data set organization
(foldoc) | data set organization
DSORG
(DSORG) An IBM term for file
structure. These include PS physical sequential, DA {direct
access}, IS indexed sequential, PO partitioned (a
library). This system dates from OS/360, and breaks down
beginning with VSAM and VTAM, where it is no longer
applied.
Sequential and indexed data sets can be accessed using either
a "basic" or a "queued" "access method." For example a
DSORG=PS file can use either BSAM (basic sequential access
method) or QSAM (queued sequential access method). It can
also be processed as a direct file using BDAM. Likewise a
library can be processed using BPAM (basic partitioned access
method), BSAM, QSAM, or BDAM. DSORG and access method are
somewhat, but not completely, orthogonal.
The "basic" access method deals with physical blocks rather
than records, and usually provides more control over the
specific device. Each I/O operation using the "basic"
access method reads or writes a single block. A "basic" read
or write starts an asynchronous I/O operation, and the
programmer is responsible for waiting for completion and
checking for errors.
The "queued" access method deals with logical records and
provides blocking and deblocking services. It is "queued"
because it provides read-ahead and write-behind services.
While a program is processing records in one input block, for
example, QSAM may be reading one or more blocks ahead. Queued
"get" or "put" operations are synchronous as far as the
programmer is concerned. The operation is complete when the
next logical record has been successfully processed.
EXCP (Execute Channel Program) is a lower-level method of
accessing data.
IBM manuals usually named "Data Administration Guide",
e.g. SC26-4505-1 for MVS/ESA DFP 3.1, provide more detail
about data set organizations and access methods.
(2005-08-08)
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