This document discusses datasets, catalogs, and VSAM datasets in IBM mainframe systems. It describes that datasets can have VSAM or non-VSAM organization, and lists the types of non-VSAM datasets like PS, PDS, PDSE. It also provides details about VSAM datasets like KSDS, ESDS, RRDS, their components and how they are defined using IDCAMS utility. Finally, it explains what catalogs are and the different types of catalogs used.

1. Datasets and Catalogs

2. Datasets
• Collection of data records which are logically connected.
• Every Dataset has an unique name which can be of 44 characters.
• Maximum 22 name segments separated by ‘.’ can be used. Overall
length is inclusive of the ‘.’.
• First segment of the name is called High Level Qualifier (HLQ),
Last segment is called Low Level Qualifier (LLQ) and the remaining
are called Middle Level Qualifiers.

3. Types of Datasets
• VSAM and Non-VSAM.
• VSAM Datasets – ESDS, KSDS, RRDS, and LDS
• Non-VSAM Datasets – PS, PDS, and PDSE

4. Non-VSAM Datasets
• PS – Physical Sequential – Contains Data stored inside it.
• PDS and PDSE – Partitioned Dataset and PDS(Extended) – Contains
sequential datasets in which the data is stored. These datasets
inside the PDS or PDSE are called PDS Members.
• PDS and PDSE are also called Libraries.

5. Non-VSAM Datasets (Contd..) – PS vs PDS
• PS – The records are grouped into BLOCKS. A Block is a basic unit that can
be read by the system into memory while processing the dataset. It is a
good idea to group as more records as possible for efficient processing of
data.
• PDS – The members are grouped into Directory Blocks. This determines
the number of members that can be created as each directory block can
have approximately 6 members.

6. Non-VSAM Datasets (Contd..) – PDS vs PDSE
PDS PDSE
Partitioned Dataset Partitioned Dataset Extended
Dataset Organisation – PO Dataset Organisation – LIBRARY
Can be stored on Tape and DASD Can be stored only on DASD
Slower Directory searches Faster Directory searches
Free space is not reused automatically,
manual compression required
Free space is automatically reused, no
manual compression required
Space is allocated in 16 extents Space is allocated in 123 extents

7. Non –VSAM Datasets (Contd..)
• Datasets are allocated space in extents of units (Cylinders, Tracks
etc.).
• Extents refer to the number of chunks of contiguous storage space
allocated at a time.
• If a dataset is allocated a space of 20 primary and 10 secondary
quantities of Cylinders, then there will be 1*20 + 15*10 = 170 Cylinders
of space allocated.

8. VSAM Datasets
• VSAM – Virtual Storage Access Method
• There are 4 types of VSAM Datasets – KSDS, ESDS, RRDS, and LDS
• Stored only on DASD for faster access.
• VSAM datasets have components like Cluster, Control Interval and Control Area.
• Cluster further has Data and Index components based on the type.
• VSAM records are grouped into Control Intervals (CI).
• CI is a smallest unit of data that can be swapped between DASD and memory for
processing.
• Cis are further grouped into Control Areas (CA).
• IDCAMS is the IBM utility used to Define, Delete, Rename, Copy etc the VSAM
datasets

9. VSAM vs Non-VSAM
Non-VSAM VSAM
PS KSDS, ESDS, RRDS, LDS
IEBGENER, IEFBR14 etc. IDCAMS
Stored on DASD and Tape Stored only on DASD
Records are grouped into Blocks Records are grouped into CI and CA

10. VSAM Datasets - KSDS
• KSDS – Key-Sequenced Dataset
• It has data and index components.
• Records in KSDS are uniquely identified by a key, arranged in key
sequential order.
• Data component contains records, where as the Index component
is used for faster access of the data in Data component.
• Records can be accessed sequentially or dynamically by supplying
the key value.
• Keyword ‘INDEXED’ is sued to create KSDS Datasets.

11. VSAM Datasets – ESDS
• Entry Sequential Dataset.
• It has only Data component.
• Records are arranged in the order they are inserted, identified by their
physical address – Relative Byte Address (RBA). If record length is 20,
then 0 is the RBA for first record(0-19), 20 is the RBA for second
record(20-39).
• Records can be accessed sequentially or dynamically by supplying the
RBA value.
• Keyword ‘NONINDEXED’ is used to create ESDS datasets.
• Records in the ESDS cannot be deleted but can be marked inactive.

12. VSAM Datasets - RRDS
• Relative Record Dataset.
• It has only Data component.
• Space allocated is divided into slots of fixed length, records of
fixed length or variable length can be inserted into the slots.
• Each record is identified by RRN – Relative Record Number which is
similar to serial numbers.
• Records can be accessed sequentially or dynamically by supplying
the RRN number.
• Keyword ‘NUMBERED’ is sued to create RRDS datasets.

13. Creation of VSAM through a JCL
//EXAMPLE JOB(VSAMJCL,XXXXXX),CLASS=A,MSGCLASS=A
//STEP1 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
DEFINE CLUSTER (NAME(EXT1RRV.VSAM.DATASET) -
INDEXED/NONINDEXED/NUMBERED - --> Specific to VSAM type
<<<Other parameters specific to type of VSAM>>>
DATA (NAME(EXT1RRV.VSAM.DATASET.DATA)) - --> For all types of VSAMs
INDEX (NAME(EXT1RRV.VSAM.DATASET.INDEX)) --> Only for KSDS VSAM
/*

14. Catalogs
• There are system critical datasets and user datasets in any environment.
• Catalogs provide the facility of isolating system datasets from user
datasets.
• A Catalog is a VSAM dataset that maintains records of all the other
datasets and the volumes in which they are stored.
• VTOC - Every Volume has a Table Of Contents that tells the system about
the physical address of the datasets stored inside it.
• In order to access a dataset, system reads the catalog to find the volume
in which it is stored and then reads the VTOC of that volume.

15. Types of Catalogs
• There are two types of catalogs – Master and User
• Master catalog maintains all the system datasets. Master Catalog is
created during the system generation and is stored in system
volume.
• User/Application datasets are separated from system datasets by
creating User catalogs. Aliases for user datasets are mapped to a
user catalog.
• Alias is nothing but the HLQ of the datasets but can be multilevel
as well.
• LISTCAT in front of a user dataset displays the user catalog.

16. Thank You!