QSAM support for MULTSDN

QSAM uses the MULTSDN value in the DCBE macro to calculate a more efficient BUFNO value for tape data sets and specific types of DASD data sets, and reduces the situations where you need to specify a BUFNO value. For concatenated data sets, QSAM uses MULTSDN to dynamically recalculate the BUFNO value when switching from one concatenated data set to the next.

QSAM accepts a MULTSDN value for the following data sets:

tape data sets
DASD data sets of the following types:
- basic format
- large format
- extended format (non-compressed)
- PDS.

For the supported types of data sets, the system uses MULTSDN to calculate a more efficient value for BUFNO when the following conditions are true:

The MULTSDN value is not zero.
DCBBUFNO has a value of zero after completion of the DCB OPEN exit routine
The data set block size is available.

When MULTSDN is specified, note that the default number of buffers may be less than what would have been derived without MULTSDN, as shown in Table 1.

Table 1. Default buffer numbers for QSAM with and without MULTSDN
Data Set Type	DCBBUFNO default without MULTSDN	DCBBUFNO default with MULTSDN
PDSE Member	1	1
Extended format data set in the compressed format	1	1
UNIX file	1	1
Extended format data set (not in the compressed format)	2 * number of stripes * number of blocks per track	MULTSDN * number of stripes * number of blocks per track
Block size equal to or greater than 32 KB (tape)	2	MULTSDN value
Block size less than 32 KB (tape)	5	MULTSDN * number of blocks in 64 KB
IBM® 2540 card reader or card punch	3	3
PS, PDS	5	MULTSDN * number of blocks per track
Others, including dummy data sets	5	5
TSO terminal	5	1

For more information, see the description of the MULTSDN parameter of the DCBE macro in z/OS DFSMS Macro Instructions for Data Sets.