||SQL Server Tips by Burleson
Data, code, and object definitions that reside in memory have a much
higher return rate than objects that must first be retrieved from
disk and placed into memory. The latest numbers on this topic
suggest that data can be accessed from RAM around 1,400 times faster
than on physical disk.
While database engines differ in the layout of the memory structures
used to hold data and other necessary items, they all contain areas
that function to speed the delivery of data and to lookup
information. The percentage of times the database can serve
information from memory instead of disk will contribute mightily
toward overall database speed. Things such as data being available
in the SQL Server cache and stored procedure definitions being
nestled into memory all play a part.
Of course, hardware design counts, too. Fast drives and load
segmentation both play a part. Contention at this level; however,
normally falls under resource availability.
The above book excerpt is from:
High-Performance SQL Server DBA
Tuning & Optimization Secrets