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Processing Techniques z/OS DFSMS Using Data Sets SC23-6855-00 |
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The information in this topic is for planning purposes only. It is not absolute or exact regarding storage requirements. You should use it only as a guideline for estimating storage requirements. Individual observations might vary depending on specific implementations and processing. System-managed buffering (SMB), a feature of DFSMSdfp, supports batch application processing. SMB takes the following actions:
You can choose or specify any of the four processing techniques
that SMB implements:
Direct Optimized (DO). The DO processing technique optimizes for totally random record access. This is appropriate for applications that access records in a data set in totally random order. This technique overrides the user specification for nonshared resources (NSR) buffering with a local shared resources (LSR) implementation of buffering. The following three options, SMBVSP, SMBDFR, and SMBHWT,
are only for processing with the Direct Optimized technique.
Note:
Sequential Optimized (SO). The SO technique optimizes processing for record access that is in sequential order. This is appropriate for backup and for applications that read the entire data set or a large percentage of the records in sequential order. Direct Weighted (DW). The majority is direct processing; some is sequential. DW processing provides the minimum read-ahead buffers for sequential retrieval and the maximum index buffers for direct requests. Sequential Weighted (SW). The majority is sequential processing; some is direct. This technique uses read-ahead buffers for sequential requests and provides additional index buffers for direct requests. The read-ahead will not be as large as the amount of data transferred with SO. To implement SMB, an application program must specify nonshared resources (NSR) buffering, ACB MACRF=(NSR). The system does not apply SMB when any VSAM data set is opened with a request for any other buffering option, MACRF=(LSR|GSR|UBF|RLS). The basis for the default technique is the application specification for ACB MACRF=(DIR,SEQ,SKP) Also, specification of the following values in the associated storage class (SC) influence the default technique:
You can specify the technique externally by using the ACCBIAS subparameter of the AMP= parameter. The system invokes the function only during data set OPEN processing. After SMB makes the initial decisions during that process, it has no further involvement. Table 1 is a guideline showing what access bias SMB chooses
for certain parameter specifications.
Note: This table can only be used as guideline to show
what Access Bias SMB will choose when ACCBIAS=SYSTEM is specified
in JCL AMP parameter, or when RECORD_ACCESS_BIAS=SYSTEM is specified
in Dataclass. There are exceptions in determining the actual Access
Bias. Other factors that can influence the decision are amount of
storage available, whether it is AIX® or Base
component, and if DSN or DDN sharing is in effect.
In the case where ACCBIAS=DO is specifically asked for on JCL AMP parameter, SMB may default to DW if there is not enough storage. To avoid this situation, there are two techniques:
If you request SMB and specify JCL AMP MSG = SMBBIAS, VSAM Open issues message IEC161I 001 to indicate which Access Bias is chosen by SMB. The following output is an example of message IEC161I 001
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