IBM System/370 145 Manual page 226

theorettcally possible for the read or write of a particular
record to be delayed indefinitely because the block multiplexer
channel is busy searching for and reading other records. That
is, utilization of more and more block. multiplexer channel time
will normally result in better overall performance but will
increase the likelihood of delayed response from anyone data set.
DATA MANAGEMENT PARAMETERS
216
Guidelines for the use of data management parameters are:
1. When organizing direct data sets to be processed using BDAM, use
fixed-length standard records and a record reference that
includes ID (relative block, relative track and ID, or actual
address).
RPS is supported only for fixed-length standard and VBS formats
without key reference because record position must be known in
order to calculate the sector number required for positioning.
However, if a key reference or a variable record format is used,
RPS support is provided for write verification and update (after
retrieval) operations.
2. Use a large block size for sequentially processed data sets
whenever possible, subject to the availability of processor
storage for buffers.
The use of RPS can provide performance gains for both short and
long disk record blocks. However, use of large rather than short
blocks reduces the total time required to read or write a given
data set because less disk space is required and fewer I/O
operations are necessary_ Note also that total throughput for a
given block multiplexer channel is improved by using blocks of
equal (or nearly equal) size for all data sets being processed on
the channel.
3. Use fixed standard records for QSAM and BSAM data sets where
possible.
The channel programs used for fixed standard records free the
channel more often than when other record formats are used. A
search for the previously read record is not used in order to
locate the next sequential record when fixed standard records are
read sequentially. The sector number of the next sequential
record is obtained by including a READ SECT9R command at the end
of the channel program used to read each record. Therefore, the
SEARCH command specifies the ID of the desired record and the
channel is free during the time it would otherwise have been busy
searching for the previously read record.
Channel time is reduced when fixed standard records are written
because the operation required to calculate the remaining number
of bytes on a track after each write is eliminated. (Note that
the disk control unit is still busy erasing to end of track after
formatting write operations even though the channel is freed
after the data record has been written.)
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
A Guide to the IBM System/370 Model 145