4.2.4. Hard Drives

All the technologies discussed so far are volatile in nature. In other words, data contained in volatile storage is lost when the power is turned off.
Hard drives, on the other hand, are non-volatile -- the data they contain remains there, even after the power is removed. Because of this, hard drives occupy a special place in the storage spectrum. Their non-volatile nature makes them ideal for storing programs and data for longer-term use. Another unique aspect to hard drives is that, unlike RAM and cache memory, it is not possible to execute programs directly when they are stored on hard drives; instead, they must first be read into RAM.
Also different from cache and RAM is the speed of data storage and retrieval; hard drives are at least an order of magnitude slower than the all-electronic technologies used for cache and RAM. The difference in speed is due mainly to their electromechanical nature. There are four distinct phases taking place during each data transfer to or from a hard drive. The following list illustrates these phases, along with the time it would take a typical high-performance drive, on average, to complete each:
  • Access arm movement (5.5 milliseconds)
  • Disk rotation (.1 milliseconds)
  • Heads reading/writing data (.00014 milliseconds)
  • Data transfer to/from the drive's electronics (.003 Milliseconds)
Of these, only the last phase is not dependent on any mechanical operation.


Although there is much more to learn about hard drives, disk storage technologies are discussed in more depth in Chapter 5, Managing Storage. For the time being, it is only necessary to keep in mind the huge speed difference between RAM and disk-based technologies and that their storage capacity usually exceeds that of RAM by a factor of at least 10, and often by 100 or more.