IBM OS/390 User Manual

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VSE SAM/VSAM data sets may also be converted to VSAM ESDS data sets.
However, this is not recommended as it requires changes to the programs.
Default Models
Both VSE and OS/390 VSAM support the MODEL parameter of the DEFINE
command. This allows the attributes of an existing file to be used during the
define of a new file. VSE/VSAM also supports three types of model data sets
through the AMS DEFINE NOALLOCATION command. OS/390 VSAM does not
support the NOALLOCATION parameter. NOALLOCATION is usually used for:
Through reserved entry names, installation defaults may be specified for one
or more VSAM file organizations. This is frequently used for SAM/VSAM
files.
A specific model for a specific data set name. This is typically used to defer
allocation until the file is opened. Space is not automatically freed when the
file is closed.
NOALLOCATION Data Sets
Through the REUSE option, temporary VSAM data sets (dynamic files) may be
defined. No space is allocated until the file is opened and space may be freed
when the file is closed. OS/390 supports temporary VSAM data sets in the
DFSMS environment.
DFSMS (Data Facility Storage Management Services) is highly recommended
and its functions can greatly ease migration of several kinds of VSAM and
non-VSAM data sets from VSE to OS/390.
JCL Implicit DEFINE
The default models allow VSE SAM/VSAM files to be defined via JCL, without the
need for a specific AMS DEFINE. In OS/390, unless DFSMS is active, all VSAM
data sets must be defined using AMS. DFSMS provides new OS/390 JCL
keywords which allow some VSAM data sets to be implicitly defined in the JCL.
Reusable Data Sets
Both VSE and OS/390 VSAM support definition of reusable data sets. Usage of
reusable data sets differs. VSE allows the REUSE option to be specified in AMS
REPRO commands, in the ACB, or through the VSE JCL. OS/390 only supports
the options specified in the ACB or through the AMS REPRO command options.
OS/390 high level languages (for example, COBOL for OS/390 & VM) permit a
reusable data set to be extended if it is opened with the EXTEND attribute, and
for sequential access. For older language compilers, a method to extend a
reusable data set under OS/390 would be to have the application write to a
temporary file, then use an AMS REPRO with the REUSE option to copy it to the
intended reusable data set. In neither case can this be controlled by JCL options
in OS/390, as it can in VSE.
Two VSE examples are shown below:
To extend a reusable data set -
/ / DLBL .....,VSAM,DISP=OLD
Chapter 5. Disk and Tape Storage Considerations
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