Determining the optimal Reality frame size

In general systems will perform best if the Reality frame size matches the host system's page size. In most systems this is 4K although, for example, Sun Solaris uses 8K.

It is not true to say reading a 2K frame on a system that uses a page size of 1K is two reads. The underlying hardware would normally do a single seek and then a single read of 2K. It is seeking that takes the time as the time to read is almost irrelevant.

The greatest increase in performance when changing frame size is when the majority of items move in group, they can then be accessed with a single seek and read. The OPTIMUM-FRAME-SIZE utility can be used to determine what percentage of items would move in group when changing the frame size.

If all items were in group in the first place then increasing the frame size may improve the sequential read performance as more items will be read in each frame, however the system is likely to do some form of read ahead under these conditions, which limits the performance gain.

Changing the frame size of the database to suit a particular application can be complicated as some files will need to reduce their modulo (files that were mainly in-group, or files that were mainly out-of-group and are still out-of-group) and some files need to increase their modulo (files that were mainly out-of-group but are mainly in-group using the new frame size). The OPTIMUM-MODULO utility can help with this.

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