William Stallings Computer Organization and Architecture 8th EditionTypes of External MemoryMagnetic DiskRead and Write MechanismsInductive Write MR ReadData Organization and FormattingDisk Data LayoutDisk VelocityDisk Layout Methods DiagramFinding SectorsWinchester Disk Format Seagate ST506CharacteristicsFixed/Movable Head DiskRemovable or NotMultiple PlatterMultiple PlattersTracks and CylindersFloppy DiskWinchester Hard Disk (1)Winchester Hard Disk (2)SpeedTiming of Disk I/O TransferRAIDRAID 0RAID 1RAID 2RAID 3RAID 4RAID 5RAID 6RAID 0, 1, 2RAID 3 & 4RAID 5 & 6Data Mapping For RAID 0Optical Storage CD-ROMCD OperationCD-ROM Drive SpeedsCD-ROM FormatRandom Access on CD-ROMCD-ROM for & againstOther Optical StorageDVD - what’s in a name?DVD - technologyDVD – WritableCD and DVDHigh Definition Optical DisksOptical Memory CharacteristicsMagnetic TapeLinear Tape-Open (LTO) Tape DrivesInternet ResourcesWilliam Stallings Computer Organization and Architecture8th EditionChapter 6External MemoryTypes of External Memory•Magnetic Disk—RAID—Removable•Optical—CD-ROM—CD-Recordable (CD-R)—CD-R/W—DVD•Magnetic TapeMagnetic Disk•Disk substrate coated with magnetizable material (iron oxide…rust)•Substrate used to be aluminium•Now glass—Improved surface uniformity–Increases reliability—Reduction in surface defects–Reduced read/write errors—Lower flight heights (See later)—Better stiffness—Better shock/damage resistanceRead and Write Mechanisms•Recording & retrieval via conductive coil called a head•May be single read/write head or separate ones•During read/write, head is stationary, platter rotates•Write—Current through coil produces magnetic field—Pulses sent to head—Magnetic pattern recorded on surface below•Read (traditional)—Magnetic field moving relative to coil produces current—Coil is the same for read and write•Read (contemporary)—Separate read head, close to write head—Partially shielded magneto resistive (MR) sensor—Electrical resistance depends on direction of magnetic field—High frequency operation–Higher storage density and speedInductive Write MR ReadData Organization and Formatting•Concentric rings or tracks—Gaps between tracks—Reduce gap to increase capacity—Same number of bits per track (variable packing density)—Constant angular velocity•Tracks divided into sectors•Minimum block size is one sector•May have more than one sector per blockDisk Data LayoutDisk Velocity•Bit near centre of rotating disk passes fixed point slower than bit on outside of disk•Increase spacing between bits in different tracks •Rotate disk at constant angular velocity (CAV)—Gives pie shaped sectors and concentric tracks—Individual tracks and sectors addressable—Move head to given track and wait for given sector—Waste of space on outer tracks–Lower data density•Can use zones to increase capacity—Each zone has fixed bits per track—More complex circuitryDisk Layout Methods DiagramFinding Sectors•Must be able to identify start of track and sector•Format disk—Additional information not available to user—Marks tracks and sectorsWinchester Disk FormatSeagate ST506Characteristics•Fixed (rare) or movable head•Removable or fixed•Single or double (usually) sided•Single or multiple platter•Head mechanism—Contact (Floppy)—Fixed gap—Flying (Winchester)Fixed/Movable Head Disk•Fixed head—One read write head per track—Heads mounted on fixed ridged arm•Movable head—One read write head per side—Mounted on a movable armRemovable or Not•Removable disk—Can be removed from drive and replaced with another disk—Provides unlimited storage capacity—Easy data transfer between systems•Nonremovable disk—Permanently mounted in the driveMultiple Platter•One head per side•Heads are joined and aligned•Aligned tracks on each platter form cylinders•Data is striped by cylinder—reduces head movement—Increases speed (transfer rate)Multiple PlattersTracks and CylindersFloppy Disk•8”, 5.25”, 3.5”•Small capacity—Up to 1.44Mbyte (2.88M never popular)•Slow•Universal•Cheap•Obsolete?Winchester Hard Disk (1)•Developed by IBM in Winchester (USA)•Sealed unit•One or more platters (disks)•Heads fly on boundary layer of air as disk spins•Very small head to disk gap•Getting more robustWinchester Hard Disk (2)•Universal•Cheap•Fastest external storage•Getting larger all the time—250 Gigabyte now easily availableSpeed•Seek time—Moving head to correct track•(Rotational) latency—Waiting for data to rotate under head•Access time = Seek + Latency•Transfer rateTiming of Disk I/O TransferRAID•Redundant Array of Independent Disks •Redundant Array of Inexpensive Disks•6 levels in common use•Not a hierarchy•Set of physical disks viewed as single logical drive by O/S•Data distributed across physical drives•Can use redundant capacity to store parity informationRAID 0•No redundancy•Data striped across all disks•Round Robin striping•Increase speed—Multiple data requests probably not on same disk—Disks seek in parallel—A set of data is likely to be striped across multiple disksRAID 1•Mirrored Disks•Data is striped across disks•2 copies of each stripe on separate disks•Read from either•Write to both•Recovery is simple—Swap faulty disk & re-mirror—No down time•ExpensiveRAID 2•Disks are synchronized•Very small stripes—Often single byte/word•Error correction calculated across corresponding bits on disks•Multiple parity disks store Hamming code error correction in corresponding positions•Lots of redundancy—Expensive—Not usedRAID 3•Similar to RAID 2•Only one redundant disk, no matter how large the array•Simple parity bit for each set of corresponding bits•Data on failed drive can be reconstructed from surviving data and parity info•Very high transfer ratesRAID 4•Each disk operates independently•Good for high I/O request rate•Large stripes•Bit by bit parity calculated across stripes on each disk•Parity stored on parity diskRAID 5•Like RAID 4•Parity striped across all disks•Round robin allocation for parity stripe•Avoids RAID 4 bottleneck at parity disk•Commonly used in network servers•N.B. DOES NOT MEAN 5 DISKS!!!!!RAID 6•Two parity calculations•Stored in separate blocks on different disks•User requirement of N disks needs N+2•High data availability—Three