Once the application and memory areas have been tuned, the next performance bottleneck can be the disk subsystem. A number of performance indicators can suggest a need to improve Oracle disk I/O. A major aim of configuring an Oracle server is to ensure that disk I/O does not become a bottleneck. While there may be some differences in the performance of disk devices from various vendors (especially if the devices are in some sort of RAID configuration), the major restraint on disk I/O is the number of disks acquired and the spread of I/O between these devices.
If possible, estimate the physical I/O that is generated by your database, and use this figure to determine the number of devices that would support the configuration. There are quite a range of areas that can be looked at in this respect.