60:15 Os Portability - IBM System/370 Manual

4. Use multiple buffers with QSAM and BSAM.
The availability of multiple buffers per data set lowers the
probability that a task will have to wait for a particular
record. QSAM is designed to initiate an I/O request whenever
a buffer becomes available, thus keeping the channel queue as
full as possible. When BSAM is used, the programmer must handle
the initiation of I/O requests.
The following summarizes the advantages of rotational position
sensing~
multiple requesting, and block multiplexing:
• System throughput increases can be achieved when multiple sequential
data sets are processed concurrently on a single block multiplexer
channel (using QSAM, QISAM, or BSAM) because a higher effective
channel data rate results.
• The number of block multiplexer channels required in a given system
configuration can be fewer than the number of selector channels
that would be required to handle the same amount of data, because
more effective channel utilization is achieved by block multiplexing
disk operations.
• The performance cost to an installation of verifying disk write
operations is sharply reduced.
• The greatest throughput improvement results from use of rotational
position sensing with high-activity, transaction-based processing,
that is, with applications that include one or more large jobs
that:
1. Use direct processing (BDAM) with fixed-length standard records
and a record reference that includes ID
2. Require a multivolume data base of small records
3. Process many additions and updates and use write verify
60:15 OS PORTABILITY
To avoid multiple system generations an OS user with multiple Model
165 systems may wish to generate a single operating system that can
be used on every Model 165 in the installation. This is possible under
the same system hardware and I/O device configuration restraints that
exist for System/360 models. That is, during the IPL procedure,
channels and I/O devices may have to be varied offline, partition sizes
may have to be redefined, etc., when the operating system is used with
a different configuration than was specified during system generation.
A user with both a System/370 Model 165 and a System/370 Model 155
or a System/360 model in an installation may also wish to generate
one operating system that can be used on both models. This approach
provides backup when one system is unavailable and can eliminate the
necessity of multiple generations.
Portability of an OS operating system between a System/370 Model
165 and a System/370 Model 155 or a System/360 model, say 50 or 65,
can be achieved by utilizing a multiple nucleus control program under
the following general conditions:
1. The system hardware and I/O device configuration of both systems
must be similar. For example, a Model 165 OS control program
generated to support block multiplexing mode and RPS direct
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