VSAM INFORMATION
INFORMATION TAKEN FROM:
VSAM ACCESS METHOD SERVICES AND PROGRAMMING
TECHNIQUES,
BY JAMES MARTIN....
. VSAM - VERY POPULAR ACCESS METHOD
.. INTRODUCED BY IBM IN THE 1970S ALONG WITH THE
IBM SERIES 370 MACHINES.
.. REPLACED OLDER ACCESS METHODS (ISAM, BDAM,
AND VIRTUAL COUNTERPARTS.)
.. IS THE UNDERLYING DATA MANAGEMENT ACCESS
METHOD THAT MOST HIGH PERFORMANCE DATA
BASE SYSTEMS FOR IBM USE.
.. A THOROUGH KNOWLEDGE OF VSAM IS REQUIRED.
.. WIDELY USED FOR MANY FORMS OF FILE ACCESS
.. MOST MAINFRAME IBM USERS STILL USE VSAM.
.. YOUR KNOWLEDGE OF VSAM WILL SUPPORT YOUR
STUDIES IN DATABASE PROCESSING AND ALL
DIRECT FILE ACCESSING USING IBM MAINFRAMES....
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VSAM DATA SET STRUCTURE:
. VSAM DATA SET IS OFTEN CALLED A "CLUSTER."
. A KSDS CLUSTER CONSISTS OF A
.. DATA COMPONENT OF CONTROL INTERVALS
CONTAINING RECORDS, FREE SPACE, CONTROL
INFORMATION,
.. INDEX COMPONENT USED TO LOCATE RECORDS.
. ESDS & RRDS CONSISTS OF DATA COMPONENT ONLY.
.. KSDS = KEY SEQUENCED DATA SET(INDEXED FILE)
.. RRDS = RELATIVE RECORD DATA SET (RELATIVE I/O)
.. ESDS = ENTRY SEQUENCED DATA SET (SEQUENTIAL FILE)
. VSAM KSDS FACILITY PROVIDES THE WIDEST RANGE OF
FEATURES AND CAPABILITIES, AND KSDS CLUSTERS ARE
MORE WIDELY USED THAN EITHER DSDS OR RRDS CLUSTERS.
. WILL DISCUSS RRDS CLUSTERS LATER
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ACCESS METHOD SERVICES (AMS)
. SUPPLIED WITH VSAM
. IS A GENERAL-PURPOSE UTILITY PROGRAM
. USED for WIDE RANGE OF FUNCTIONS TO SUPPORT VSAM
DATA SETS AND THEIR
APPLICATIONS.
SUCH FUNCTIONS INCLUDE:
1. DEFINING, ALTERING, AND DELETING VSAM DATA SETS
2. LISTING AND MAINTAINING VSAM CATALOG ENTRIES
3. LOADING, COPYING, AND PRINTING VSAM DATA SETS
4. CONSTRUCTING ALTERNATE INDEXES
5. RECOVERING FROM VARIOUS TYPES OF ERRORS
6. PORTING DATA SETS BETWEEN OPERATING SYSTEMS
THE JCL SUPPORTING VSAM ACTIVITIES IS COMPLEX.
VSAM TRIES TO SIMPLIFY THE JCL AND CREATES MANY OF THE
SUPPORT UNDERLYING DATA STRUCTURES (LIKE CATALOGS,
INDEXES, AS NEEDED FOR VSAM) AS WELL AS PROVIDING SEVERAL
UTILITY PROGRAMS THAT ARE DESIGNED TO ACCESS VSAM FILES,
SUCH AS VSAM PRINT, VSAM REPRO, AND MORE.
BUT, IN ORDER FOR VSAM TO DO THESE THINGS FOR YOU, YOU
MUST SET UP YOUR OWN VSAM ENVIRONMENT, WHICH INCLUDES
FORMATTING YOUR OWN ADISK@ IN A MANNER THAT IS
ACCESSIBLE BY THE SERVICES PROVIDED BY VSAM.
YOU MUST THEN DEFINE YOUR MASTER CATALOG, SO VSAM CAN
FIND YOUR FILES.
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YOU MUST ALLOCATE THE PHYSICAL SPACE ON DISK FOR YOUR
CLUSTER (DATA PORTION AND INDEX PORTION).
YOU MUST NAME THESE FILES AND REQUEST A CERTAIN AMOUNT
OF DISK SPACE.
YOU MUST TELL THE VSAM UTILITIES (THAT DEFINE YOUR CLUSTER
FOR YOU) YOUR RECORD SIZE, HOW MANY TRACKS OF DISK TO
ALLOCATE FOR YOUR FILE, THE LOCATION AND SIZE OF YOUR
RECORD KEY, THE NAMES OF YOUR DATA AND INDEX FILES,
AND MORE.
GIVEN THAT YOU HAVE A CATALOG, HAVE ALLOCATED SPACE,
HAVE DESCRIBED SEVERAL OF YOUR RECORD AND FILE
PARAMETERS, THEN, UPON REQUEST, VSAM CAN CREATE A VSAM
FILE DIRECTLY FOR YOU FROM YOUR SORTED SEQUENTIAL FILE.
(YOU MAY CREATE YOUR OWN FILE VERY EASILY TOO
WITHOUT THE VSAM REPRO BY SIMPLY CREATING THE
FILE (AFTER SPACE, ...., HAS BEEN NAMED) VIA A COBOL
PROGRAM. REMEMBER: ACCESS IS SEQUENTIAL, OPEN
FOR OUTPUT, ORG IS INDEXED, RECORD KEY IS
WHATEVER....(AND ALTERNATE KEYS SPECIFIED, IF ANY).
ONCE THIS FILE IS CREATED, YOU ARE ON YOUR WAY WITH THE
COBOL INSTRUCTIONS AND TECHNIQUES DISCUSSED IN CLASS
AND IN YOUR TEXTBOOK.
BUT, IN ITS INFINITE KINDNESS, VSAM ALSO GIVES YOU MANY
UTILITIES THAT YOU MAY USE AS NEEDED.
A FEW INCLUDE:
A PRINT FACILITY,
A DELETE FACILITY,
A LIST CATALOG FACILITY,
A VERIFY FACILITY, AND OTHERS.
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VSAM STORAGE STRUCTURES
. KSDS HAS AN INDEX COMPONENT AND A DATA
COMPONENT
. PHYSICAL I/Os DONE WITH BLOCKS IN CONVENTIONAL
ACCESS METHODS.
IN VSAM, PHYSICAL I/O DONE IN CONTROL INVERVALS.
. A CONTROL INTERVAL CONTAINS
1. RECORDS
2. CONTROL INFORMATION (record descriptors)
3. FREE SPACE (FOR EXPANSION)
R
E
C
1
RE
C
2
R
E
C
3
R
E
C
4
FREE
SPACE
C
T
L
I
N
F
O
CONTROL INTERVAL
AT LOAD TIME:
CONTROL INTERVALS ARE CREATED
RECORDS WRITTEN INTO THEM.
CONTROL INTERVALS GROUPED INTO CONTROL AREAS.
CONSIDER THE FOLLOWING DRAWING:
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INDEX COMPONENT
…
INDEX SET
SEQUENCE
SET
. . .
DATA COMPONENT
CONTROL
INTERVALS
. . .
CONTROL AREA
CONTROL AREA
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CONTROL AREA
. THE INDEX IS USED TO LOCATE RECORDS IN THE DATA
COMPONENT.
. INDEX COMPONENT CONSISTS OF:
.. INDEX SET AND
.. SEQUENCE SETS
. ONE SEQUENCE SET IS NEEDED FOR EACH CONTROL AREA
. ONE ENTRY WITHIN EACH SEQUENCE SET FOR EACH
CONTROL INTERVAL WITHIN THAT CONTROL AREA.
. THE RECORDS IN EACH CONTROL INTERVAL ARE
MAINTAINED IN PHYSICAL SEQUENCE BY KEY VALUE, AND
SEQUENCE SET ENTRIES MAINTAIN THE LOGICAL SEQUENCE
OF THE CONTROL INTERVALS WITHIN EACH CONTROL AREA.
. CONTROL INTERVALS IN A CONTROL AREA ARE NOT
NECESSARILY IN PHYSICAL SEQUENCE ACCORDING TO THE
KEY VALUES OF THE RECORDS CONTAINED IN THEM. THE
SEQUENCE SETS ORDER THE CONTROL INTERVALS WITHIN
CONTROL AREAS.
. IF THERE IS MORE THAN ONE CONTROL AREA, THEN THERE
IS AN INDEX SET IN ADDITION TO THE SEQUENCE SETS.
. AN ENTRY IN THE INDEX SET POINTS TO EACH SEQUENCE
SET. SO ONCE THERE IS MORE THAN ONE CONTROL AREA,
THERE IS MORE THAN ONE SEQUENCE SET.
. THUS WE HAVE THE INDEX SET TO MANAGE THE SEQUENCE
SETS.
.
THUS TO ACCESS RECORDS, WE HAVE A TREE STRUCTURE,
INDEX SET => SEQUENCE SET => CONTROL INTERVAL
(TWO LEVELS OF INDICES) THIS HIERARCHY GENERALIZES....
CAN HAVE MULTIPLE INDEX SETS FOR EXTREMELY LARGE
KSDSs.
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PLEASE NOTE (TO KEEP THINGS IN PERSPECTIVE) THAT KSDS
CONTROL AREAS ARE QUITE LARGE AND MAY CONTAIN
THOUSANDS OF RECORDS.
A KSDS WOULD HAVE TO BE VERY LARGE TO REQUIRE TWO
LEVELS OF INDEX SET RECORDS AND WOULD HAVE TO CONTAIN
LITERALLY MILLIONS OF RECORDS BEFORE THREE LEVELS OF
INDEX SET RECORDS WOULD BE REQUIRED.
MORE NOTES:
. ONE SEQUENCE SET PER CONTROL AREA
. SEQUENCE SET ENTRY POINTS TO HIGHEST RECORD IN
CONTROL INTERVAL
. FREE SPACE IN CONTROL INTERVALS (FOR DATA AND
SEQUENCE SETS TOO.)
. CAN ALSO HAVE FREE CONTROL INTERVALS IN A CONTROL
AREA (FOR GROWTH AND CONTROL INTERVAL SPLITTING)
THINK: (WHILE NOT ABSOLUTELY TRUE, THIS MAY HELP...)
CONTROL INTERVALS: RECORDS IN BLOCK OR TRACK
CONTROL AREAS: TRACKS (NOT NECESSARILY CONTIGUOUS)
IN A CYLINDER.
. CONTROLLED/ACCESSED BY SEQUENCE SETS
INDEX SETS: POINT TO DIFFERENT CYLINDERS....
CONSIDER THE EXERCISE FOR THE NEXT DRAWING....
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KEY VALUES EXTREMELY EXAGGERATED!!
I1
INDEX
SET
SEQUENCE
S1
62
9/S1 S2
FREE
SETS
S2
3 9
D1 D2
D1
1
I2
36 62
D3 D4
FREE
D2
3
FREE
S3
5
FREE
D3
9
FREE
35 36
CONTROL INTERVALS
D4
FREE
42 43 62 FREE
CONTROL INTERVALS
CONTROL AREAS
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ADD REC6 (MOVED IN PLACE WITHIN CONTROL
INTERVAL
SEQUENCE SET UNCHANGED
DELETE REC 1 REC 3 AMOVED@ ONTO REC 1
SEQUENCE SET UNCHANGED
DELETE REC 3
CONTROL INTERVAL RETURNED TO AVAILABLE SPACE
SEQUENCE SET ENTRY MODIFIED.
ADD REC 7 AND REC8 (NO ROOM)
CONTROL INTERVAL SPLIT
. A CONTROL INTERVAL IS ALLOCATED
. ROUGHLY HALF RECORDS GO INTO EACH
SEQUENCE SET ALTERED
(TO REFLECT HIGHEST RECORDS IN THE TWO
CONTROL INTERVALS.)
10
S1
7 9
D2 D5
FREE
D2
5
D5
7
FREE
8
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9
FREE
ADDITIONAL FACTS:
ISAM:
ISAM WAS A STATIC WAY OF DOING THINGS
USED LOGICAL DELETES
REQUIRED AREORGANIZATION@ PERIODICALLY
VERY HARDWARE DEPENDENT
DOWNTIME DURING REORGANIZATION
VSAM:
DYNAMIC AND FLEXIBLE
AUTOMATICALLY REORGANIZES ITSELF
DON=T TALK ABOUT ATRACKS@ AND ACYLINDERS@
ON SURFACE - JUST CLUSTERS OR Cis
DISK SPACE CAN BE FRAGMENTED
NO REASON FOR ADOWN TIME@ FOR
REORGANIZATION
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