COMP 206: Computer Architecture and ImplementationDisk Systems: CharacteristicsExample: Parameters of A Single Disk DriveTechnical Details (1)Technical Details (2)Why Disks Are Bit-SerialTrack DensitiesSector FormatDisk MiscellaneaEnsembles of DisksImproving Bandwidth with Disk ArraysImproving Availability with Disk ArraysRAID-0RAID-1Combining RAID-0 and RAID-1RAID-3RAID-2RAID-4RAID-5Removable MediaInterfacing I/O Subsystem to CPUBus: Different OptionsExamples of Bus StandardsDoes I/O Performance Matter?Does CPU Performance Matter?Does Performance Matter?1COMP 206:COMP 206:Computer Architecture and Computer Architecture and ImplementationImplementationMontek SinghMontek SinghMon, Nov 21, 2005Mon, Nov 21, 2005 Mon, Nov 28, 2005 Mon, Nov 28, 2005Topic: Topic: Storage Systems (Disk Technology)Storage Systems (Disk Technology)2Disk Systems: CharacteristicsDisk Systems: CharacteristicsCapacity [bytes]Capacity [bytes]Mainframes typically have Mainframes typically have 3.7 GB of disk storage per 3.7 GB of disk storage per MIPSMIPSPCs rend to require PCs rend to require 5 MB of disk storage per MIPS5 MB of disk storage per MIPSBandwidth (throughput)Bandwidth (throughput)Bytes transferred per unit timeBytes transferred per unit timeService rateService rateNumber of service requests satisfied per unit timeNumber of service requests satisfied per unit timeSupercomputer I/O requests tend to involve large Supercomputer I/O requests tend to involve large amounts of data; transaction processing systems tend to amounts of data; transaction processing systems tend to involve small onesinvolve small onesResponse time (latency)Response time (latency)Time between start and completion of an eventTime between start and completion of an eventCost [$/MB]Cost [$/MB]3Example: Parameters of A Single Disk Example: Parameters of A Single Disk DriveDriveDiameter ("form factor") 1.8, 2.5, 3, 3.5, 5.25 inchesSpeed of rotation 3600-7200 rpm60-120 rpsTime of one revolution 8.33-16.66 msSector size 32-4096 BTracks per inch 2000-4000Bits per inch 90000-150000 b/inchAreal density 200-600 Mb/sq inchData rate 5-17.5 MBpsMTBF 250000-1000000 hoursCapacity 170-9000 MBCost 0.1-1.5 $/MB4Technical Details (1)Technical Details (1)PlattersPlatters2-4mm thick, made of an aluminum alloy2-4mm thick, made of an aluminum alloyTypically 1-6 platters in a hard diskTypically 1-6 platters in a hard diskBoth sides of each platter generally usedBoth sides of each platter generally usedOne side of one platter dedicated to information to One side of one platter dedicated to information to guide the servomechanisms that control speed of guide the servomechanisms that control speed of rotation and head movementrotation and head movementEach recording surface has its own read-write headEach recording surface has its own read-write headOnly one operational at any timeOnly one operational at any timeData transfer to and from disk is bit-serialData transfer to and from disk is bit-serialRotationRotationSpeed of rotation carefully controlled by Speed of rotation carefully controlled by servomechanismservomechanismE.g., 7200 rpm E.g., 7200 rpm 1% or even 1% or even 1 rpm1 rpmAll disk drives are synchronized in some disk arraysAll disk drives are synchronized in some disk arraysAngular positions are identical (within tolerance) at any timeAngular positions are identical (within tolerance) at any time5Technical Details (2)Technical Details (2)Flying heightFlying heightRead/write heads do not touch rotating disk surfaceRead/write heads do not touch rotating disk surfaceCareful aerodynamic design keeps small constant Careful aerodynamic design keeps small constant distance (distance (0.2 0.2 ))Disk moves with approximate linear speed of 700 Disk moves with approximate linear speed of 700 inches/secinches/sec18m/s, 40 miles/hour18m/s, 40 miles/hourSmaller flying height Smaller flying height  higher writing density higher writing densityHence, platters and heads sealed hermetically in clean spaceHence, platters and heads sealed hermetically in clean spaceCalled Winchester drives for historical reasonsCalled Winchester drives for historical reasons““Parking the heads”Parking the heads”Before disk is allowed to slow down and stop, heads are Before disk is allowed to slow down and stop, heads are moved over an area of the disk not used for recording, moved over an area of the disk not used for recording, where they are allowed to come into contact with the where they are allowed to come into contact with the disk surfacedisk surface6Why Disks Are Bit-SerialWhy Disks Are Bit-SerialHigh TPI value makes parallel access disks High TPI value makes parallel access disks unworkableunworkableReading heads move as rigid unitReading heads move as rigid unitOne of them (the one over the servo disk) supposedly One of them (the one over the servo disk) supposedly defines radial (track) positiondefines radial (track) positionEnsemble is not truly rigid, and various reasons (like Ensemble is not truly rigid, and various reasons (like thermal dilations) prevent all heads being positioned over thermal dilations) prevent all heads being positioned over same track simultaneously, repeatedly, and reliablysame track simultaneously, repeatedly, and reliablyOnly one head is positioned accurately at a time: servo Only one head is positioned accurately at a time: servo guides the assembly to approximate position of requested guides the assembly to approximate position of requested track, reading head does final positioning using a high track, reading head does final positioning using a high frequency signal between data tracksfrequency signal between data tracksSector ID always contains track number for confirmationSector ID always contains track number for confirmationPortable disks use shock sensors to prevent Portable disks use shock sensors to prevent overwriting of adjacent tracks caused by jarring of overwriting of adjacent tracks caused by jarring of R/W headR/W head7Track DensitiesTrack DensitiesUntil recently, only innermost track was recorded Until recently, only innermost track was recorded with maximum densitywith maximum densityAll other tracks contained same number of sectors and bytesAll other tracks contained same number of sectors and bytesRecently, manufacturers are using bands