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14.2. Main versus secondary Storage¶

Computer storage gadgets are generally classified intoprimary warehouse or key memory top top the one hand, andsecondary warehouse or peripheral storage on the other.Primary memory usually refers to Random access Memory (RAM),while second storage refers to tools such ashard decaying drives, hard state drives, removable “USB” drives,CDs, and DVDs.Primary memory additionally includes registers, cache, and video clip memories,but us will overlook them because that this discussion since their existencedoes not affect the principal differences in between primary andsecondary memory.

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Along with a faster CPU, every brand-new model of computer seems come comewith more main memory.As storage size continues to increase, is it feasible thatrelatively slow disk storage will be unnecessary?Probably not, because the desire to save and procedure larger filesgrows at least as quick as key memory size.Prices because that both main memory and peripheral storage tools havedropped dramatically in recent years, together demonstrated byTable 14.2.1.However, the cost per unit that disk drive storage is about twoorders that magnitude less than RAM and has to be formany years.


Table 14.2.1

Price to compare table for part writable digital data storagemedia in usual use.Prices room in united state Dollars/MB.


<eginspliteginarraylhline extbfMedium& 1996 & 1997 & 2000 & 2004 & 2006 & 2008 & 2011\hline extbfRAM& $45.00 & 7.00 & 1.500 & 0.3500 & 0.1500 & 0.0339 & 0.0138\ extbfDisk& 0.25 & 0.10 & 0.010 & 0.0010 & 0.0005 & 0.0001 & 0.0001\ extbfUSB drive& -- & -- & -- & 0.1000 & 0.0900 & 0.0029 & 0.0018\ extbfFloppy& 0.50 & 0.36 & 0.250 & 0.2500 & -- & -- & --\ extbfTape& 0.03 & 0.01 & 0.001 & 0.0003 & -- & -- & --\ extbfSolid State& -- & -- & -- & -- & -- & -- & 0.0021\hlineendarrayendsplit>

There is now a wide range of removable media available fortransferring data or save on computer data offline in relative safety.These include floppy disks (now largely obsolete), writable CDs andDVDs, “flash” drives, and also magnetic tape.Optical warehouse such together CDs and DVDs prices roughly half the price ofhard disc drive an are per megabyte, and have become practical for useas backup storage in ~ the past few years.Tape offered to be lot cheaper than various other media, and also was the preferredmeans of backup, yet are no so popular now as various other media havedecreased in price.Flash drives expense the many per megabyte, but due to their storagecapacity and also flexibility, quickly replaced floppy disks together theprimary storage device for transporting data between computer whendirect network deliver is not available.

Secondary storage gadgets haveat least two other benefits over lamb memory.Perhaps many importantly, disk, flash, and optical media arepersistent,meaning the they room not erased native the media once the strength isturned off.In contrast, RAM offered for main memory is typically volatile—all info is shed with the power.A 2nd advantage is that CDs and also USB drivescan quickly be transferred in between computers.This gives a convenient method to take info from one computerto another.

In exchange for decreased storage costs, persistence, andportability, second storage devices pay a punish in terms ofincreased access time.While no all accesses come disk take the same amount that time(more ~ above this later), the common time required to accessibility a byte ofstorage native a disk journey in 2011 is approximately 9 ms(i.e., 9 thousandths of a second).This could not it seems to be ~ slow, but compared to the time requiredto accessibility a byte from key memory, this is fantastically slow.Typical access time native standard an individual computer ram in2011 is around 5-10 nanoseconds(i.e., 5-10 billionths that a second).Thus, the moment to accessibility a byte that data indigenous a disk drive is aboutsix orders of magnitude better than that required toaccess a byte from main memory.While decaying drive and also RAM accessibility times space both decreasing, theyhave done so at roughly the very same rate.The loved one speeds have actually remained the very same for over several decades,in the the difference in accessibility time between RAM and also adisk drive has actually remained in the variety between a factor of 100,000 and1,000,000.

To gain some intuition for the definition of this rate difference,consider the moment that it might take because that you to look increase the entry fordisk drives in the index of this book, and also then revolve to theappropriate page.Call this your “primary memory” access time.If the takes you around 20 sealtoalsimce.org to do this access, thenan access taking 500,000 times longer would requiremonths.

It is interesting to keep in mind that while handling speeds have actually increaseddramatically, and hardware prices have dropped dramatically, diskand memory access times have actually improved by much less than an stimulate of magnitudeover the past 15 years.However, the situation is really much far better than the modest speedupwould suggest.During the same time period, the dimension of both disk andmain memory has actually increased by over 3 orders the magnitude.Thus, the access times have actually actually decreased in the challenge of amassive boost in the thickness of this storage devices.

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Due come the relatively slow access time because that data on disc as compared tomain memory, good care is required to produce efficient applicationsthat process disk-based information.The million-to-one ratio of disk access time versus main memory accesstime provides the following dominance of big importance as soon as designingdisk-based applications:

Minimize the variety of disk accesses!

There are typically two approaches to minimizing disk accesses.The very first is come arrange information so that if girlfriend do access data fromsecondary memory, friend will acquire what you need in together fewaccesses as possible, and preferably ~ above the very first access.File structure is the term used for adata framework that organizes data save on computer in secondary memory.File structures must be arranged so regarding minimize the requirednumber of decaying accesses.The other way to minimize disk accesses is to save informationpreviously re-cover (or retrieve extr data v each accessibility atlittle additional cost) that have the right to be offered tominimize the need for future accesses.This requires the ability to guess: v accuratelywhat information will be required later and also store it in main memorynow.This is described as caching.


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