SAS(R) 9.1.3 Companion for z/OS

214 Optimize I/O for Sequential Libraries Chapter 10
Sequential Processing Pattern
Use the following recommendations to optimize performance when using a sequential
access pattern:
3 Choose a library block size that corresponds to half-track blocking, that is, two
blocks per track. For example, specify:
option blksize(3380)=half blksize(3390)=half;
3 Select a BUFNO value that is an even number between 6 and 10. Setting BUFNO
to a value from 10 to 30 might result in small additional gains in the number of
bytes transferred per unit of elapsed time; however, this gain might come at the
expense of monopolizing the channel or the device. Consult with your system
administrators to evaluate the likelihood of this problem occurring, as well as the
impact on the system.
3 Choose a larger BUFNO value than the default value. Start with 10, although it
might be helpful to increase the page size to 30. The performance benefit will vary
depending upon the cache scheme that is employed by the controller on which the
data library resides.
3 Consider using the In-Memory File (IMF) feature for a SAS file that will be
accessed across many SAS steps (DATA / procedure) if the file is small enough to
fit into the available region size. Load the file using the SASFILE statement prior
to the SAS steps that will process the file. The file will be read and, if necessary,
written only once. Without IMF, the file would be read once per step. See
“SASFILE Statement” on page 401 for more information on how to reserve enough
space to hold the entire data set in memory while it is being processed.
Random Processing Pattern
Use the following criteria to optimize performance when using a random access pattern:
3 Choose a library block size of 6K, if that block size is practical. However, for some
DASD controller configurations, half-track blocking performs nearly as well as a
6K block size for random access. Half-track blocking, which results in fewer
inter-block gaps, allows more data to be packed on a track.
3 If necessary, explicitly set the member page size, the BUFSIZE, equal to the
library block size.
3 Consider using the SASFILE statement to load a repetitively accessed file, such as
a master file, into memory. The elapsed time for such operations is dramatically
reduced by IMF because all of the member pages that need to be accessed must be
read into memory only once. However, take care to ensure that the region size for
the job is large enough to contain the file being loaded. It might also be necessary
to consult with your z/OS system administrator to ensure that the job is protected
against having the working-set size for its virtual storage trimmed.
Optimize I/O for Sequential Libraries
Sequential format bound libraries are those libraries that are processed with the TAPE
engine.
3 Use the default BUFSIZE when you access sequential format bound libraries. The
default BUFSIZE is always the most appropriate choice.