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Mysteries of Buffer Optimization Solved!
By: Bill Hitefield
In some respects, IBM’s access methods (especially VSAM) are like software black boxes – you pass a request to them, they go off into the Enchanted Forest, and sometime later, data is either retrieved or written by your application. As application developers, we have some modicum of control over when, why, and how often we make requests to the access method. It is that second part of the process, the “sometime later,” that we view as a part of application performance that we seemingly have no means of influencing. For most applications, if the “sometime later” variable were lessened, the aggregate overhead experienced by an application would decrease.
The access method and buffers When the access method processes the data set on behalf of the application, it builds a buffering environment. Think of buffers as “temporary housing” for data records. The access method uses this temporary housing as a staging area for the data until it is needed for the program. As application owners, we know how our application accesses the data (sequentially, randomly, or some combination). If we combine that knowledge with knowledge of how the access method avails itself of the buffers in each of those access types, then we can define a unique buffering environment for each data set accessed by our application, and thus ensure optimal performance by the access method. Many people already do this. There are multiple sources that make that correlation (for example, IBM’s “VSAM Demystified,” Redbooks SG24-6105). Those sources allow them to determine the needed buffering environment and then make the needed alterations to their system (JCL, …) to define that specific environment. Improvements in application execution time can be quite dramatic. In some instances, over 90% of the EXCP SVCs issued against a data set are not just optimized, they are eliminated. This can result in tremendous savings. This sounds like something that everybody should be doing, doesn’t it? If so, then why don’t they? For an IT site, the task does not end with that one set of definitions. Applications change over time, data sets change over time, and the access pattern that the application uses changes over time. In their due diligence, the site then must periodically monitor those definitions to ensure that they are current and that they in fact provide the needed relief. The manpower and time required to initially perform this task and then monitor the results quickly becomes something that is not insignificant. This causes most sites to lose heart in this ongoing contest with the access method. A good buffer optimizer such as VELOCI-Raptor can virtually eliminate this concern.
The buffer optimizer’s place in the process This is where a buffer optimization product, such as VELOCI-Raptor (also known as VR), performs its magic. When an application opens a data set, VR will combine its knowledge of access method behavior, the access declarations made by the application, along with the current status of the data set (number of records, CI-size, and so forth), and will determine the best buffering environment for that particular access to the data set. Since all of this information is used each time the data set is opened, there is no need to have an ongoing task to monitor the performance definitions that were (without the buffer optimization product) entered manually. The buffering product does what a performance specialist could have done and does it automatically each time a data set is opened, for every application, day or night, over and over, even on weekends. The manpower constraint facing most sites is now no longer a concern.
A final consideration – communicating with the buffer optimizer
For a site to make effective use of any buffer optimization solution, it must be able to communicate its wishes to the chosen solution. What candidates are to be selected? Once selected, what is to be done with them? Communication with any buffering solution can be broken down into several levels of user involvement as follows:
- Low-level: Some solutions require guidance at a very low level, resulting in a high level of input from the user. Not only must the user identify the candidates for optimization, they must also define (and monitor) the type of optimization that is to be performed. At this level, the buffering solution does not make that determination. An example of this would be the use of JCL parameters to implement a rudimentary buffering solution. The user must not only select which candidates are to be processed, they must also implement that selection in each place that candidate is referenced. While this method may get the job done, it requires a great deal of effort and sophistication on the part of the user.
- Mid-level: Solutions at this level require the same sort of definitions (i.e., identification of the candidates and a selection of the optimization type) but require them to be issued only once. The buffering product will then apply the requested actions each time the candidate data set is accessed. Some buffering solutions may allow you to define buffering options that can be associated with various criteria. You can specify a specific option that is to be used for a certain data set, or you can specify a solution that is to globally apply to a selected storage class. While this is an improvement over the previous level, the user still must make a determination as to which buffering solution is to be implemented.
- High-level: At this highest level, the best of all features is uniquely embodied within VELOCI-Raptor. VR exclusively provides:
- The user’s only involvement is simply the identification of candidates, and,
- The product is able, in and of itself, to determine the optimal buffering solution based upon real-time criteria 24/7/365.
- At this level, users only need to concern themselves with the “what” of their activities, not the “how.” This removes a tremendous workload from the user.
In literature lore, most mysteries end with “the butler did it.” In the mystery of buffer optimization, the answer is “the buffering product did it.” A good buffering product does what a performance specialist can do, but does it automatically – and does it all the time. VELOCI-Raptor was designed with the premise that an application is not static for long. Something will change, and that something will impact performance. VR is uniquely prepared for that, will detect it, and will make the needed adjustments to the optimal buffering scheme. VELOCI-Raptor has a mantra – “The fastest I/O is the one you do not have to issue.” Its goal is to provide a buffering scheme such that a majority of application I/O requests can be satisfied via “temporary housing” – the buffers. When considering a buffering product, think about the following:
- The product should have a realistic and intelligent approach to buffer optimization. It should avail itself of all possible solutions and not limit itself to any particular one.
- The product should automatically respond to dynamics in your operating environment. You should not have to monitor its selections.
- All the product should require of you is the identification of candidates. It should be sophisticated enough to determine the optimal solution on its own.
I hope that by now the mystery of the “buffer optimizer” has been removed and we now have a clear picture of the need for something that will help us optimize the buffering environment in which VSAM works with our applications. Our understanding of the role of a buffer optimizer now helps us determine whether or not one is right for us. We also have some new insight as to how to differentiate the good optimizers from the ordinary. Is a buffer optimizer right for you? I would offer that there is a high probability that you should consider one. If so, I would suggest that you give VELOCI-Raptor a good look. You will be happy you did. No mystery involved.
Appendix: Going deeper – Buffer optimization and IBM
The buffer optimizer and IBM’s buffering schemes IBM allows for the use of different buffering technologies. The default is NSR – Non-Shared Resources. While this buffering scheme excels in sequential access to a data set, it seems to merely tolerate random access. IBM also offers LSR – Local Shared Resources. This buffering scheme is pretty much the opposite of NSR in how it responds to applications. It excels in random access to a data set, and it tolerates sequential access. For a performance specialist to implement LSR, the approach is not quite as straightforward as the process described earlier. The implementation of LSR requires more internal knowledge of the application. LSR has certain expectations and requirements that must be met by the requestor of its services. For all of its beauty in easily processing random requests, LSR is fairly stringent in its implementation. In their implementation of LSR, this means that the performance specialist must be diligent not only in selecting the proper applications but also in making the needed changes to implement the use of LSR for an application. Buffering products like VELOCI-Raptor are designed to do just that. When the data set is opened, they will determine whether the optimal scheme is NSR or LSR. If LSR is selected, VELOCI-Raptor will provide the necessary interface to ensure that the application abides by the restrictions set by LSR itself. VELOCI-Raptor then extends this support by determining whether or not the application abides by its own declared intentions. For example, the application may declare “ACCESS IS RANDOM” when the data set is opened, yet it may process the data set sequentially. If so, the buffering strategy defined either by the performance specialist or by the buffering product is no longer optimal. VELOCI-Raptor monitors that application’s access and can determine when a different buffering scheme may have been more beneficial – and then implement that buffering scheme. The buffer optimizer vs IBM access method replacement Consider the components of the operating system which involve themselves with the processing of an I/O request for your applications:
- The access method – VSAM itself. VSAM has the responsibility of performing the actual I/O operands required to process your application’s data; either acquiring data from a disk drive, or writing data to a disk drive. The access method uses the buffering environment presented to it.
- The buffer optimizer. The buffer optimizer does not get involved with the actual I/O requests themselves. Instead, the buffer optimizer attempts to provide the best possible buffering environment for the access method (VSAM).
The access method and the buffer optimizer partner together in processing an application’s data. The access method provides the needed services, and the buffer optimizer ensures the optimal use of those services. What happens when the lines get blurred? Can the access method replace the buffer optimizer? Or, can the buffer optimizer replace the access method? Some programmatic solutions to I/O performance have attempted to do one or both by either implementing the use of an alternate IBM access method or by completely replacing an IBM access method with a proprietary access method and data format. While these solutions (the use of alternate access methods, whether IBM or proprietary) may initially seem to be viable solutions, closer investigation does reveal some areas of concern. Implementing alternate IBM access methods: Historically, proposed solutions involve the use of other methodologies and access methods when performing specific tasks (such as loading a VSAM data set).
- Design: The design behind this methodology involves the replacement of normal VSAM processing with a faster interface being used to perform the processing.
- Concerns: Since VSAM processing is actually bypassed, the solution must internally provide all processing normally performed by VSAM (for example, key compression, index level processing, …). Solutions based upon this design are susceptible to internal changes in IBM processing. Failure to account for those changes could lead to data corruption and/or loss.
Replacing IBM access methods with proprietary access methods: In this scenario, the IBM VSAM access method is completely replaced with a proprietary access method and data format.
- The design behind this methodology stresses the superiority of the proprietary access method over IBM’s VSAM access method.
- Most buffer optimization products work within the framework provided by VSAM and the operating system. As such, they do not modify the actual format and organization of the user data.
- Access method replacement products require a user to convert their data sets from IBM’s VSAM format to a proprietary, non-VSAM format. All data sets using the replacement access method must first be converted to this non-VSAM format.
- All converted data sets are now in a proprietary data format and cannot be accessed by the native VSAM access method.
- While the VSAM access method is provided as an integral part of the operating system, access method replacement products can be expensive.
- To access the data sets on a system where the access method replacement product is not active, the data set must first be converted back to IBM VSAM format.
- Native IBM VSAM can no longer be used to access the data.
- Trialing and implementation of such products is more complex and labor-intensive than the processes associated with a buffer optimization product.
- Since each data set must be converted from VSAM into a format acceptable to the proprietary access method, each prospective data set must be converted prior to its use with the access method replacement product.
- For test data, this may not be a major issue. But, with production data, this leads to the necessity of parallel copies of the prospective data sets – to keep production data separate from the test data (which is converted into the proprietary data format for the trial process).
- The access method replacement product must be installed and active in order to access the data. This could be a concern during disaster recovery.
- Support for your application’s data access is no longer through IBM (since VSAM is not being used).
- Since VSAM is not being used, support for new IBM features in VSAM may not be possible in an access method replacement product, or may not be implemented on a timely basis by the access method replacement product vendor, thus limiting their availability to a user.
- Return on investment:
- Because of the resources invested in the access method replacement, in both the real cost of the product and the man-power invested in the trialing of, conversion to, and maintenance of the product, a long-term commitment may be necessary in order to see a return on your investment.
Bill Hitefield has been involved with IBM mainframes since the late 1970s. He has concentrated on VSAM buffer optimization for the past 30 years and is currently the Product Manager for VELOCI-Raptor with Dino-Software. VELOCI-Raptor (VR) is a dynamic performance optimization tool designed to dramatically improve system performance while significantly reducing system overhead by automating tuning activities. VR reduces overhead by optimally allocating buffers for VSAM and non-VSAM files to provide the best throughput and response time for I/O requests. For more information on VELOCI-Raptor, visit https://velociraptordinosoftware.com. About Dino-Software Dino-Software Corporation develops enterprise-wide solutions for the management, analysis, protection, and repair of complex z/OS mainframe environments. Dino-Software has long been acknowledged for its superiority in ICF catalog management and technical support, helping organizations ensure their business-critical assets remain online and recoverable in a disaster. Learn more about Dino-Software and its z/OS mainframe storage solutions at https://www.dino-software.com.