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Let the System Select QSAM BUFNO or BSAM or BPAM NCP z/OS DFSMS Using Data Sets SC23-6855-00 |
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In QSAM, the value of BUFNO determines how many buffers will be chained together before I/O is initiated. The default value of BUFNO is described in Constructing a Buffer Pool Automatically. When enough buffers are available for reading ahead or writing behind, QSAM attempts to read or write those buffers in successive revolutions of the disk. If you do not set a non-zero value for MULTSDN on the DCBE macro or BUFNO on the DCB macro before completion of the DCB OPEN exit, then OPEN provides a default value for BUFNO as described in Constructing a Buffer Pool Automatically. If you code a MULTSDN value when the following conditions
occur, the system calculates a more efficient BUFNO value for QSAM
or NCP value for BSAM or BPAM:
If the preceding criteria are met,
For better performance with BSAM and BPAM, use the technique described in Using Overlapped I/O with BSAM and Figure 3. For sequential data sets and PDSs, specifying a nonzero MULTACC value on a DCBE macro can result in more efficient channel programs. You can also code a nonzero MULTSDN value. If MULTSDN is nonzero and DCBNCP is zero, OPEN determines a value for NCP and stores that value in DCBNCP before giving control to the DCB open exit. If MULTACC is nonzero and your program uses the WAIT or EVENTS macro on a DECB or depends on a POST exit for a DECB, then you must precede that macro or dependence by a CHECK or TRUNC macro. Note:
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Copyright IBM Corporation 1990, 2014
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