Phần cứng - Chapter 6: External memory

William Stallings Computer Organization and Architecture6th EditionChapter 6External MemoryTypes of External MemoryMagnetic DiskRAIDRemovableOpticalCD-ROMCD-Recordable (CD-R)CD-R/WDVDMagnetic TapeMagnetic DiskDisk substrate coated with magnetizable material (iron oxiderust)Substrate used to be aluminiumNow glassImproved surface uniformityIncreases reliabilityReduction in surface defectsReduced read/write errorsLower flight heights (See later)Better stiffnessBetter shock/damage resistanceRead and Write MechanismsRecording and retrieval via conductive coil called a headMay be single read/write head or separate onesDuring read/write, head is stationary, platter rotatesWriteCurrent through coil produces magnetic fieldPulses sent to headMagnetic pattern recorded on surface belowRead (traditional)Magnetic field moving relative to coil produces currentCoil is the same for read and writeRead (contemporary)Separate read head, close to write headPartially shielded magneto resistive (MR) sensorElectrical resistance depends on direction of magnetic fieldHigh frequency operationHigher storage density and speedInductive Write MR ReadData Organization and FormattingConcentric rings or tracksGaps between tracksReduce gap to increase capacitySame number of bits per track (variable packing density)Constant angular velocityTracks divided into sectorsMinimum block size is one sectorMay have more than one sector per blockDisk Data LayoutDisk VelocityBit near centre of rotating disk passes fixed point slower than bit on outside of diskIncrease spacing between bits in different tracks Rotate disk at constant angular velocity (CAV)Gives pie shaped sectors and concentric tracksIndividual tracks and sectors addressableMove head to given track and wait for given sectorWaste of space on outer tracksLower data densityCan use zones to increase capacityEach zone has fixed bits per trackMore complex circuitryDisk Layout Methods DiagramFinding SectorsMust be able to identify start of track and sectorFormat diskAdditional information not available to userMarks tracks and sectorsGap1IdGap2DataGap3Gap1IdGap2DataGap3TrackSyncByteHeadSectorCRCSyncByteDataCRCST506 format (old!)Foreground readingFind othersCharacteristicsFixed (rare) or movable headRemovable or fixedSingle or double (usually) sidedSingle or multiple platterHead mechanismContact (Floppy)Fixed gapFlying (Winchester)Fixed/Movable Head DiskFixed headOne read write head per trackHeads mounted on fixed ridged armMovable headOne read write head per sideMounted on a movable armRemovable or NotRemovable diskCan be removed from drive and replaced with another diskProvides unlimited storage capacityEasy data transfer between systemsNonremovable diskPermanently mounted in the driveMultiple PlatterOne head per sideHeads are joined and alignedAligned tracks on each platter form cylindersData is striped by cylinderreduces head movementIncreases speed (transfer rate)Multiple PlattersCylindersFloppy Disk8”, 5.25”, 3.5”Small capacityUp to 1.44Mbyte (2.88M never popular)SlowUniversalCheapObsolete?Winchester Hard Disk (1)Developed by IBM in Winchester (USA)Sealed unitOne or more platters (disks)Heads fly on boundary layer of air as disk spinsVery small head to disk gapGetting more robustWinchester Hard Disk (2)UniversalCheapFastest external storageGetting larger all the timeMultiple Gigabyte now usualRemovable Hard DiskZIPCheapVery commonOnly 100MJAZNot cheap1GL-120 (a: drive)Also reads 3.5” floppyBecoming more popular?All obsoleted by CD-R and CD-R/W?SpeedSeek timeMoving head to correct track(Rotational) latencyWaiting for data to rotate under headAccess time = Seek + LatencyTransfer rateTiming of Disk I/O TransferRAIDRedundant Array of Independent Disks Redundant Array of Inexpensive Disks6 levels in common useNot a hierarchySet of physical disks viewed as single logical drive by O/SData distributed across physical drivesCan use redundant capacity to store parity informationRAID 0No redundancyData striped across all disksRound Robin stripingIncrease speedMultiple data requests probably not on same diskDisks seek in parallelA set of data is likely to be striped across multiple disksRAID 1Mirrored DisksData is striped across disks2 copies of each stripe on separate disksRead from eitherWrite to bothRecovery is simpleSwap faulty disk & re-mirrorNo down timeExpensiveRAID 2Disks are synchronizedVery small stripesOften single byte/wordError correction calculated across corresponding bits on disksMultiple parity disks store Hamming code error correction in corresponding positionsLots of redundancyExpensiveNot usedRAID 3Similar to RAID 2Only one redundant disk, no matter how large the arraySimple parity bit for each set of corresponding bitsData on failed drive can be reconstructed from surviving data and parity infoVery high transfer ratesRAID 4Each disk operates independentlyGood for high I/O request rateLarge stripesBit by bit parity calculated across stripes on each diskParity stored on parity diskRAID 5Like RAID 4Parity striped across all disksRound robin allocation for parity stripeAvoids RAID 4 bottleneck at parity diskCommonly used in network serversN.B. DOES NOT MEAN 5 DISKS!!!!!RAID 6Two parity calculationsStored in separate blocks on different disksUser requirement of N disks needs N+2High data availabilityThree disks need to fail for data lossSignificant write penaltyRAID 0, 1, 2RAID 3 & 4RAID 5 & 6Data Mapping For RAID 0Optical Storage CD-ROMOriginally for audio650Mbytes giving over 70 minutes audioPolycarbonate coated with highly reflective coat, usually aluminiumData stored as pitsRead by reflecting laserConstant packing densityConstant linear velocityCD OperationCD-ROM Drive SpeedsAudio is single speedConstant linier velocity1.2 ms-1Track (spiral) is 5.27km longGives 4391 seconds = 73.2 minutesOther speeds are quoted as multiplese.g. 24xQuoted figure is maximum drive can achieveCD-ROM FormatMode 0=blank data fieldMode 1=2048 byte data+error correctionMode 2=2336 byte dataRandom Access on CD-ROMDifficultMove head to rough positionSet correct speedRead addressAdjust to required location(Yawn!)CD-ROM for & againstLarge capacity (?)Easy to mass produceRemovableRobustExpensive for small runsSlowRead onlyOther Optical StorageCD-Recordable (CD-R)WORMNow affordableCompatible with CD-ROM drivesCD-RWErasableGetting cheaperMostly CD-ROM drive compatiblePhase changeMaterial has two different reflectivities in different phase statesDVD - what’s in a name?Digital Video DiskUsed to indicate a player for moviesOnly plays video disksDigital Versatile DiskUsed to indicate a computer driveWill read computer disks and play video disksDogs Veritable DinnerOfficially - nothing!!!DVD - technologyMulti-layerVery high capacity (4.7G per layer)Full length movie on single diskUsing MPEG compressionFinally standardized (honest!)Movies carry regional codingPlayers only play correct region filmsCan be “fixed”DVD – WritableLoads of trouble with standardsFirst generation DVD drives may not read first generation DVD-W disksFirst generation DVD drives may not read CD-RW disksWait for it to settle down before buying!CD and DVDMagnetic TapeSerial accessSlowVery cheapBackup and archiveDigital Audio Tape (DAT)Uses rotating head (like video)High capacity on small tape4Gbyte uncompressed8Gbyte compressedBackup of PC/network servers