The slowest thing in any computer system is the disk I/O. Disk I/O almost always takes more time than any other system processing activity. When we begin concentrating our design efforts on efficiency, focus first on minimizing disk I/O.

The most critical elements are the design of the keys, the number of keys, the design of the SIFT fields, the number of SIFT fields, the design of the filters, and the frequency of access of data (especially FlowFields). If our system will have five or ten users processing a few thousand order lines per day, and is not heavily modified, we probably won't have much trouble. However, if we are installing a system with one or more of the following attributes (any of which can have a significant effect on the amount of disk I/O), we will need to be very careful with our design and implementation:

• Large concurrent user count
• High transaction volumes, especially in data being Posted
• Large stored data volumes, especially resulting from customizations or setup option choices
• Significant modifications
• Very complex business rules