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Database Essentials
Administrative Planning : Disk capacity : Understanding cache usage

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, the cache becomes saturated after a short period of time. When your cache is saturated, performance reverts to conventional disk speeds. When estimating performance, consider as performing at conventional speeds.
Consider that an index rebuild can quickly saturate the disk cache because it performs many modifications in a short time frame. The potential performance benefit of a disk cache should not alter the layout of these disks—they should be treated as identical to any other disk.