How do you optimize VSAM buffer space allocation for sequential access?

1 What is buffer space allocation?

Buffer space allocation is the process of assigning memory space to hold VSAM records during processing. Buffer space allocation can affect the performance of your VSAM applications, as it determines how many records can be read or written at a time, and how often the buffers need to be refreshed. Buffer space allocation can be specified at the dataset level, or at the system level using system parameters.

2 How does sequential access work?

Sequential access is a way of accessing VSAM records in the order they are stored in the dataset. Sequential access is often used for batch processing, report generation, or backup and restore operations. Sequential access can be achieved by using the COBOL OPEN and CLOSE statements, or by using the Access Method Services (AMS) utility. Sequential access can also be specified by using the AMP parameter in the JCL DD statement.

3 What are the buffer space allocation parameters?

There are two main parameters that control buffer space allocation for sequential access: BUFNO and BUFND. BUFNO is the number of buffers allocated for each control interval (CI), which is a logical unit of storage in a VSAM dataset. BUFND is the number of buffers allocated for each control area (CA), which is a group of CIs. Both BUFNO and BUFND can be specified in the DD statement, or in the AMS DEFINE or ALTER commands.

4 How do you optimize buffer space allocation for sequential access?

The optimal buffer space allocation for sequential access depends on several factors, such as the size of the records, the size of the CIs, the size of the CAs, the number of concurrent users, and the available memory. However, some general guidelines can be applied to ensure an efficient process. For instance, using a large BUFNO value can reduce the number of I/O operations per CI; a good rule of thumb is to use BUFNO=CI/RECSIZE, where CI is the size of the control interval in bytes, and RECSIZE is the average size of the records in bytes. Similarly, a small BUFND value can reduce memory consumption per CA; a good rule of thumb is to use BUFND=1, unless you have a very large CA size or a very high record update rate. Additionally, BUFNI and BUFNDI parameters should be used to specify buffer space allocation for indexed sequential access; a good rule of thumb is to use BUFNI=2 and BUFNDI=1, unless you have a very large index or a very high key search rate. Finally, BUFSP parameter should be used for spanned records; a good rule of thumb is to use BUFSP=2, unless you have a very high spanned record rate.

5 How do you monitor and measure buffer space allocation performance?

You can use various tools and reports to monitor and measure the performance of your buffer space allocation for sequential access. The VSAM Statistics report, generated by using the AMS PRINT command with the STATISTICS option or the IDCAMS utility with the LISTCAT command, indicates the number of CIs and CAs processed, the number of buffer hits and misses, and the buffer hit ratio for each VSAM dataset. The VSAM Buffer Tuning report, generated by using the AMS TUNE command or the IDCAMS utility with the TUNE command, provides recommended buffer space allocation parameters for each VSAM dataset based on current workload and configuration. Additionally, the IBM z/OS Performance Toolkit's VSAM Performance Monitor allows you to collect and analyze real-time performance data for your VSAM datasets, such as the buffer hit ratio, I/O rate, and CPU utilization. This tool can be used to identify and resolve performance bottlenecks, as well as simulate and compare different buffer space allocation scenarios.