DABACON Buffer

The Database Constructor (DABACON) buffer is an area of data storage space within each module, reserved for holding pages of data which have been read from databases or which are to be written to databases. Data transfers between the databases and the module always take place in exact multiples of one page.

The larger the DABACON buffer, the higher the chance that a previously used page can be found in the buffer and reused, rather than having to be physically fetched from disk again. Therefore, other things being equal, as the buffer size is increased so disk I/O should decrease, possibly at the expense of slightly increased CPU usage. However, as the buffer is increased the computer may be forced to do more paging itself, to and from the swap space. Normally, DABACON paging is more efficient than the computer paging as DABACON has extra knowledge on what has been accessed. Thus if the computer is having to do a lot of paging to swap, then normally it is best to reduce the DABACON buffer size. It must be emphasized that there are no firm rules which determine the best buffer size for a particular module/computer. Each machine will have an optimum combination of main memory size, disk I/O capacity, DABACON Buffer size etc., which will probably vary throughout the life of the project. The AVEVA base products System Managers will learn by experience the best buffer sizes for general usage on their computers.

The default buffer size is 5,120,000 words (20 Mbyte) for all modules and this will often prove to be a suitable compromise. On an installation where the computer has plenty of spare memory, no paging, but overloaded disk channels, it may be useful to increase the buffer size. A typical example in which the use of a larger buffer may be justified is when doing a clash check.