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IBM eServer zSeries is a brand name of IBM which was designated to all IBM mainframes in 2000 with the e depicted in IBM's well-known red trademarked symbol.
Nonetheless, IBM didn't give the previous servers of the same line (being called IBM S/390) new names, so the term zSeries now popularly refers only to IBM's first line of 64-bit mainframes, starting with the first model of the new 64-bit mainframes the IBM zSeries 900 or, for short, z900. The z900 was (compared to its predecessors) a very powerful machine which introduced 64-bit computing into the mainframe world using the newly-designed z/Architecture. At the same time, the new servers provided more than twice the performance of previous models.
In July 2005, IBM rebranded the server line again, to System z, but again not renaming previous servers. At the same time IBM announced a new brand name (System z9) using it to announce the latest generation System z9-109 servers. (See System z9 for information on the successor models to zSeries.)
The IBM eServer zSeries line succeeded the IBM System/390 (S/390 for short) which were based on the ESA/390 architecture while ensuring full compatibility to it. Thus, zSeries servers are the direct, lineal descendents of the IBM System/360, announced in 1964, and the 1970s System/370. Applications written for the 24-bit-addressing/32-bit-data System/360 can still run, unmodified, on the newest System z9 over four decades later, with few exceptions.
The term "zSeries" is still the most popular way to refer to all IBM mainframe servers, probably because as of 2006 the old S/390 IBM mainframe servers can be considered history.
In its 64-bit mode the new CPU become free from the legacy of the 31-bit addressing limits of its predecessors.
Major features of the eServer zSeries family:
Based on z/Architecture (64-bit real and virtual addresses)
Offers up to 32 central processors (CPs) per frame (rack) coupled in up to a 32-frame Sysplex — with each frame physically separated up to 100 kilometers
S/390 ESA (31-bit) applications are fully compatible with z/Architecture
Some models introduced multiple I/O channel subsystems (exceeding the previous 256 channel limit) and zAAPs
The System z9 servers add on top of that:
Up to 54 central processors (CPs) per frame (rack)
The System z9-109 Model S54, with up to 54 "characterizable" PUs (processors), is currently the most powerful IBM mainframe available, reportedly capable of performing approximately 18,660,000,000 core instructions per second. A single S54 can typically process one billion or more business transactions per day — double the throughput of its predecessor. The 54 PUs can be characterized (configured) for a variety of purposes including general purpose processing (CPs), zAAPs, IFLs, and ICFs (coupling facilities).
A direct comparison of zSeries servers with other computing platforms is difficult. zSeries servers are unique in providing processing power for business-critical, high volume transaction processing and databases. For example, zSeries servers offload such functions as I/O processing, cryptography, memory control, and various service functions (such as accounting and logging) to dedicated processors. These "extra" processors are in addition to the (up to) 54 main CPs per frame. The zSeries servers also effectively execute every instruction twice in order to assure processing integrity. If the instruction results differ, the zSeries server retries the instruction. If the instruction still fails, the zSeries/z9 server will shut down the failing processor and shift workload, "in flight," to any surviving processors, including one or more spares. The IBM mainframe then "calls home" (automatically places a service call to IBM), and an IBM service technician soon arrives with a replacement part (possibly even a new processor book, consisting of a group of processors). With System z9 servers, the technician installs the new book and removes the old one without interruption to running applications. (Note that IBM mainframe processors have a reported 40 year MTBF.) Similar design redundancies exist in memory, I/O, power, cooling, and other subsystems. All these features exist at the hardware and microcode level, without special application programming. The same concepts extend to coupled frames separated by up to 100 kilometers in a Geographically Dispersed Parallel Sysplex.
zSeries servers are used by IBM customers for business-critical installations for large organisations which need very high availability, where unscheduled downtime of service costs more than the saving which cheaper high-availabilty setups could provide and at traditional mainframe shops which already have mainframe applications at the center of their business processes like banks and insurance companies. For such organisations which have to consider a very high price for system failures and service outages, zSeries machines can provide a lower total cost of ownership than other platforms, especially when running a variety of business-critical applications concurrently (so-called mixed workload). Overall mainframes like the zSeries are mostly used in government, financial services, retail, and manufacturing.
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