ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.45 VSAM: System-managed buffering (SMB)
Only for SMS-managed extended format data sets
and NSR buffering mode
RECORD_ACCESS_BIAS in DATACLASS
ACCBIAS subparameter of AMP, in JCL DD
statement
For ACCBIAS equal to SYSTEM, VSAM decisions
based on MACRF parameter of ACB and MSR in
storage class
Optimum number of index and data buffers
For random access, VSAM changes buffering
management algorithm from NSR to LSR
Figure 4-55 System-managed buffering (SMB)
System-managed buffering (SMB)
SMB is a feature of VSAM that was introduced in DFSMS V1R4. SMB enables VSAM to:
► Determine the optimum number of index and data buffers
► Change the buffer algorithm as declared in the application program in the ACB MACRF
parameter, from NSR (sequential) to LSR (least recently used - LRU)
Usually, SMB allocates many more buffers than are allocated without SMB. Performance
improvements can be dramatic with random access (particularly when few buffers were
available). The use of SMB is transparent from the point of view of the application; no
application changes are needed.
SMB is available to a data set when all the following conditions are met:
► It is an SMS-managed data set.
► It is in extended format (DSNTYPE = EXT in the data class).
► The application opens the data set for NSR processing.
SMB is invoked or disabled through one of the following methods:
1. The Record Access Bias data class field.
2. The ACCBIAS subparameter of AMP in the JCL DD statement. JCL information takes
precedence over data class information.
Chapter 4. Storage management software 179