Chapter 9: Peripheral Devices: Magnetic DisksMagnetic Disk DrivesMulti-Platter Hard Disk DriveMultiple PlattersSimplified View of Disk Track and Sector OrganizationSimplified View of Individual Bits Encoded on a Disk TrackDisk Layout Methods DiagramTypical Hard Disk Sector OrganizationWinchester Disk Format: Seagate ST506Disk FormattingStatic Disk CharacteristicsDynamic Disk CharacteristicsRAIDData Mapping For RAID 0RAID - 1RAID 0, 1, 2RAID – 2RAID - 3RAID: 4RAID 3 & 4RAID: 5 & 6RAID 5 & 6Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallChapter 9: Peripheral Devices: Magnetic Disks Basic Disk Operation Performance Parameters and History of Improvement Example disks RAID (Redundant Arrays of Inexpensive Disks) Improving Reliability Improving PerformanceComp Sys Design & Arch 2nd Ed © 2004 Prentice HallMagnetic Disk Drives High density and non-volatile Densities similar to semiconductor RAM on an inexpensive medium No power required to retain stored information Motion of medium supplies power for sensing More random access than tape: direct access Different platters selected electronically Track on platter selected by head movement Cyclic sequential access to data on a track Structured address of data on disk Drive: Platter: Track (cylinder): Sector: ByteComp Sys Design & Arch 2nd Ed © 2004 Prentice HallMulti-Platter Hard Disk DriveComp Sys Design & Arch 2nd Ed © 2004 Prentice HallMultiple PlattersComp Sys Design & Arch 2nd Ed © 2004 Prentice HallSimplified View of Disk Track and Sector Organization An integral number of sectors are recorded around a track A sector is the unit of data transfer to or from the diskComp Sys Design & Arch 2nd Ed © 2004 Prentice HallSimplified View of Individual Bits Encoded on a Disk Track Inside tracks are shorter & thus have higher densities or fewer words All sectors contain the same number of bytes Inner portions of a platter may have fewer sectors per track; multiple zone recording. Small areas of the disk are magnetized in different directions• Change in magnetization direction is what is detected on readComp Sys Design & Arch 2nd Ed © 2004 Prentice HallDisk Layout Methods DiagramSame number of bits in each zone, but more zones as you go to outer cylinders.Stallings – Computer Organization & ArchitectureComp Sys Design & Arch 2nd Ed © 2004 Prentice HallTypical Hard Disk Sector Organization Serial bit stream has header, data, & error code Header synchronizes sector read and records sector address Data length is usually power of 2 bytes Error detection/correction code needed at endComp Sys Design & Arch 2nd Ed © 2004 Prentice HallWinchester Disk Format: Seagate ST506Stallings – Computer Organization & ArchitectureComp Sys Design & Arch 2nd Ed © 2004 Prentice HallDisk Formatting Disks are pre-formatted with track and sector address written in headers Disk surface defects may cause some sectors to be marked unusable for the softwareComp Sys Design & Arch 2nd Ed © 2004 Prentice HallStatic Disk Characteristics Areal density of bits on surfacedensity = 1/(bit spacing × track spacing) Maximum density: density on innermost track Unformatted capacity: includes header and error control bits Formatted capacity:sector track surface# of surfacescapacity =bytes sectors tracks×××Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallDynamic Disk Characteristics Seek time: time to move heads to cylinder Track-to-track access: time to adjacent track Rotational latency: time for correct sector to come under read/write head Average access time = seek time + rotational latency Burst rate (maximum transfer bandwidth) burst rate =revssecsectorsrevbytessector××Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID Raid LEVEL 0: Speed improvement only, NO reliability improvement Data is "striped" across several disks so they can be accessed in parallel independent of each other. “Strips” may be physical blocks, sectors, or other units. If there is a reasonable probability that multiple disk requeststarget different disks, they can be accessed in parallel (i.e., overlap seek and rotational delay) thus lowering average access time. Very good when accessing sequential/contiguous data.Redundant Array of Inexpensive/Independent DisksReliability and Redundancy for Improved PerformanceComp Sys Design & Arch 2nd Ed © 2004 Prentice HallData Mapping For RAID 0Stallings – Computer Organization & ArchitectureComp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID - 1 RAID Level 1: "Mirroring," writing the exact same data to two different disk drives. Reliability: If one drive fails, the other can be used. Performance: Can seek on both drives at the same time, accept the data from the first one to obtain the data and abort the other request. Negatives: Must write data to two disks, slowest one can determine performance. Costly; need twice as many disks for the same amount of data.Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID 0, 1, 2Stallings – Computer Organization & ArchitectureComp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID – 2RAID 2 & 3 General: All disks in the array are used for every I/O request. Disks are synchronized so each disk head is over the same disk position at every point in time.RAID Level 2: Data is striped at the bit or word level across several disks. Reliability: Additional ECC (Error Correcting Codes) bits to recover data if one drive fails (e.g., Hamming SECDEC) are calculated. The ECC bits are placed on additional drives. The number is proportional to the log of the number of data disks. Performance: Data bandwidth improves and correponds to the number of data disks.Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID - 3RAID Level 3: Striped as in level 2 at the byte level. Reliability: A simple parity bit is computed for each bit position. A single disk (improvement over RAID-2) holds the parity bits in the calculated position. If a parity error is detected: The disk holding the error can be replaced & data rewritten. The error can be corrected on the fly by performing the proper calculation. Performance: Overall bandwidth is increased through parallel access of block of data.Comp Sys Design & Arch 2nd Ed © 2004 Prentice HallRAID: 4RAID Level 4: Similar to level 3- Reliability:
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