A sysparm that can have a dramatic effect on CA Disk performance and the amount of memory that is required is IOTRACKS. IOTRACKS controls the maximum number of tracks read or written at one time by the CA Disk EXCP access method, which is used for all but exception case handling of non-VSAM data sets. See the sysparm description for IOTRACKSxx in the chapter "Sysparms."
Ideally, optimum performance is achieved when IOTRACKS allows a cylinder to be processed by a single I/O request; for example, 15 tracks for 3380 devices. The default value for IOTRACKS accomplishes this. Test different values of this sysparm to find the best to suit your environment. As with most performance issues, there can be several factors to consider, such as the memory constraints, I/O contention, and page and swap activity.
First consider the size of these buffers themselves. A 15-track buffer on a 3380 is (15)*(47,476) or 712,140 bytes. CA Disk double buffers, which means 1,424,280 bytes (or roughly 1.4 megabytes). For the Move/Copy function, which is reading and writing to disk at the same time, this means 2.8 megabytes merely for basic I/O data buffers. Memory is required for many other items as well, such as space for the loaded programs themselves and buffers for reading VTOCs, PDS directories, and control information from the CA Disk parameter library. Many of these memory tables reside above the 16 MB line. A five- to six-megabyte region is usually sufficient for all CA Disk jobs, including Move/Copy. If real memory is very limited, however, this region requirement can be too high, and must be lowered by reducing the value of sysparm IOTRACKS.
Buffers are acquired based upon the values specified, but a single I/O request never exceeds a cylinder in size or crosses a cylinder boundary. If the number of tracks in a data set (or technically the number of tracks in each extent) is less than a cylinder in size, part of the buffer always remains unused (wasted). It follows that if the average data set size in tracks is less than the value specified for the appropriate sysparm, you can set the sysparm to a lower value to reduce memory constraints without noticeably increasing the number of I/Os (which degrades performance).
If the amount of available real memory is limited, having this sysparm set to a high value can cause jobs to be swapped frequently and/or paging rates to be high, which degrades performance instead of improves it.
If the to and from volumes are on the same channel when using the Move/Copy function to relocate data sets, the job can perform better by setting IOTRACKS to a much smaller value, perhaps even 1. This is due to the inherent channel contention in this particular case, combined with the paging and swapping problem previously described. Again, testing within your environment demonstrates whether this case warrants special treatment.
Another sysparm to see when considering memory requirements is IOMAXRECnnnnnn. This sysparm sets the maximum allowable record size for data sets to be processed by CA Disk. It is used to acquire buffer space to contain the records during backup and data compression/decompression. The higher this value is set, the greater the memory requirements. Set this sysparm to the lowest value possible that still allows CA Disk to process. The default value is 65,000.
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