what is a mainframe computer
2006-12-14 11:18:53,from:WOWwhat is a mainframe computer?
A very large and expensive computer capable of supporting hundreds, or even thousands, of users simultaneously. In the hierarchy that starts with a simple microprocessor (in watches, for example) at the bottom and moves to supercomputers at the top, mainframes are just below supercomputers. In some ways, mainframes are more powerful than supercomputers because they support more simultaneous programs. But supercomputers can execute a single program faster than a mainframe. The distinction between small mainframes and minicomputers is vague, depending really on how the manufacturer wants to market its machines.
What is a Mainframe?
Mainframes used to be defined by their size, and they can still fill a room, cost millions, and support thousands of users. But now a mainframe can also run on a laptop and support two users. So today's mainframes are best defined by their operating systems: Unix and Linux, and IBM's z/OS, OS/390, MVS, VM, and VSE. Mainframes combine four important features: 1) Reliable single-thread performance, which is essential for reasonable operations against a database. 2) Maximum I/O connectivity, which means mainframes excel at providing for huge disk farms. 3) Maximum I/O bandwidth, so connections between drives and processors have few choke-points. 4) Reliability--mainframes often allow for "graceful degradation" and service while the system is running.
Mainframes combine three important features:
1) Maximum reliable single-thread performance: Some processes, such as the merge phase of a sort/merge (sorting can be subdivided...) MUST be run single thread. Other operations (balancing b-trees, etc) are single thread and tend to lock out other accesses. Therefore, single thread performance is critical to reasonable operations against a DataBase (especially when adding new rows).
2) Maximum I/O Connectivity: Mainframes excel at providing a convenient paradigm for HUGE disk farms; While SAN devices kind of weaken this to some degree, SAN devices mimic the model of the Mainframe in connectivity "tricks" (at least internally).
3) Maximum I/O Bandwidth: Despite the huge quantities of drives that may be attached to a mainframe, the drives are connected in such a way that there are very few choke-points in moving data to/from the actual processor complex.
All system architectures are best at different jobs; Each is a set of compromises. Mainframes are more expensive because the compromises are less, well, compromised. The CPU performance is not always greater (in MIPS) than other processes, but the actual priority here is not raw performance but reliability. Mainframes, due to their great cost (and trouble in amortizing this across outages) often allow for "graceful degradation" and servicing while the system is running. While this is not a universal trait, it's interesting to see this priority setting the line in the sand between performance / price.