Release 10.1A: OpenEdge Getting Started:
Database Essentials

Understanding cache usage

Disk vendors provide for caching on their disk arrays to increase performance. However, there is a limit to how effective this cache can be. If your system is doing hundreds of thousands of reads per hour on your system, the cache becomes saturated after a short period of time. Under these conditions, the system quickly degrades to conventional disk speeds. It is important to consider all disks, regardless of manufacturer’s published guidelines, as performing at conventional speeds. Otherwise, you will experience a larger problem than you would expect when the cache is saturated.

For example, an index rebuild is a process that saturates the disk cache quickly. This process performs many modifications in a short time frame. The cache is nice to have and will provide a benefit, but do not let the benefit sway you from regarding the layout of these disks as identical to any other disk.