Data set (IBM mainframe): Difference between revisions

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{{confused|Passive data structure}}

A '''partitioned data set''' ('''PDS''')<ref>{{cite webbook

| title = z/OS DFSMS Using Data Sets Version 2 Release 3

|url=https://searchdatacenter.techtarget.com/answer/Print-Partitioned-Data-Set-PDS-sequentially

| id = SC23-6855-30

|title=Print Partitioned Data Set (PDS) sequentially

| date = October 2, 2018

|quote=.. members of a Partitioned Data Set (PDS)}}</ref> is a data set containing multiple ''members'', each of which holds a separate sub-data set, similar to a [[directory (file systems)|directory]] in other types of [[file system]]s. This type of data set is often used to hold ''load modules'' (old format bound executable programs), source program libraries (especially Assembler macro definitions), and [[Job Control Language]]. A PDS may be compared to a [[ZIP (file format)|Zip]] file or [[COM Structured Storage]].

| section = Structure of a PDS

| url = https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3sc236855/$file/idad400_v2r3.pdf

| mode = cs2

}}</ref>

|quote=.. members of a Partitioned Data Set (PDS)}}</ref> is a data set containing multiple ''members'', each of which holds a separate sub-data set, similar to a [[directory (file systems)|directory]] in other types of [[file system]]s. This type of data set is often used to hold ''load modules'' (old format bound executable programs), source program libraries (especially Assembler macro definitions), and [[Job Control Language]]. A PDS may be compared to a [[ZIP (file format)|Zip]] file or [[COM Structured Storage]].

A Partitioned Data Set can only be allocated on a single volume and have a maximum size of 65,535 tracks.

Besides members, a PDS consistscontains also of theira directory. Each member can be accessed directlyindirectly usingvia the directory structure. Once a member is located, the data stored in that member isare handled in the same manner as a PS (sequential) data set.

Whenever a member is deleted, the space it occupied is unusable for storing other data. Likewise, if a member is re-written, 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 is to perform frequent file compression.<ref name=Stephens>{{cite book|last1=Stephens|first1=David|title=What On Earth is a Mainframe?|date=Oct 2008|publisher=Lulu.com|isbn=978-1-4092-2535-5|page=52|url=https://books.google.com/books?id=1NMYOOW3gHMC|accessdate=May 11, 2018}}</ref> Compression, which is done using the [[IEBCOPY]] utility,<ref>{{cite webbook

| title = z/OS DFSMSdfp Utilities Version 2 Release 3

|url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.1.0/com.ibm.zos.v2r1.ieac600/iea3c6_Data_Sets_with_Partitioned_Organization.htm

| id = SC23-6864-30

|title=Data Sets with Partitioned Organization

| date = July 17, 2017

|quote=.. compressing using IEBCOPY}}</ref> moves all members to the front of the data space and leaves free usable space at the back. (Note that in modern parlance, this kind of operation might be called [[defragmentation]] or [[garbage collection (computer science)|garbage collection]]; [[data compression]] nowadays refers to a different, more complicated concept.) PDS files can only reside on [[direct access storage device|DASD]], not on [[magnetic tape]], in order to use the directory structure to access individual members. Partitioned datasets are most often used for storing multiple [[job control language]] files, [[IBM mainframe utility programs|utility]] control statements, and executable modules.

| publisher = IBM Corporation

| section = Compressing a Partitioned Data Set

| quote = A partitioned data set will contain unused areas (sometimes called gas) where a deleted member or the old version of an updated member once resided. This unused space is only reclaimed when a partitioned data set is copied to a new data set, or after a compress-in-place operation successfully completes. It has no meaning for a PDSE and is ignored if requested.

| url = https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3sc236864/$file/idau100_v2r3.pdf

| mode = cs2

}}</ref>

|quote=.. compressing using IEBCOPY}}</ref> moves all members to the front of the data space and leaves free usable space at the back. (Note that in modern parlance, this kind of operation might be called [[defragmentation]] or [[garbage collection (computer science)|garbage collection]]; [[data compression]] nowadays refers to a different, more complicated concept.) PDS files can only reside on [[direct access storage device|DASD]], not on [[magnetic tape]], in order to use the directory structure to access individual members. Partitioned datasets are most often used for storing multiple [[job control language]] files, [[IBM mainframe utility programs|utility]] control statements, and executable modules.

An improvement of this scheme is a Partitioned Data Set Extended (PDSE or PDS/E, sometimes just ''libraries'') introduced with DFSMSdfp for [[MVS/XA]] and [[MVS/ESA]] systems. A PDS/E library can store program objects or other types of members, but not both. BPAM cannot process a PDS/E containing program objects.