ABCs of z/OS System Programming Volume 3 - IBM Redbooks

BSAM support
For BSAM in z/OS V1R9, if you code a nonzero MULTACC value, OPEN calculates a default
number of READ or WRITE requests that you are suggesting the system queue more
efficiently. OPEN calculates the number of BLKSIZE-length blocks that can fit within 64 KB,
then multiplies that value by the MULTACC value. If the block size exceeds 32 KB, then OPEN
uses the MULTACC value without modification (this can happen only if you are using LBI, the
large block interface). The system then tries to defer starting I/O requests until you have
issued this number of READ or WRITE requests for the DCB. BSAM will never queue (defer)
more READ or WRITE requests than the NCP value set in OPEN.
For BSAM, it will work as documented for DASD:
► If you code a nonzero MULTACC value, OPEN will calculate a default number of read or
write requests that you are suggesting the system queue more efficiently.
► The system will try to defer starting I/O requests until you have issued this many read or
write requests for the DCB.
Note: BSAM will never queue or defer more read or write requests than the number of
channel programs (NCP) value set in OPEN.
QSAM support
For QSAM in z/OS V1R9, if you code a nonzero MULTACC value, OPEN calculates a default
number of buffers that you are suggesting the system queue more efficiently. OPEN
calculates the number of BLKSIZE-length blocks that can fit within 64 KB, then multiplies that
value by the MULTACC value. If the block size exceeds 32 KB, then OPEN uses the
MULTACC value without modification (this can happen only if you are using LBI, the large
block interface). The system then tries to defer starting I/O requests until that number of
buffers has been accumulated for the DCB. QSAM will never queue (defer) more buffers than
the BUFNO value that is in effect.
Note: If you code a nonzero MULTACC value, OPEN will calculate a default number of
buffers that you are suggesting the system queue more efficiently.
QSAM support for MULTSDN parameter
You can use the MULTSDN parameter of the DCBE macro with QSAM. In previous releases,
QSAM ignored the MULTSDN parameter. This new support for MULTSDN allows the system
to calculate a more efficient default value for DCB's BUFNO parameter, and reduces the
situations where you need to specify a BUFNO value. The user can use MULTSDN to give a
hint to OPEN so it can calculate a better default value for QSAM BUFNO instead of 1, 2, or 5.
The user will not have to be dependent on device information such as blocks per track or
number of stripes. 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 these supported data set types, 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.
Chapter 4. Storage management software 157