Data stored on IBM disks can be organized in a number of ways, which
are referred to as data set types. IBM software supports the following
data set types:
-
-
Partitioned Organization (PO)
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-
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Virtual Storage Access Method (VSAM)
VSAM data sets can be
one of the following:
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Entry-Sequenced Data Set (ESDS)
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Key-Sequenced Data Set (KSDS)
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Relative-Record Data Set (RRDS)
In each data set type
except VSAM, the records are organized in a unique way, depending
on their purpose. Each type of data set organization has one or more
special access methods. (For example, a data set that uses DA organization
is characterized by a predictable relationship between the key of
a record and the address of that record on a DASD device.) The programmer
establishes this relationship and must supply most of the logic required
to locate the individual records.
VSAM is a multifunction, all-purpose
access method. VSAM is different from the other data set types because
it provides a functional equivalent for most of the other data set
organizations, as follows:
-
ESDS organization is the functional
equivalent of Physical Sequential organization (PS).
-
KSDS organization is the functional
equivalent of Indexed Sequential organization (IS).
-
RRDS organization is the functional
equivalent of Direct Access organization (DA).
The types of data set
organizations that you access with VSAM differ from others for two
reasons:
-
They are device independent.
-
They can be both sequentially and
directly accessed.
You access a record
by addressing the record in terms of its displacement (in bytes) from
the beginning of the data set, by its key, or by its record number.
The root of the VSAM
access method is the VSAM catalog, which is a disk area for defining
data sets and disk space and for maintaining information about each
VSAM data set. VSAM catalogs and data sets are created and managed
with IBM Access Method Services (AMS), a multifunction service program.