CSE 4/521Introduction to Operating

SystemsLecture19– MassStorageStructureI

(OverviewofMass-StorageStructure,DiskStructure,DiskScheduling)Summer2018

Overview

• OverviewofMass-StorageStructure• DiskStructure• DiskScheduling

Objective:• Todescribethephysicalstructureofsecondarystoragedevicesanditseffectson

theusesofthedevices• Toexplaintheperformancecharacteristics ofmass-storagedevices• Toevaluatediskschedulingalgorithms

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Recap

• VirtualMem:AllocationofFrames• FixedAllocation(Equal&ProportionalAllocation)• PriorityAllocation• Globalvs.LocalAllocation

• VirtualMem:Thrashing• CausesofThrashing• AddressingThrashingthroughWorking-SetModel• AddressingThrashingthroughPage-FaultFrequency

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Questions1. Whatisthecauseofthrashing?Howdoesthesystem

detectthrashing?Oncedetected,whatcanthesystemdotoeliminatethrashing?(Easy)

2. Isitpossibleforaprocesstohavetwoworkingsets,onerepresentingdata andanotherrepresentingcode?(Medium)

3. Considerasystemthatallocatespagesofdifferentsizes toitsprocess.Whataretheadvantagesofsuchapagingscheme?(Open-ended)

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Overview

• OverviewofMass-StorageStructure• DiskStructure• DiskScheduling

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Overview of Mass Storage Structure• Magneticdisksprovidebulkofsecondarystorageofmoderncomputers• Drivesrotateat60to250timespersecond• Transferrateisrateatwhichdataflowbetweendriveandcomputer

• Positioningtime(random-accesstime)istimetomovediskarmtodesiredcylinder (seektime)andtimefordesiredsector torotateunderthediskhead(rotationallatency)

• Headcrashresultsfromdiskheadmakingcontactwiththedisksurface-- Thatʼsbad

• DriveattachedtocomputerviaI/Obus• Bussesvary,includingEIDE, ATA,SATA,USB,Fibre Channel,SCSI,SAS,Firewire

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Moving-head Disk Mechanism

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Overview of Mass Storage Structure:Hard Disk

• Rangefrom30GBto3TBperdrive• Performance• TransferRate– theoretical– 6Gb/sec• EffectiveTransferRate– real–1Gb/sec• Seektimefrom3msto12ms–9mscommonfordesktopdrives• Averageseektimemeasuredorcalculatedbasedon1/3oftracks• Latency basedonspindlespeed• 1/(RPM/60)=60/RPM

• Averagelatency=½latency

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(From Wikipedia)

• AccessLatency=Averageaccesstime=averageseektime+averagelatency• Forfastestdisk3ms+2ms=5ms• Forslowdisk9ms+5.56ms=14.56ms

• AverageI/Otime=averageaccesstime+(amounttotransfer/transferrate)+controlleroverhead

• Forexampletotransfera4KBblockona7200RPMdiskwitha5msaverageseektime,1Gb/sectransferratewitha.1mscontrolleroverhead=• 5ms+4.17ms+0.1ms+transfertime=• Transfertime=4KB/1Gb/s= 32Kb/(Gb/s)=0.031ms• AverageI/Otimefor4KBblock=9.27ms+.031ms=9.301ms

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Overview of Mass Storage Structure:Hard Disk Performance

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Overview of Mass Storage Structure:The First Commercial Disk Drive

1956IBM RAMDAC computer included the IBM Model 350 disk storage system

5M (7 bit) characters50 x 24” plattersAccess time = < 1 second

2018

• Nonvolatilememoryusedlikeaharddrive• Manytechnologyvariations

• CanbemorereliablethanHDDs• MoreexpensiveperMB• Maybehaveshorterlifespan• Lesscapacity• Butmuchfaster• Bussescanbetooslow->connectdirectlytoPCIforexample• Nomovingparts,sonoseektimeorrotationallatency

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Overview of Mass Storage Structure:Solid-State Disks

• Wasearlysecondary-storagemedium• Evolvedfromopenspoolstocartridges

• Relativelypermanentandholdslargequantitiesofdata• Accesstimeslow

• Randomaccess~1000timesslowerthandisk• Mainlyusedforbackup,storageofinfrequently-useddata,transfermediumbetweensystems

• Keptinspoolandwoundorrewoundpastread-writehead• Oncedataunderhead,transferratescomparabletodisk

• 140MB/secandgreater• 200GBto1.5TBtypicalstorage

• CommontechnologiesareLTO-{3,4,5}andT10000

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Overview of Mass Storage Structure:Magnetic Tape

Overview

• OverviewofMass-StorageStructure• DiskStructure• DiskScheduling

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• Diskdrivesareaddressedaslarge1-dimensionalarraysoflogicalblocks,wherethelogicalblockisthesmallestunitoftransfer• Low-levelformattingcreateslogicalblocksonphysicalmedia

• The1-dimensionalarrayoflogicalblocksismappedintothesectorsofthedisksequentially• Sector0isthefirstsectorofthefirsttrackontheoutermostcylinder

• Mappingproceedsinorderthroughthattrack,thentherestofthetracksinthatcylinder,andthenthroughtherestofthecylindersfromoutermosttoinnermost

• Logicaltophysicaladdressshouldbeeasy• Exceptforbadsectors• Non-constant#ofsectorspertrackviaconstantangularvelocity

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Disk Structure

Overview

• OverviewofMass-StorageStructure• DiskStructure• DiskScheduling

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Disk Scheduling

• Theoperatingsystemisresponsibleforusinghardwareefficiently— forthediskdrives,thismeanshavingafastaccesstimeanddiskbandwidth

• Minimizeseektime• Seektime» seekdistance

• Diskbandwidthisthetotalnumberofbytestransferred,dividedbythetotaltimebetweenthefirstrequestforserviceandthecompletionofthelasttransfer

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Disk Scheduling

• TherearemanysourcesofdiskI/Orequest• OS• Systemprocesses• Usersprocesses

• OSmaintainsqueueofrequests,perdiskordevice• IdlediskcanimmediatelyworkonI/Orequest,busydiskmeansworkmustqueue• Optimizationalgorithmsonlymakesensewhenaqueueexists

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Disk Scheduling• NotethatdrivecontrollershavesmallbuffersandcanmanageaqueueofI/Orequests(ofvarying“depth”)• SeveralalgorithmsexisttoscheduletheservicingofdiskI/Orequests• Theanalysisistrueforoneormanyplatters• Weillustrateschedulingalgorithmswitharequestqueue(0-199)

98,183,37,122,14,124,65,67Headpointer53

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Disk Scheduling:FCFS

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Illustration shows total head movement of 640 cylinders

Disk Scheduling:SSTF

• ShortestSeekTimeFirstselectstherequestwiththeminimumseektimefromthecurrentheadposition

• SSTFschedulingisaformofSJFscheduling;maycausestarvationofsomerequests

• Illustrationshowstotalheadmovementof236cylinders

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Credits for slides

Silberschatz,GalvinandGagne

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