Mainframes used to be defined by their size, and they can still fill a room, cost millions, and support thousands of users. So today's mainframes are best defined by their operating systems: 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.

It was once defined that a mainframe, was a room or more of computer equipment. To describe a mainframe as a host system whose OS origins predate the PC and primarily used a text dumb terminal model for user interaction. That is no longer the case in the general sense of the words. Yes, mainframes can still occupy a room full of equipment and cost million of dollars and support thousands of users. Today, a mainframe can also run in a laptop and support only a couple of users.

I would define a mainframe today as a operating system. Namely, IBM's z/OS MVS/ESA (OS/390), VM/ESA & VSE/ESA. All are considered as a mainframe operating system in the old sense of the word. Some people may argue that UNIX / LINUX is a mainframe operating system. In the true sense, they are not a mainframe operating systems.

The mainframe operating system can support the UNIX operating system, known as USS (UNIX System Services) from IBM, also true of LINUX. UNIX / LINUX can have OS/390 run on it powered by INTEL chips. So, the concept of a mainframe is a room full of equipment is not true any more.

For purposes of this website, we will define a mainframe as one that runs, z/OS (OS/390 or MVS), Linux, VM and VSE, in today's environment, not yesterdays. It's time to move on...

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.

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