PDSE version

Beginning with z/OS Version 2, z/OS supports two formats of PDSEs: version 1 and version 2. Version 2 PDSEs, introduced in z/OS Version 2, take advantage of a number of internal design changes to improve space utilization in the data set, reduce CPU processing and I/O, and provide better index searches. Start of change They also support generations for members, as described in PDSE Member Generations. End of change All PDSEs created before z/OS Version 2 are by definition version 1 PDSEs, and Version 1 remains the default for newly allocated PDSEs. To create a version 2 PDSE, specify DSNTYPE=(LIBRARY,2) on the DD statement or the equivalent on the TSO ALLOCATE command or dynamic allocation. Alternatively, the system programmer can specify the PDSE_VERSION keyword in a IGDSMSxx member of SYS1.PARMLIB. Outwardly, version 1 and 2 PDSEs appear the same; and they present no external changes to the user.

Version 2 PDSEs can only be created on z/OS Version 2 systems; they can be read and written to on previous releases of z/OS, but cannot be allocated there.

The goal of version 2 PDSEs is to provide improved PDSE performance overall. Start of change

There is one situation where an application might perform better with a version 1 PDSE (this situation is not considered likely): End of change

The PDSE has a RECFM of V, VB, or U
The PDSE members are large
The application points to a record at the end of the member which it has not previously read.

You can do the following to specify the version for new PDSE data set allocations:

Code a version number after the LIBRARY parameter on the DSNTYPE keyword in a DD statement or TSO ALLOCATE command. For example, the following specifies a PDSE version level of 2:
```
DSNTYPE=(LIBRARY,2)
```
Coding a DSNTYPE of (LIBRARY,1) specifies that the data set will be a version 1 PDSE. You can also specify a version number of 0 or omit the number to take the default version number.
Use the PDSE_VERSION keyword in IGDSMSxx to specify a default version number for data sets that are allocated with a DSNTYPE value of LIBRARY. For example, the following sets a default of version 2 for new PDSE allocations:
```
PDSE_VERSION(2)
```
The following rules apply to the PDSE_VERSION keyword:
- PDSE_VERSION(2) sets the default to LIBRARY,2
- PDSE_VERSION(1) sets the default to LIBRARY,1
- The default value when not specified is 1
- The only valid values are 1 and 2.

The version specified using DSNTYPE takes precedence over the PDSE_VERSION specified in IGDSMSxx, if both are specified and have different values.

Note: PDS remains the default value for DSNTYPE even if PDSE_VERSION is specified in the IGDSMSxx member.