Hard Drives - Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

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Retrieving data is just as straightforward:
1. The address of the desired data is presented to the address connections.
2. The read/write connection is set to read mode.
3. The desired data is read from the data connections.
While these steps seem simple, they take place at very high speeds, with the time spent on each step
measured in nanoseconds.
Nearly all RAM chips created today are sold as modules. Each module consists of a number of in-
dividual RAM chips attached to a small circuit board. The mechanical and electrical layout of the
module adheres to various industry standards, making it possible to purchase memory from a variety
of vendors.
Note
The main benefit to a system using industry-standard RAM modules is that it tends to keep the cost
of RAM low, due to the ability to purchase the modules from more than just the system manufacturer.
Although most computers use industry-standard RAM modules, there are exceptions. Most notable
are laptops (and even here some standardization is starting to take hold) and high-end servers.
However, even in these instances, it is likely that third-party RAM modules are available, assuming
the system is relatively popular and is not a completely new design.

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.
Chapter 4. Physical and Virtual Memory