Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Also see for ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE: Installation guide Installation guide Tuning guide Installation guide Manual

Contents

48 Chapter 4. Physical and Virtual Memory2. The address at which the data is to be stored is presented to the address connections.3. The read/write connection is set to write mode.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 stepmeasured 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 themodule adheres to various industry standards, making it possible to purchase memory from a varietyof vendors.NoteThe main benefit to a system using industry-standard RAM modules is that it tends to keep the costof 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 notableare 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, assumingthe system is relatively popular and is not a completely new design.4.2.4. Hard DrivesAll the technologies discussed so far are volatile in nature. In other words, data contained in volatilestorage 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 thepower is removed. Because of this, hard drives occupy a special place in the storage spectrum. Theirnon-volatile nature makes them ideal for storing programs and data for longer-term use. Anotherunique aspect to hard drives is that, unlike RAM and cache memory, it is not possible to executeprograms 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 atleast an order of magnitude slower than the all-electronic technologies used for cache and RAM. Thedifference in speed is due mainly to their electromechanical nature. There are four distinct phasestaking place during each data transfer to or from a hard drive. The following list illustrates thesephases, along with the time it would take a typical high-performance drive, on average, to completeeach:• 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.