2Types of External Memory Magnetic DiskRAID (Redundant Array of Independent Disks)RemovableOpticalCD-ROMCD-Recordable (CD-R)CD-R/WDVDMagnetic TapeMagnetic disks – most important component of external memory range from PC to mainframe and super computerRAID disk technology used to achieve great performance and higher availabilityOptical storage technology become increasingly important for all types of computer systems. CD and DVD are becoming increasingly important
3Disk substrate coated with magnetizable material (iron oxide…rust) Magnetic DiskDisk substrate coated with magnetizable material (iron oxide…rust)Substrate used to be aluminium or aluminium alloy materialNow glassImproved surface uniformityIncreases reliabilityReduction in surface defectsReduced read/write errorsLower flight heightsBetter stiffnessReduce disk dynamicsBetter shock/damage resistanceA disk is a circular platter constructed of nonmagnetic material called substrateTraditionally, substrate has been an aluminium or aluminium alloy materialMore recently, glass substrate have been reduced5 advantages of using glass
4Read and Write Mechanisms Recording & retrieval via conductive coil called a headMay be single read/write head or separate onesDuring read/write, head is stationary, platter rotatesWrite:Current 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 writeUsed in floppy disk systemsRead (contemporary):Separate read head, close to write headConsists of partially shielded magnetoresistive (MR) sensorElectrical resistance depends on direction of magnetic fieldHigh frequency operationHigher storage density and speedWrite: electric pulses sent to write head, and the resulting magnetic pattern recorded on surface below with different pattern for + and – currentRead : exploit the fact that magnetic field moving relative to a coil produces electrical current in the coil. When surface passes under the head, it generates current of the same polarity as the one already recordedRead contemporary: by passing current thru MR sensor, resistance changes are detected as voltage signals. MR Design allows high frequency operation, which equates to greater storage densities and operating speeds.
5Inductive Write MR Read Inductive Write/MR Read HeadCurrent induces magnetic field across the gap, in turn magnetizes small area of recording mediumReversing current direction reverses the direction of magnetization on the recording medium
6Data Organization and Formatting Concentric 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 blockHead is relatively small device capable of reading from and writing to a portion of a platter rotating beneath it.
7Disk Data LayoutTo avoid imposing unreasonable precision requirement on the systemError due to misalignment or simply interference of magnetic fieldHundreds of sectors per track. May be fixed or variable length. Most contemporary systems, fixed-length are usedDisk data layoutAdjacent tracks are separated by gaps. This to prevent/minimizes errorData are transferred to and from the disk in sectors
8Disk VelocityBit near centre of rotating disk passes fixed point slower than bit on outside of diskIncrease spacing between bits in different tracksRotate 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 capacity (density)Each zone has fixed bits per trackMore complex circuitry (allows for greater overall storage capacity)Density = in bits per linear inchSome way must found to compensate for the variation in speed so that the head can read all the bits at the same rateInformation can then be scanned at the same rate by rotating the disk at the fixed speed, known as CAVConstant = fixed speedZones = multi zone
9Disk Layout Methods Diagram The layout of a disk using CAV (b) The nature of multiple zone recording
10Must be able to identify start of track and sector Format disk Finding SectorsMust be able to identify start of track and sectorFormat diskAdditional information not available to userMarks tracks and sectors
11Winchester Disk Format Seagate ST506 Each track contains 30 fixed length sectors of 600 bytes eachEach sector holds 512 bytes of data plus control infos useful to the disk controllerID field used to locate a particular sector & SYNCH byte delimits the beginning of the fieldTrack# identifies a track on a surface & Head# identifies a head because this disk has multiple surfacesID and data field, each contain an error detecting codeID field, unique identifier/addressSYNCH byte, a special bit pattern
12Physical Characteristics Head motionFixed (rare) or movable headDisk portabilityRemovable or fixedSidedSingle or double (usually) sidedPlatterSingle or multiple platterHead mechanismContact (Floppy)Fixed gapFlying gap (Winchester)
13Fixed/Movable Head Disk Fixed headOne read-write head per trackHeads mounted on fixed ridged arm, extends across all the tracksMovable headOne read write head per sideMounted on a movable armArm can be extended/retracted for positioning above the trackFixed head – very rare today
14Sided/Double SidedMost disks: magnetizable coating applied to both sideOnly cheap disk use single-sided
15Removable or Not Removable disk Nonremovable disk Can be removed from drive and replaced with another diskProvides unlimited storage capacityEasy data transfer between systemsEg: floppy disk, ZIP cartridgeNonremovable diskPermanently mounted in the driveEg: hard disk in a PC
16Heads are joined and aligned Multiple PlatterOne head per sideHeads are joined and alignedAligned tracks on each platter form cylindersData is striped by cylinderreduces head movementIncreases speed (transfer rate)
17Multiple Platters One head per side Heads are joined and aligned Aligned tracks on each platter form cylindersData is striped by cylinderreduces head movementIncreases speed (transfer rate)
18Tracks and CylindersCylinder: set of all the tracks in the same relative position on the platterAll the shaded tracks are part of one CylinderHead mechanism- physical contact with medium during read/write operation
19Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow Universal Cheap Up to 1.44Mbyte (2.88M never popular)SlowUniversalCheapObsolete?
20Winchester 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 robust
21Winchester Hard Disk (2) UniversalCheapFastest external storageGetting larger all the time250 Gigabyte now easily available
22Access time = Seek + Latency Transfer rate SpeedSeek timeMoving head to correct track(Rotational) latencyWaiting for data to rotate under headAccess time = Seek + LatencyTransfer rateTime to/from the disk depends on the rotation speed of the diskT = b/rN (b= no. Of bytes to be transferredr = rotation speedN = no of bytes on a track)
23Timing of Disk I/O Transfer Timing of a Disk I/O transferdisk I/O operation depends on the computer system, OS, nature of I/O channel and disk controller hardware
24Assignment#2 – RAID What is RAID? RAID has a total of seven levels. Briefly explain for each of them.Due date: 31Oct2011