to contain Total Cost of Ownership (TCO), dictates inventing smarter architectures that allow for growth at a component level. IBM understood this early on and introduced its Seascape® architecture, and brought the ESS into the marketplace in 1999 based on this architecture.

12.1.1 Speed gap between server and disk storage

Disk storage evolved over time from simple structures to a string of disk drives attached to a disk string controller without caching capabilities. The actual disk drive—with its mechanical movement to seek the data, rotational delays and actual transfer rates from the read/write heads to disk buffers—created a speed gap compared to the internal speed of a server with no mechanical speed brakes at all. Development went on to narrow this increasing speed gap between processor memory and disk storage server with more complex structures and data caching capabilities in the disk storage controllers. With cache hits in disk storage controller memory, data could be read and written at channel or interface speeds between processor memory and storage controller memory. These enhanced storage controllers, furthermore, allowed some sharing capabilities between homogenous server platforms like S/390-based servers. Eventually disk storage servers advanced to utilize a fully integrated architecture based on standard building blocks as introduced by IBM with the Seascape architecture. Over time, all components became not only bigger in capacity and faster in speed, but also more sophisticated; for instance, using an improved caching algorithm or enhanced host adapters to handle many processes in parallel.

12.1.2 New and enhanced functions

Parallel to this development, new functions were developed and added to the next generation of disk storage subsystems. Some examples of new functions added over time are dual copy, concurrent copy, and eventually various flavors of remote copy and FlashCopy. These functions are all related to managing the data in the disk subsystems, storing the data as quickly as possible, and retrieving the data as fast as possible. Other aspects became increasingly important, like disaster recovery capabilities. Applications demand increasing I/O rates and higher data rates on one hand, but shorter response times on the other hand. These conflicting goals must be resolved, and are the driving force to develop storage servers such as the new DS8000 series.

With the advent of the DS8000 and its server-based structure and virtualization possibilities, another dimension of potential functions within the storage servers is created.

These storage servers grew with respect to functionality, speed, and capacity. Parallel to their increasing capabilities, the complexity grew as well. The art is to create systems which are well balanced from top to bottom, and these storage servers scale very well. Figure 12-1 on page 255 shows an abstract and simplified comparison of the basic components of a host server and a storage server. All components at each level need to be well balanced between each other to provide optimum performance at a minimum cost.

254DS8000 Series: Concepts and Architecture