Introduction to MVS 1 - What is it?
===================================

I started becoming interested in MVS maybe a year, 18 months or so
ago. A couple of decades as a sysadmin (among other hats) had made me
kind of weary of Unix OSes and I was looking around for something
else.

The availability of public-access systems [1] [2], in addition to
seemingly-usable emulation [3] caught my eye. That, along with *the*
most extensive documentation[4] I've seen for *any* system, and the
eye-watering apparent complexity settled things. It was going to be
MVS for me. Here was something I could get my teeth into.

The OS
------

IBM MVS (Multiple Virtual Storage) runs on mainframe systems, and
should not to be confused with DEC VMS [5] which runs on DEC VAX
minis, Alpha and Itanium servers (and is being ported to x86).

Wikipedia[6] has *much* more on MVS, but here's a brief summary.

One of a small number of IBM mainframe OS families, MVS is part of
the OS/360 line, first introduced in 1966 for use on the System/360
machine. The timeline is more or less as follows:

1966  MFT                        (24-bit memory addressing)
1967   |       MVT
1968 MFT II     |
       |        |
1972 OS/VS1   OS/VS2
                |
1974          OS/VS2 R2(MVS)
                         |
1980                  MVS/SP
                         |
1983                  MVS/XA     (31-bit memory addressing)
                         |
1988                  MVS/ESA
                         |
1995                  OS/390
                         |
2000                   z/OS      (64-bit memory addressing)
             
Present                z/OS 2.4

As the dominant platform for large-scale commerce and finance, the OS
(and underlying hardware) has three major attributes:

- Reliability (continuous uptimes measured in years under very high
  load)
- Backwards compatibility (most 24-bit applications will run unmodified
  on even the latest version of the OS)
- Enormous I/O throughput (think banks, utilities, large retailers)

Major Components
----------------

- JES2, JES3

One or the other, the vast majority of MVS installations choose
JES2. A JES (Job Entry Subsystem) is the primary means of getting work
into the system.

- SMF and RMF

The System Management Facility and the Resource Measurement Facility
provide fine-grained and comprehensive recording and reporting of
system activity and performance. The introduction of sub-capacity
licensing for MVS requires that customers send monthly usage reports
generated from SMF to IBM for billing purposes (MVS is rented).

- RACF

The Resource Access Control Facility is an extensive security and
access control system. If you've gone to all the expense of an MVS
system, the chances are your data is extremely important to you.

- SMS

SMS (System Managed Storage) handles automated policy-based
hierachical storage management. When your MVS estate could run to
thousands of disks and tapes spread across multiple sites, why not let
the system do most of the storage management work for you?

- TSO/E

The Time-Sharing Option provides interactive facilities for users
ie. a CLI. Similar to background processes/cron/etc in Unix, CLI
applications can also be run in batch.

- Unix System Services

A POSIX-compliant UNIX subsystem integrated into MVS. And you thought
WSL was a novelty - MVS has had Unix support since 1993.


[1] https://mtm2018.mybluemix.net
[2] https://moshix.dynu.net
[3] https://www.hercules-390.org
[4] https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zosInternetLibrary?OpenDocument
[5] https://en.wikipedia.org/wiki/VMS
[6] https://en.wikipedia.org/wiki/MVS