IBM OS/390 User Manual
Chapter 5. Disk and Tape Storage Considerations
The VSE/SP and VSE/ESA systems and MVS and OS/390 systems have some
conceptual similarities and data compatibilities for disk and tape files. This
chapter will discuss the similarities and differences between the VSE and OS/390
environments in the following areas:
conceptual similarities and data compatibilities for disk and tape files. This
chapter will discuss the similarities and differences between the VSE and OS/390
environments in the following areas:
•
5.1, Access Method Similarities and Differences
•
5.2, Data Set Naming Considerations
•
5.3, Storage and Space Management
•
5.4, Tape Similarities and Differences
•
5.5, DASD Similarities and Differences
•
5.6, VSAM Differences
These topics will be discussed in order within this chapter.
5.1 Access Method Similarities and Differences
5.1.1 Access Methods
An access method is a set of user application programming interfaces (APIs),
utility programs, other programming, and format standards which provide users
with the ability to readily store and retrieve data within a computer system.
utility programs, other programming, and format standards which provide users
with the ability to readily store and retrieve data within a computer system.
The VSE/ESA and OS/390 operating systems each support a number of access
methods, with varying levels of compatibility. Further, details of the
implementation are different between the operating systems, even if the function
and the external format of the data are the same.
methods, with varying levels of compatibility. Further, details of the
implementation are different between the operating systems, even if the function
and the external format of the data are the same.
Access methods used for disk and tape storage in the VSE system include the
following:
following:
SAM
Sequential Access Method -- used for disk and tape devices. Records
are stored and/or retrieved in the order presented.
are stored and/or retrieved in the order presented.
In OS/390, the most similar access methods are QSAM or SAM-E.
ISAM
Indexed Sequential Access Method -- formerly used for disk devices,
when records were to be maintained (logically) in ascending key
sequence, but might need to be retrieved in arbitrary order (by key).
Obsolete, replaced by VSAM in most VSE environments over twenty
years ago.
when records were to be maintained (logically) in ascending key
sequence, but might need to be retrieved in arbitrary order (by key).
Obsolete, replaced by VSAM in most VSE environments over twenty
years ago.
In OS/390, may still be supported, but not recommended.
VSAM
Virtual Storage Access Method -- used for disk devices. Records can
be stored and/or retrieved in the order presented, or in key or
address order.
be stored and/or retrieved in the order presented, or in key or
address order.
In OS/390, DFP/VSAM is the most similar access method.
VSAM will be discussed in a separate sub-chapter below.
Copyright IBM Corp. 1998
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