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This mechanism eliminates the need for using any "delimiter" byte value to separate records. The file is an abstraction of a collection of records, in contrast to the unstructured "stream" of bytes found in systems found in smaller computers such as Unix, Windows, or MacOS. This allows data to be of any type, including binary integers, floating point, or characters, without introducing a false end-of-record condition.
== Partitioned Datasets ==
For example, a '''PDS''' or '''Partitioned Data Set''' is a dataset containining multiple ''members'', each of which holds a separate sub-data set, similar to a [[directory (file systems)|directory]] in other types of [[file system]]. This type of dataset is used to hold executable programs, or '''Load Modules'''. PDS's are also used to store source program libraries, especially Assembler macro definitions.
A PDS consists of a directory and a group of small, related sequential files stored together in a single dataset. Each sequential file is known as a member of the PDS and is accessed directly using the directory structure. Once a member is located, the data stored in that member is handled in the same manner as a PS (sequential) file.
Whenever a member is deleted, the space it occupied is unusable for storing other data. Likewise, if a member is updated, it is stored in a new spot at the back of the PDS and leaves wasted “dead” space in the middle. The only way to recover “dead” space in a PDS is to perform frequent file compression which moves all members to the front of the data space and leaves free usable space at the back. PDS files can only reside on disk in order to use the directory structure to access individual members. They are most often used for storing job JCL, utility control statements and executable modules.
Since MVS/XA there is also the Partitioned DATA set Extended (PDSE).
PDS/E file structure is similar to PDS files and is used to store the same types of data. However, PDS/E files have a better directory structure which does not require pre-allocation of directory blocks when the PDS/E is defined (and therefore does not run out of directory blocks if not enough were specified). Also, PDS/E stores members in such a way that no compression is needed to reclaim dead space. PDS/E files can only reside on disk in order to use the directory structure to access individual members. PDS/E files are also referred to as Libraries.
[[category:IBM storage devices]]
[[Category:Data management]]
[[ja:データセット (IBMメインフレーム)]]
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