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14.2. Primary versus secondary Storage¶

Computer storage gadgets are commonly classified intoprimary warehouse or main memory on the one hand, andsecondary storage or peripheral warehouse on the other.Primary memory usually refers to Random accessibility Memory (RAM),while secondary storage refers to tools such ashard disk drives, heavy state drives, removable “USB” drives,CDs, and DVDs.Primary memory likewise includes registers, cache, and video memories,but we will overlook them for this discussion since their existencedoes not affect the major differences in between primary andsecondary memory.

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Along v a much faster CPU, every brand-new model of computer seems come comewith more main memory.As storage size proceeds to increase, is it possible thatrelatively slow-moving disk storage will be unnecessary?Probably not, due to the fact that the desire to store and procedure larger filesgrows at least as fast as main memory size.Prices for both key memory and peripheral storage devices havedropped substantially in recent years, as demonstrated byTable 14.2.1.However, the cost per unit of disk journey storage is around twoorders the magnitude less than RAM and also has to be formany years.

Table 14.2.1

Price comparison table for some writable digital data storagemedia in usual use.Prices space in united state Dollars/MB.

\<\beginsplit\beginarrayr\hline\textbfMedium& 1996 & 1997 & 2000 & 2004 & 2006 & 2008 & 2011\\\hline\textbfRAM& \$45.00 & 7.00 & 1.500 & 0.3500 & 0.1500 & 0.0339 & 0.0138\\\textbfDisk& 0.25 & 0.10 & 0.010 & 0.0010 & 0.0005 & 0.0001 & 0.0001\\\textbfUSB drive& -- & -- & -- & 0.1000 & 0.0900 & 0.0029 & 0.0018\\\textbfFloppy& 0.50 & 0.36 & 0.250 & 0.2500 & -- & -- & --\\\textbfTape& 0.03 & 0.01 & 0.001 & 0.0003 & -- & -- & --\\\textbfSolid State& -- & -- & -- & -- & -- & -- & 0.0021\\\hline\endarray\endsplit\>

There is now a wide variety of removable media accessible fortransferring data or save on computer data offline in loved one safety.These include floppy disks (now greatly obsolete), writable CDs andDVDs, “flash” drives, and magnetic tape.Optical warehouse such as CDs and DVDs costs roughly half the price ofhard disc drive space per megabyte, and also have come to be practical for useas back-up storage in ~ the past couple of years.Tape provided to be lot cheaper than other media, and was the preferredmeans that backup, but are no so well-known now as various other media havedecreased in price.Flash drives cost the many per megabyte, however due to your storagecapacity and flexibility, quickly replaced floppy disks as theprimary storage device for moving data in between computer whendirect network transfer is no available.

Secondary storage tools haveat least two other advantages over ram memory.Perhaps many importantly, disk, flash, and also optical media arepersistent,meaning the they are not erased native the media once the strength isturned off.In contrast, RAM provided for key memory is commonly volatile—all info is lost with the power.A 2nd advantage is that CDs and USB drivescan conveniently be transferred in between computers.This offers a convenient means to take details from one computerto another.

In exchange for diminished storage costs, persistence, andportability, an additional storage gadgets pay a punish in terms ofincreased access time.While not all accesses come disk take it the same amount of time(more ~ above this later), the common time required to accessibility a byte ofstorage indigenous a disk drive in 2011 is approximately 9 ms(i.e., 9 thousandths the a second).This can not it seems ~ slow, yet compared to the time requiredto access a byte from key memory, this is fantastically slow.Typical access time from standard an individual computer ram in2011 is about 5-10 nanoseconds(i.e., 5-10 billionths the a second).Thus, the time to access a byte the data from a disk drive is aboutsix orders of magnitude greater than that compelled toaccess a byte from main memory.While disc drive and also RAM accessibility times room both decreasing, theyhave excellent so at about the exact same rate.The relative speeds have remained the exact same for over several decades,in the the difference in accessibility time in between RAM and adisk drive has actually remained in the variety between a factor of 100,000 and1,000,000.

To gain some intuition for the significance of this speed difference,consider the time that it could take because that you come look up the entrance fordisk cd driver in the table of contents of this book, and also then rotate to theappropriate page.Call this her “primary memory” access time.If that takes you about 20 seconds to do this access, thenan accessibility taking 500,000 times longer would requiremonths.

It is exciting to note that while handling speeds have increaseddramatically, and hardware prices have dropped dramatically, diskand memory access times have improved by much less than an bespeak of magnitudeover the past 15 years.However, the case is yes, really much better than the modest speedupwould suggest.During the exact same time period, the size of both disk andmain memory has increased by over 3 orders of magnitude.Thus, the access times have actually decreased in the confront of amassive increase in the thickness of these storage devices.

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Due come the reasonably slow accessibility time because that data on disc as compared tomain memory, great care is compelled to develop efficient applicationsthat process disk-based information.The million-to-one ratio of disk access time versus main memory accesstime renders the following dominance of great importance when designingdisk-based applications:

Minimize the number of disk accesses!

There are normally two viewpoints to minimizing disk accesses.The very first is come arrange details so the if friend do access data fromsecondary memory, girlfriend will obtain what you require in together fewaccesses as possible, and preferably on the first access.File structure is the term provided for adata structure that organizes data stored in secondary memory.File structures should be arranged so as to minimize the requirednumber of disk accesses.The other method to minimize disk accesses is to conserve informationpreviously retrieved (or retrieve additional data through each access atlittle extr cost) that have the right to be provided tominimize the require for future accesses.This requires the ability to guess accuratelywhat information will be needed later and store it in main memorynow.This is referred to as caching.

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