data set organization

data set organization
DSORG

   <operating system, storage> (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)