Computer Systems - Andrew Blundell · Computer Systems Lecturer: Andrew ... Hard Drives use spinning magnetic disks which the data ... , files on a hard disk drive will most certainly

ComputerSystems

Level3Unit2ComputerSystemsLecturer:AndrewBlundell

Unit2Storage

SystemStorage


Datacanbeheldin3non-volatileways

1. MagneticStorage2. Optical3. Flash

SystemStorage


Method1:Magnetic ThismethodusesMagnetic disksandareeitherfixedorremovable.ExamplesoffixedareHardDriveandremovableareFloppyDisks

Method2:OpticalDrivesThismethodsusesalasertoreadawritedata.ExamplesincludeCD/DVD

Method3:FlashMemoryThismethodusesFlashmemory.ExamplesincludeUSBthumbdrivesandSSDs

Non-VolatileStorage


Non-VolatilestoragedoesnotlosedatastoredonitwhenthepowerisremovedItsusuallycheapandslow

HardDiskDrives(HDD)


HardDrivesareaincrediblyefficientcomputermemorydevicethatusessimplemagnetismtostorevastamountsofinformation

HardDrivesusespinningmagneticdiskswhichthedata(1and0)isstoredon

Thisdataisreadandwrittenbyamechanicalarmswhichasthediskspinsmovesacrossthedisk

HardDiskDrives(HDD)


1.Actuatorthatmovestheread-writearm.Inolderharddrives,theactuatorsweresteppermotors.Inmostmodernharddrives,voicecoilsareusedinstead.Astheirnamesuggests,thesearesimpleelectromagnets,workingratherlike themovingcoils thatmakesoundsinloudspeakers.Theypositiontheread-writearmmorequickly,precisely,andreliably thansteppermotorsandarelesssensitive toproblemssuchastemperaturevariations.2.Read-writearmswingsread-writeheadbackandforthacrossplatter.3.Centralspindleallowsplattertorotateathighspeed.4.Magneticplatterstoresinformationinbinaryform.5.Plugconnectionslinkharddrivetocircuitboardinpersonalcomputer.6.Read-writeheadisatinymagnetontheendoftheread-writearm.7.Circuitboardonundersidecontrolstheflowofdatatoandfromtheplatter.8.Flexible connectorcarriesdatafromcircuitboardtoread-writeheadandplatter.9.Smallspindleallowsread-writearmtoswingacrossplatter.

HardDiskDrives(HDD)


Thedataisstoredinaveryorderlypatternoneachplatter.Bitsofdataarearrangedinconcentric,circularpathscalledtracks.Eachtrackisbrokenupintosmallerareascalledsectors.Partoftheharddrivestoresamapofsectorsthathavealreadybeenusedupandothersthatarestillfree.(InWindows, thismapiscalledtheFileAllocationTableorFAT.)Whenthecomputerwantstostorenewinformation,ittakesalookatthemaptofindsomefreesectors.Thenitinstructstheread- writeheadtomoveacrosstheplattertoexactlytherightlocationandstorethedatathere.Toreadinformation, thesameprocessruns inreverse.

FileSystems


• Formattinginstallsafilesystemontothepartition.•FileSystemsareamethodoforganizingyourfilesandfoldersontotheharddrive.•TheFilesystemtelltheoperatingsystemwhereitemsareheldsoitcanfindthem•Thinkofafilingcabinet.Filearearrangedinaspecificorderwithindicatortoeachsection.

FileSystems


HardDriveIssues


Diskfragmentationisaresultofhowacomputerusesstorageonaharddrive.Astimegoeson, filesonaharddiskdrivewillmostcertainlystarttofragment.Whenafileis'fragmented',partsofitarestoredatvariouslocationsonaharddiskdrive.Theendresultisthatinordertoaccessafile,acomputerneedstoaccessdifferentpartsofaharddiskdrive.Becauseofthemechanicalnatureofaharddiskdrive,theneedtoaccessdifferentpartscandramaticallyslowdowntheoverallperformance

AFragmentedHardDiskDriveshowing filesbeingSpreadallovertheHardDiskFile

HardDriveIssues


Thepicturetotheleftillustrateswhatafragmenteddiskwouldlooklikewhenfilesthatbelong togetheraresavedtodifferentareasoftheharddrive.Thepicturetotherightillustrateswhatadiskwouldlook likewhenthefilesthatbelong togetheraredefragmentedusingadefragmentingsoftwareapplication.

Afragmentedfiledoesnotloseanydata,norisitunreliableorinsecure.Inotherwords, fragmentationdoesnotcauseanyseriousproblems.Ontheotherhand,whenacomputeraccessesafragmentedfile, ittakeslonger thanaccessingonethatisnotfragmented.

HardDriveIssues


Whatproblemscouldamechanicalharddrivehave?•BadSectors- Aareaoftheharddiskdrivethatcannotholddatareliablybecauseofdamageorerror.Causedbybumping orbanging thedisk•LostCluster- Alostclusterisaclusterthattheoperatingsystemhasclassedasbeinginuse,butactuallycontainsnodata.- TheScanDiskutilitywithinWindowsisdesigned tosearchforlostclustersandmakethemavailabletothefilesystemagain.

Partitioning


Partitioning istheprocessofelectronicallysubdividing thephysicalharddriveintogroupsofcylinderscalledpartitions.Aharddrivemusthaveatleastonepartition, butmayhavemany.

Eachpartition isassignedadriveletter.i.e.C:orD:

Primary Partitions =Bootable(MustbesetActive).Extended Partitions =NonBootable(ContainLogicalDrives).

SolidStateDrive(SSD)


AnSSDdoesnothaveaDisktoreadandwritedata,itinsteadreliesonanembeddedprocessorcalledacontrollertoperformoperationsrelatedtoreadingandwritingdata.TheyareverysimilartoaUSBflashdrive.

RAID


RedundantArrayofIndependentDisks.

ASystemthatemploystwoormoredrivesincombinationforfaulttolerance,performanceorboth.

RAIDprovidesamethodofaccessingmultipleindividualdisksasifthearraywereonelargerdisk.

RAIDeitherspreadsorduplicatesthedataacrossmultipledisks,therebyeitherimprovingaccesstimeorreducingtheriskoflosingalldataifonedrivefails.

TypicallyRAIDisusedinlargefileservers,andapplicationservers,wheredataaccessibilityiscritical,andfaulttoleranceisrequired.

TherearenumberofdifferentRAIDlevels

BuyingaHardDiskDrive


CapacityHowmuchdatatheHardDrivecanstore,usuallyinGigabytesorTerabytesCacheRandomAccessMemory (RAM) thatstorescopiesoffrequentlyuseddisksectorsinsotheycanbereadwithoutaccessingtheslowerdiskRead/WriteSpeedThereadspeedishowlong ittakestoreadsomething fromthedrive-- so,howlong ittakestoopenafilethat’sstoredonthedrive.Thewritespeedishowlong ittakestosavesomethingon tothedrive.InterfaceTheconnector theharddriveusestocommunicatewiththemotherboard usuallySATAorIDEFormFactorThesizeofthedisk.2.5"(smallfor laptops)or3.5“(DesktopSize)

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Computer Systems - Andrew Blundell · Computer Systems Lecturer: Andrew ... Hard Drives use spinning magnetic disks which the data ... , files on a hard disk drive will most certainly