Introduction to z/OS

Intro to z/OS
Steve Warren
IBM
November 2020
Session 5AO

Introduction
• This presentation will cover (at the 50,000 foot level) many different
areas (but not all) of z/OS
• Don’t be overwhelmed. There is a ton of material here. The purpose
of this session is to acquaint you with some of the major concepts of
z/OS
z/OS Concepts #CC 2

Agenda
Why Z Matters and Why z/OS
Hardware/LPAR
z/OS Components
Software Stack
App Dev, App Exec and Mgmt Envs
DASD
Data Sets / Allocation
PDS & PDSE
VTOC & Catalogs
SMS
TSO/E
ISPF
z/OS UNIX/ISHELL/OMVS/Remote
Address Spaces & Modes
Storage & DAT
Batch Processing/JES/JCL
SDSF
Job Flow
System Log
IPL
Sysplex/GDPS
Serialization
Managing Workloads
z/OS Concepts #CC 3

Why Z Matters
z/OS Concepts #CC 5
Utilities Retail
Government Financial
Travel
Healthcare
Automotive
Telecoms
Weather

Why Z Matters
Mainframes process 30 billion
business transactions per day
z/OS Concepts #CC 15
Mainframes process 1.3 million
CICS transactions. Every second. Every day.
Mainframes enable $6 trillion
in card payments annually
80 percent of the world’s corporate
data originates on mainframes
91 percent of CIOs said new customer-
facing apps are accessing the mainframe

Why z/OS?
• Ideally suited for processing large workloads for many concurrent users
• Highest security ratings for an OS
• Highest reliability
• Low TCO compared to other enterprise competitors
• Designed for:
• Serving 1000s of users concurrently
• I/O and numeric intensive computing
• Processing very large heterogeneous workloads
• Running mission critical applications securely

z/OS Lineage
• At the beginning there was:
• OS/360 (1966)
• Configuration options PCP, MFT and MVT
• OS/VS2 R1 (~1972)
• Retroactively renamed to SVS
• OS/VS2 R2 (~1974)
• Also called MVS
• MVS/370 (~1977)
• MVS/SP V1
• Optionally DFDS and DFEF
• MVS/XA (1983)
• MVS/SP V2 or V3 and MVS/DFP
• MVS/ESA (~1990)
• MVS/SP V4 or V5
• Either MVS/DFP or DFSMS/MVS
• OS/390 (1996)
• z/OS (2001)
Since 2001 the name
has remained z/OS
This is the longest
that it has been called
one name!
Since 2001 the name
has remained z/OS

IBM Z Server – the IBM z15

IBM Z Server – Multiple LPARs
CP, Memory, I/O
PR/SM™
LPAR
LPAR
LPAR
LPAR
LPAR
LPAR
LPAR
Support Element (SE)
Up to 85
LPARs can be
defined
Each LPAR can
be allocated
different
amounts of
resources (up to
16TB memory)

IBM Z Hardware and z/OS
Support Element (SE)
CP, Memory, I/O
PR/SM™
z/OS®
ICF
–
CFCC
z/VSE®
z/TPF
z/VM®
z/OS®
Linux
on
z
Systems®
Linux
on
z
Systems®
Linux
on
z
Systems®
Linux
on
z
Systems®
z/OS (a.k.a. Multiple Virtual
Storage - MVS)
Where we will be spending our
time

23
z/OS Elements,
Features, and
Components

z/OS Concepts – Elements, Features,
Components
• z/OS is a collection of elements
• Each element consists of a collection of modules (called components)
Examples:
IOS, RSM, GRS, Contents,
Allocation, Consoles, NIP,
Scheduler, SMF, BCPii, Logger

Components
• Base elements are always included in z/OS
• BCP (Base Control Program)
• DFSMSdfp (Data Facility Storage Management Subsystem)
• Deliver essential operating system functions
Other Base Elements:
Communication Server, z/OSMF,
HLASM, TSO, ISPF, JES2,
Metal C Runtime Library

Components
• Base elements are always included in z/OS
• BCP (Base Control Program)
• DFSMSdfp (Data Facility Storage Management Subsystem)
• Deliver essential operating system functions
• Optional features installed in addition to base elements
• Requested separately from base elements
• Can be priced or free
Optional Features
Examples:
Security Server, SDSF,
XL C/C++, RMF, JES3,
DFSMShsm, DFSMSdss

IBM z/OS Software Stack
z/OS®
Operating System
Middleware
Applications
Provides critical set of
system services made
possible by the
z/Architecture® and
firmware

z/OS®
Operating System
Middleware
Applications
Software that is
neither operating
system code nor an
end-user application

SAP is the registered trademark of SAP SE in Germany and in several other countries.
z/OS®
Operating System
Middleware
Applications
Db2® Data Base 2
CICS® Customer Information Control System
IMS™ Information Management System
WAS WebSphere® Application Server
IBM MQ IBM Message Queuing
SAP® Systems, Applications and Products in Data
Processing
TSO/E Time Sharing Option/Extensions

z/OS®
Operating System
Middleware
Applications Provide end-user
interface to product
solutions
ISPF Interactive System Productivity Facility
SA z/OS IBM System Automation for z/OS

32
Application Development Environments,
Application Execution Environments and
z/OS Management Environments

Application Development Env
z/OS supports many programming languages as well as
modern IDEs and environments

z/OS supports many programming languages as well as modern
IDEs and environments
• Assembler
• COBOL
• C/C++
• PL/I
• Fortran
• REXX
• JAVATM
• Python
• Node.js
• Scala

Legacy interactive interface

Integrated Development
Environment (IDE)

Integrated Development
Environment (IDE)
Other Modern Application
Development Tools

Application Execution Env
z/OS Concepts
#CC
39
Traditional z/OS
application environments,
including middleware
z/OS UNIX application
environment
z/OS Container Extensions
application environment
Java Virtual Machine
application environment

Management Environments
z/OS Management Facility (z/OSMF) provides modern interface
to help manage your z/OS systems

IBM Speak – DASD
• DASD – Direct Access Storage Device
• Think “Hard Drive”

IBM Speak – DASD
• DASD – Direct Access Storage Device
• Think “Hard Drive”
• a.k.a. DASD Volume
• Volume needs a label
• Volume Serial Number (VOLSER)
• 1 to 6 characters

IBM Speak – Data Set
• Data set
• Think “File”
• Contains data in different structured formats
• Need to choose the format of data set and reserve
space (allocate dataset) prior to using for the first time
• Resides on DASD, Tape
• z/OS has types of data sets
• Legacy Data set
• Unix file
IBM prefers the
spelling “data set” to
“dataset”

DSORG=PS (Physically Sequential)
Data
Editing the data set gives you all
the data in the data set

Directory
DSORG=PO (Partitioned)
DSORG=PS (Physically Sequential)
Data
Member 2 Data
Member 1 Data Member 3 Data
The PDS contains a directory to
locate the members

PDS & PDSE
• PDSE data sets can be used in place of nearly all PDS data sets
• PDSE designed to address running out of space issues
• Compression is not necessary
• PDSE directory expands to fit the members

IBM Speak – Data Set Name
• Data set name (DSN or DSName)
• Think “File Name”

• 1 to 22 segments
• segments separated by a period
• each segment limited to 8 characters
FRED.ASSEMBLE.SOURCE

• 1 to 22 segments
• segments separated by a period
• each segment limited to 8 characters
• Up to 44 characters (includes periods)
• High-level qualifier (HLQ) is typically your userid
High-level
qualifier
(HLQ)
Low-level
qualifier

IBM Speak – Data Set Allocation
• Data set allocation
• Think “Create Space” for a data set on disk

IBM Speak – Data Set Allocation
• Data set allocation
• Think “Create Space” for a data set on disk
• Need:
• Data set name
• VOLSER (i.e., the DASD volume) where the space should be
allocated
• Data set attributes (Size, RECFM, LRECL, BLKSZE, DSORG)

IBM Speak – VTOC & Catalogs
VOL100
Allocate space for data set
on volume VOL100
VOLSER

VOL100
VTOC
DASD volume has a Volume
Table of Contents (VTOC) that
locates a data set on the
volume

VOL100
VTOC
VOL002
SYS1.USERCAT
is on VOL100
To simplify find the data set,
the name and location
information is placed in a
catalog

VOL100
VTOC
VOL002
SYS1.USERCAT
is on VOL100
There can be many catalogs
so how do we locate the
correct one?

VOL100
VTOC
VOL001
SYS1.MSTRCAT
FRED.* data sets
cataloged in
SYS1.USERCAT on VOL002
VOL002
SYS1.USERCAT
is on VOL100
We look in the “Master”
catalog (of which there is
only one)

VOL100
VTOC
VOL001
SYS1.MSTRCAT
FRED.* data sets
cataloged in
VOL002
SYS1.USERCAT
is on VOL100
The master catalog contains an HLQ of
FRED and indicates that all of Fred’s
data sets are cataloged in the user
catalog named SYS1.USERCAT

VOL100
VTOC
VOL001
SYS1.MSTRCAT
FRED.* data sets
cataloged in
VOL002
SYS1.USERCAT
is on VOL100
Now we can tell z/OS to find
FRED.ASSEMBLE.SOURCE without
giving any more location information

z/OS Concepts – SMS
• Data set management is tedious & error prone

• Data set management is tedious & error prone
• IBM introduced SMS (System-Managed-Storage) to address this concern

• SMS - automated approach to managing storage resources

• SMS - automated approach to managing storage resources
• Uses software to manage
• data security
• data placement
• migration – move to tape
• backup
• recall – move from tape to DASD
• recovery – restore damaged data set
• deletion
• encryption
• compression
• And …

Interacting with z/OS – TSO
• End users (sometimes tens of thousands of them) use the
system
• TSO/E (Time Sharing Option/Extensions) allows users to log on and
interactively share resources
• Supports limited set of basic commands
• Sometimes called using TSO in its “native mode”

• End users (sometimes tens of thousands of them) use the
system
• TSO/E (Time Sharing Option/Extensions) allows users to log on and
interactively share resources
• Supports limited set of basic commands
• Sometimes called using TSO in its “native mode”

You provide your userid
and password

Interacting with z/OS – ISPF
Because TSO/E
native mode
supports limited
functions, most
users go right to ISPF

• ISPF (Interactive System Productivity Facility)
• Full panel application
• Panels are hierarchical

• ISPF (Interactive System Productivity Facility)
• Full panel application
• Panels are hierarchical
• Navigated via keyboard
• F7 and F8 scroll backward and forward
• Enter (not right-ctrl) moves cursor to next input field
• Home moves cursor to first input field
• ISPF use of Function Keys
• Provides text editor and browser
• Data set utilities
• Allocation
• Deletion
• Locating and Listing
• etc.

Specify the data set
name (or use wildcards)
that you want to display
and press Enter

Side note:
To list the available
commands, enter a
slash “ / “

Enter the number
of the function you
desire

You write your
program!

Interacting with z/OS – UNIX
• z/OS UNIX a certified UNIX operating system by the Open Software
Foundation
• z/OS UNIX® provides another interactive way to access z/OS
• UNIX-compliant applications can be recompiled on z/OS unmodified and run
directly on z/OS
• Before we examine some UNIX functions, we need to understand the
z/OS UNIX file system
• In particular, the zFS (zSeries File System)
UNIX is a registered trademark of The Open Group

Interacting with z/OS – UNIX zFS
• Path name identifies a file
• Consists of directory names and a file name
• Up to 1023 characters
• Directories and file name separated by a forward-slash ( / )
/dir1/dir2/dir3/MyFile

Interacting with z/OS – UNIX zFS
• Path name identifies a file
• Consists of directory names and a file name
• Up to 1023 characters
• Directories and file name separated by a forward-slash ( / )
/dir1/dir2/dir3/MyFile
• Names are case sensitive

• z/OS UNIX provides three main environments
• ISHELL & z/OS UNIX Directory List Utility
• ISPF panel interfaces to z/OS UNIX System Services
• Good for users familiar with TSO and ISPF

• OMVS
• The z/OS UNIX shell
• Users of current UNIX systems find the z/OS UNIX shell environment familiar

• OMVS
• The z/OS UNIX shell
• Users of current UNIX systems find the z/OS UNIX shell environment familiar
• Remote UNIX interfaces
• z/OS UNIX can be accessed thru standard UNIX interfaces remotely
• Telnet, SSH, NFS

88
ISHELL & z/OS UNIX
Directory List Utility

Interacting with z/OS – ISHELL
This panel is
tailorable so your
data may be
displayed differently

Interacting with z/OS – ISHELL

Interacting with z/OS – z/OS UNIX Directory List Utility
ISPF Option 3.17

Interacting with z/OS – z/OS UNIX Directory List Utility

Interacting with z/OS – OMVS
Here we have
our helowrld
file

We use the
oedit command
to edit the file

Interacting with z/OS - Telnet
• Remote command execution
• Connection may be encrypted
• Automatically converts EBCDIC on the mainframe side to ASCII on the
user side
• Telnet client is needed
• Userid/password of valid z/OS userid required

Interacting with z/OS – SSH
• Remote command execution
• Connection is encrypted
user side
• SSH client is needed
• Public/private encryption key pair needs to be generated
• No userid/password needs to be specified
• Public key needs to be stored on z/OS

Interacting with z/OS – SSH

Interacting with z/OS – NFS
• Network file system
• Acts as a file server to remote systems
• Client sees data sets or files as if they are local resources
• Access to both traditional z/OS data sets and z/OS UNIX files
• Connection may be encrypted
user side
• NFS client is needed
• mvslogin command required to logon to z/OS
• mount command used to make a connection between a drive letter of
local system and z/OS data sets or z/OS UNIX directories

Interacting with z/OS – NFS
c:znfs-client-utils>mvslogin mvshost smith
GFSA968I UNIX uid=502/gid=1000 for user JSmith obtained
from local passwd file.
Password required
GFSA973A Enter MVS password for SMITH: ********
GFSA955I SMITH logged in ok.
c:znfs-client-utils>mount mvshostmvssmith J:
J: is now successfully connected to mvshostmvssmith
The command completed successfully.
c:znfs-client-utils> J:
j:>

z/OS Address Spaces
z/OS®
z/OS is structured around address spaces
Address spaces are ranges of addresses in
virtual storage
Each user gets an address space containing
the same range of addresses

z/OS Address Spaces
z/OS®
virtual storage

z/OS Address Spaces
Wilma
0
16 MB
2 GB
16 EB
Barney
Fred
Each user’s
address space has
the same range of
addresses
0 – 16 EB
(exabyte)
1 EB = 1 billion
gigabytes (1018)

z/OS Address Spaces
PCAuth
RASP
Master
Besides an
address space for
each user, z/OS
creates address
spaces (~35) for
some of its
internal
components
Trace
DumpSrv
XCFAS
GRS

z/OS Address Spaces
SDSFAUX
VTAM
SDSF
TCAS
TCPIP
XCFAS
GRS
PCAuth
RASP
Master
Trace
DumpSrv
Then address
spaces for
middleware
and
applications
are created

111
What makes up an
Address Space?

What Makes Up an Address Space?
Prefixed Save Area (PSA)
System code and data
common to all address spaces
z/OS system data for the address space
Not available to z/OS. Used by JAVA™
Shared and common data areas
~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
z/OS has storage ranges that
contain code/data that is

Prefixed Save Area (PSA)
System code and data
z/OS system data for the address space
Not available to z/OS. Used by JAVA™
Shared and common data areas
~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
Private Area
Private Area
Private Area
Private Area
What is left is
the “Private
Area” that is
available for
your use

~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
But how much
can your
program see
(i.e., address)?

Addressing Modes
16 MB
0
An address space size
is limited to the
amount of memory
that can be addressed
In 1977, an address
space was limited to
16 MB

Addressing Modes
2 GB
0
In 1981, an address
space size increased to
a size of
2 GB

Addressing Modes
2 GB
16 MB
“The
Line”
0
The concept of
“The Line” was
introduced

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
Programs not changed to
support 31-bit addressing
had to run and could only
access data that was
“Below the line”
Data

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
MyPgm31
Programs changed to
support 31-bit addressing
could run and could also
access data that was
“Above the line”
Data
Data

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
MyPgm31
Needed a way to indicate
which addressing mode the
program supported
Programs now have to
specify:
AMODE 24 or
AMODE 31
Data
Data

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
MyPgm31
Since AMODE 31 programs
could run (i.e., be loaded)
above or below the line, a
way was needed to request
where the program should
reside
Data
Data

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
MyPgm31
Programs now have to
specify a residency mode:
RMODE 24 or
RMODE 31
so z/OS knows where to
load the program
Data
Data

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm24
MyPgm31
AMODE 31
RMODE 31
Data
Data
AMODE 24
RMODE 24

Addressing Modes
2 GB
16 MB
“The
Line”
0
MyPgm31
Data
Data
AMODE 31
RMODE 24
This request is
also possible

Addressing Modes
2 GB
16 MB
“The
Line”
0
Data AMODE 24
RMODE 31
This request is impossible!
Branches within MyPgm24 would
fail since the target location is in 31-
bit storage but the address being
used is a 24-bit address
MyPgm24

Addressing Modes
~
~
~
~
16 MB
“The
Line”
16 EB
0
In 2000, an address
space size in creased
to 64-bit or
16 EB

Addressing Modes
~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
The concept of
“The Bar” was
introduced

Addressing Modes
~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
AMODE 64 is introduced
and data can now be
accessed “Above the
Bar”
While RMODE 64 is
supported by
z/Architecture, z/OS does
not support it
MyPgm6424
MyPgm6431
Data
Data
Data

Addressing Modes
~
~
~
~
2 GB
"The
Bar"
16 MB
“The
Line”
16 EB
0
MyPgm6424
MyPgm6431
Data
Data
With z/OS V2R3 and
beyond, z/OS supports
RMODE 64
With restrictions
Programs will be loaded
“Above the Bar”
MyPgm6464
Data

Storage
• Programs are written to utilize a given amount of memory.

Storage
• The amount of memory required by all the running tasks is usually
much greater than the amount of real storage available.

Storage
• The amount of memory required by all the running tasks is usually
much greater than the amount of real storage available.
• This is facilitated by the use of virtual storage.

Storage
When a task is active, its memory
is brought into real storage.

Storage
The rest can stay in auxiliary
storage, enabling the remaining
real storage to be used for other
programs.

Storage
• Virtual storage is divided into 4KB pieces called pages.
• Real storage is divided into 4KB pieces called frames.
• Auxiliary storage is divided into 4KB pieces called slots.

138
Dynamic Address
Translation (DAT)

Address Translation
• When a program is running, the processor must be able to
locate its virtual pages in real storage.

Address Translation
• When a program is running, the processor must be able to
locate its virtual pages in real storage.
• Address translation is the process that enables a processor
to convert virtual addresses into real addresses.

Batch Processing
• z/OS is also ideal for batch jobs
• Workloads that run in background
• Little or no human interaction

Batch Processing
• Many customer core applications, such as payroll, are performed
through batch processing (jobs)
• Jobs run without end user interaction
• Run as resources permit
• JCL (Job Control Language) is used to control the operation of each
job

Batch Processing
Example
of JCL

Batch Processing
JCL statements
start with a // in
column 1

Batch Processing
• An initiator (system program)
• Processes JCL
• Creates environment in an address space
• Runs the job in that address space

Batch Processing
• Management of jobs & resources are shared between z/OS and JES
(Job Entry Subsystem)

Batch Processing
• Management of jobs & resources shared between z/OS and JES
(Job Entry Subsystem)
• JES
• Receives jobs into system
• Queue jobs waiting to be executed
• Manages priority
• Schedules for processing
• Controls output processing

Job Flow
Typical
Job Flow

Batch Processing – SDSF
We want to see the output
status of our job
Option “ST” is used

This is the job
we just ran

To see the
output, type “S”
for select

Messages issued by
z/OS on behalf of
your job are
displayed

z/OS Concepts – SYSLOG
• What is the System Log?
• Better known as SYSLOG
• a.k.a. Hardcopy Log (at one time this was a printer)
• Chronological listing of messages about z/OS system activity and other major
middleware software products
• Issued system commands and their responses
• When an unexpected system problem occurs, the SYSLOG is the first
place to look to gather information about the problem

z/OS Concepts – SYSLOG
Message ID of a message
Every message starts with an ID
(up to 10 characters)
Enables you to look up message in
documentation

159
IPL and System
Address Space
Initialization

IBM Speak - IPL
• IPL – Initial Program Load
• Think “Boot the System”
• Starting z/OS

IBM Speak - IPL
Use a special console
connected to the internal
network of mainframe boxes
(HMC) and
activate the “Load Profile”
Profile contains DASD
device number to IPL

IBM Speak - IPL
HMC
“current”
screen
image

IBM Speak - IPL
Load Profile
Device address
to IPL
0980
0CE3W1

IBM Speak - IPL
IPL processed in
phases
Each phase
builds on the
previous

IBM Speak - IPL
0980
SYSRS1
To “Boot” or “IPL”
z/OS, the device
number of the
SYSRES (system
residence volume)
is specified

IBM Speak - IPL
JES2
RACF
RESOLVER
LLA
VLF
HZR
JES2AUX
MSI – several z/OS
component address
spaces created
After MSI, queued
START commands are
processed
Additional z/OS
component address
spaces are created

IBM Speak - IPL
Important address spaces started at Master Scheduler Initialization
time (MSI):
• SMF (System Management Facility)
Collector of system and job-
related information used for:
billing users, reporting reliability,
analyzing the configuration,
evaluating activity, profiling system
resource use, maintaining system
security.

IBM Speak - IPL
time (MSI):
• System Logger
A set of services that allow an
application to write, browse, and
delete log data. Designed to
merge data from multiple
instances of an application,
including merging data from
different systems across a sysplex.

IBM Speak - IPL
time (MSI):
• System Logger
• BCPii (Base Control Program internal interface)
A set of services that allow
special z/OS applications a way of
programmatically controlling the
mainframe hardware
configuration

IBM Speak - IPL
Important started address spaces
• JES (Job Entry Subsystem a.k.a. JES2 or JES3)
Receives jobs, schedules them for
processing and handles their output

IBM Speak - IPL
• RACF® (Resource Access Control Facility a.k.a. z/OS Security Server)
Security program used to protect
resources

IBM Speak - IPL
• ICSF (Integrated Cryptographic Service Facility)
Provides the z/OS Cryptography interfaces
to applications: Enciphering, Deciphering,
Hashing, and Generating/verifying
digital signatures. Implements this via
both crypto hardware and software.

IBM Speak - IPL
• VTAM® (Virtual Telecommunications Access Method a.k.a. z/OS Communications Server)
Implements SNA (Systems Network
Architecture) with API (Application Program
Interface) for communicating with devices
and programs

IBM Speak - IPL
• TCAS (Terminal Control Address Space)
Communication interface for
TSO/E (Time Sharing
Option/Extensions)

IBM Speak - IPL
• TCP/IP (Transmission Control Protocol/Internet Protocol a.k.a. z/OS Communications
Server: IP)
Networking protocols for computer
communications

IBM Speak - IPL
• TCP/IP (Transmission Control Protocol/Internet Protocol a.k.a. z/OS Communications
Server: IP)
Once TCP/IP is initialized,
the IPL is considered
complete

IBM Speak - IPL
• Of course there is still more work to be done
• Middleware and application products like the following need to be
started:
• SA z/OS (System Automation for z/OS)
• DB2® (Data Base 2)
• CICS® Transaction Server for z/OS (Customer Information Control System)
• IMS™ (Information Management System)
• WAS (WebSphere® Application Server)

IBM Speak – Parallel Sysplex
• A sysplex is
• A collection of z/OS systems that cooperate
• A Parallel Sysplex
• Is a sysplex that uses multisystem data-sharing technology
• Allows direct, concurrent read/write access to shared data from all systems
without impacting performance or data integrity
• Work requests that are associated with a single workload
• Can be dynamically distributed for parallel execution on systems
• Based on available processor capacity
•

IBM Speak – Parallel Sysplex
• In many ways a Parallel Sysplex appears as a single large system
• The collection of z/OS systems cooperate to:
• Process workloads
• Provide higher availability
• Provide easier systems management
• Provide improved growth

Parallel Sysplex
This sysplex consists of two z/OS
images, each on their own CPC

Parallel Sysplex
The z/OS
components that
manage the
sysplex are:
XCF (cross system
coupling facility)
XES (cross-system
extended services)

Parallel Sysplex
XCF allows authorized programs on one
system to communicate with programs on
the same system or on other systems

Parallel Sysplex (7 of 13)
XES enables applications and subsystems to
take advantage of the coupling facility

Parallel Sysplex
CF (Coupling Facility) provides:
• Data sharing across the systems in a sysplex
• Maintaining the integrity and consistency of
shared data
• Maintaining the availability of a sysplex

Parallel Sysplex
STP (Server Time Protocol) is required
to synchronize the TOD (time-of-day)
clocks for systems in a sysplex

Parallel Sysplex
Shared CDS (Couple Data
Sets) records status
information for the sysplex

Parallel Sysplex
Applications written to take advantage of a
parallel sysplex can have multiple
instances running on any/all z/OS images
Data accessible from all images

Parallel Sysplex
Application is still
available if a failure in
one image/CPC occurs

z/OS Concepts – Serialization
• Customers using z/OS may runs many concurrent programs and
many threads of the same program simultaneously.
• Think credit card transactions!

• What if these programs need to update resources at the same time?

• Data sets
• Virtual resources
• Lists
• Queues
• Data areas (control blocks)

• Data sets
• Virtual resources
• Lists
• Queues
• Data areas (control blocks)
• How do we ensure data integrity and fair access to these resources?

• Global Resource Serialization (GRS) is the z/OS component designed
to protect the integrity of resources in a multitasking, multi-host
environment

• Global Resource Serialization (GRS) is the z/OS element designed to
protect the integrity of resources in a multitasking, multi-host
environment
• Coordinates access to resources used by more than one program

• Global Resource Serialization (GRS) is the z/OS element designed
to protect the integrity of resources in a multitasking, multi-host
environment
• Coordinates access to resources used by more than one program
• Uses ENQs and Latches to scope resources at various levels:
• STEP – synchronize within a single address space
• SYSTEM – synchronize single system apps
• SYSTEMS – synchronize multisystem apps
• SYSPLEX – synchronize across a sysplex
• Custom (Latches) – synchronize within a multitasking, or multi-threaded
application

• Program obtains ENQ or Latch before reading or updating
protected resource

• Program obtains ENQ or Latch before updating protected
resource
• Determines if it should be exclusive (write access) or
shared (read/only access)

z/OS Concepts – Managing Workloads
• One of the strengths of the IBM Z platform and the z/OS operating
system is the ability to run multiple workloads at the same time
within one z/OS image or across multiple images.

• z/OS needs to:
• Prioritize work
• Use the installation resources as efficiently as possible
• Maintain the highest possible throughput
• Achieve the best possible system responsiveness.

• z/OS needs to:
• Prioritize work
• Use the installation resources as efficiently as possible
• Maintain the highest possible throughput
• Achieve the best possible system responsiveness.
• Dynamic workload management is accomplished through the
Workload Management (WLM) component of the z/OS operating
system

• WLM allows a customer to define performance goals and assign a
business importance to each goal.

• WLM allows a customer to define performance goals and assign a
business importance to each goal.
• Goals are defined in business terms, and the system decides how
much resource, such as CPU and storage, should be given to the work
to meet its goal.

• WLM algorithms use the service definition information and internal
monitoring feedback to check how well they are doing in meeting the
goals.
• Algorithms periodically adjust the allocation of resource as the workload level
changes.

• WLM algorithms use the service definition information and internal
monitoring feedback to check how well they are doing in meeting the
goals.
• Algorithms periodically adjust the allocation of resource as the workload level
changes.
• For each system, WLM manages the system resources.
• Coordinates and shares performance information across the
sysplex.
• How well it manages one system is based on how well the other systems are also doing
in meeting the goals. If there is contention for resources, it makes the appropriate trade-
offs based on the importance of the work and how well the goals are being met.

• WLM can dynamically start and stop server address spaces to
process work from application environments.
• On a single system or across the sysplex
• Batch initiators can be managed
• Can dynamically manage the number of batch initiators for one or more job classes to
meet the performance goals of the work.

• WLM can dynamically start and stop server address spaces to
process work from application environments.
• On a single system or across the sysplex
• Batch initiators can be managed
• Can dynamically manage the number of batch initiators for one or more job classes to meet
the performance goals of the work.
• WLM also collects real-time performance data and delay monitoring.
• Available for performance monitors and reporters for integration into detailed
reports.

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Systems.
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Introduction to z/OS

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