NonKey Indexes - used to find data that meets a certain criteria. You may have many indexes defined for a relation. Check for significant variety in values. Check your DBMS for the maximum number of indexes allowed. Be careful when null values may occur.
Indexes do add overhead to processing.
Here, an array of drives appears to programs as a single device. RAID 0 striping only. Fastest, but lowest security. Slightly slower than one drive, but more security. RAID 2 striping and dedicated disks for error correction.