ABCs of z/OS System Programming Volume 3 - IBM Redbooks

Chapter 2. Data set basics
2
A data set is a collection of logically related data; it can be a source program, a library of
macros, or a file of data records used by a processing program. Data records (also called
logical records) are the basic unit of information used by a processing program. By placing
your data into volumes of organized data sets, you can save and process the data efficiently.
You can also print the contents of a data set, or display the contents on a terminal.
You can store data on secondary storage devices, such as:
► A direct access storage device (DASD)
The term DASD applies to disks or to a large amount of magnetic storage media on which
a computer stores data. A volume is a standard unit of secondary storage. You can store
all types of data sets on DASD.
Each block of data on a DASD volume has a distinct location and a unique address, thus
making it possible to find any record without extensive searching. You can store and
retrieve records either directly or sequentially. Use DASD volumes for storing data and
executable programs, including the operating system itself, and for temporary working
storage. You can use one DASD volume for many separate data sets, and reallocate or
reuse space on the volume.
► A magnetic tape volume
Only sequential data sets can be stored on magnetic tape. Mountable tape volumes can
reside in an automated tape library. For information about magnetic tape volumes, see
z/OS DFSMS: Using Magnetic Tapes, SC26-7412. You can also direct a sequential data
set to or from spool, a UNIX file, a TSO/E terminal, a unit record device, virtual I/O (VIO),
or a dummy data set.
The Storage Management Subsystem (SMS) is an operating environment that automates the
management of storage. Storage management uses the values provided at allocation time to
determine, for example, on which volume to place your data set, and how many tracks to
allocate for it. Storage management also manages tape data sets on mountable volumes that
reside in an automated tape library. With SMS, users can allocate data sets more easily.
The data sets allocated through SMS are called system-managed data sets or SMS-managed
data sets.
© Copyright IBM Corp. 2010. All rights reserved. 15