Contained in this redbook is a close-up, performance related view of the different hardware architectures IBM offers in its RS/6000 and ^ pSeries systems, including system, processor, memory, storage, and network architectures. One chapter is dedicated to general sizing rules for a number of environments such as database sizing, IBM HTTP server sizing, Net.Commerce sizing, and Lotus Domino sizing. The reader will also find a description of the Industry benchmarks that are performed on IBM systems as well as an overview on AIX performance tools.This redbook is an update to the successful first Edition of “Understanding IBM RS/6000 Performance and Sizing,” that was published in 1997. Obsolete information was taken out, still relevant information was updated, and new information was added to this new Edition. While the book was produced, IBM RS/6000 was re-branded to IBM^ pSeries. As a conclusion to that, the given information applies to both brands, even though only one of the brand names might be mentioned, except for the processor and system architectures, which only apply in part to the IBM^ pSeries models.