IBM System/370 Guide

The read or write of a particular record may be delayed one
rotation because the channel is busy servicing another channel
program. The probability of a particular record being delayed
is a function of the percentage of channel busy time. As block
multiplexer channel utilization increases, the probability that
individual channel programs will be delayed increases. It is
theoretically 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
92
Following are guidelines for establishing data management parameters:
1. When organizing direct data sets to be processed using BDAM,
use fixed-length standard or VBS 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 blocks
rather than short 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 in order to locate the
next se~Jential record is not used when fixed standard records
are read sequentially. The sector number of the next sequential
record is obtained by including a READ SEeTOR 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 standard records are written, or
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.)

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English