In z/OS® V1R9, the MULTACC parameter of the DCBE macro is expanded, to optimize performance for tape data sets with BSAM, and to support QSAM with optimized performance for both tape and DASD data sets. The calculations used to optimize performance for BSAM with DASD data sets are also enhanced.
For BSAM in 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 QSAM in 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.
IBM® recommends setting MULTACC to one half of the MULTSDN value.
If you code a MULTACC value that is too large for the system to use, the system ignores the excess amount. However, the absolute upper limit for MULTACC is 255.
For more information, see the description of the MULTACC parameter of the DCBE macro in z/OS DFSMS Macro Instructions for Data Sets.