Page 1CS252/PattersonLec 5.11/31/01CS252Graduate Computer ArchitectureLecture 5: I/O Introduction: Storage Devices & RAIDJanuary 31, 2001Prof. David A. PattersonComputer Science 252Spring 2001CS252/PattersonLec 5.21/31/01Motivation: Who Cares About I/O?• CPU Performance: 60% per year• I/O system performance limited by mechanicaldelays (disk I/O)< 10% per year (IO per sec)• Amdahl's Law: system speed-up limited by theslowest part!10% IO & 10x CPU => 5x Performance (lose 50%)10% IO & 100x CPU => 10x Performance (lose 90%)• I/O bottleneck:Diminishing fraction of time in CPUDiminishing value of faster CPUsCS252/PattersonLec 5.31/31/01Big Picture: Who cares about CPUs?• Why still important to keep CPUs busy vs. IOdevices ("CPU time"), as CPUs not costly?– Moore's Law leads to both large, fast CPUs but also to verysmall, cheap CPUs– 2001 Hypothesis: 600 MHz PC is fast enough for OfficeTools?– PC slowdown since fast enough unless games, new apps?• People care more about about storing informationand communicating information than calculating– "Information Technology" vs. "Computer Science"– 1960s and 1980s: Computing Revolution– 1990s and 2000s: Information Age• Next 3 weeks on storage and communicationCS252/PattersonLec 5.41/31/01I/O SystemsProcessorCacheMemory - I/O BusMainMemoryI/OControllerDisk DiskI/OControllerI/OControllerGraphicsNetworkinterruptsinterruptsCS252/PattersonLec 5.51/31/01Storage Technology Drivers• Driven by the prevailing computing paradigm– 1950s: migration from batch to on-line processing– 1990s: migration to ubiquitous computing» computers in phones, books, cars, video cameras, …» nationwide fiber optical network with wireless tails• Effects on storage industry:– Embedded storage» smaller, cheaper, more reliable, lower power– Data utilities» high capacity, hierarchically managed storageCS252/PattersonLec 5.61/31/01Outline• Disk Basics• Disk History• Disk options in 2000• Disk fallacies and performance• Tapes• RAIDPage 2CS252/PattersonLec 5.71/31/01Disk Device Terminology• Several platters, with information recorded magnetically on bothsurfaces (usually)• Actuator moves head (end of arm,1/surface) over track (“seek”),select surface, wait for sector rotate under head, then read orwrite– “Cylinder”: all tracks under heads• Bits recorded in tracks, which in turn divided into sectors (e.g.,512 Bytes)PlatterOuterTrackInnerTrackSectorActuatorHeadArmCS252/PattersonLec 5.81/31/01Photo of Disk Head, Arm,ActuatorActuatorArmHeadPlatters (12){SpindleCS252/PattersonLec 5.91/31/01Disk Device PerformancePlatterArmActuatorHeadSectorInnerTrackOuterTrack• Disk Latency = Seek Time + Rotation Time + TransferTime + Controller Overhead• Seek Time? depends no. tracks move arm, seek speed of disk• Rotation Time? depends on speed disk rotates, how far sector isfrom head• Transfer Time? depends on data rate (bandwidth) of disk (bitdensity), size of requestControllerSpindleCS252/PattersonLec 5.101/31/01Disk Device Performance• Average distance sector from head?• 1/2 time of a rotation– 10000 Revolutions Per Minute ⇒⇒ 166.67 Rev/sec– 1 revolution = 1/ 166.67 sec ⇒⇒ 6.00 milliseconds– 1/2 rotation (revolution) ⇒⇒ 3.00 ms• Average no. tracks move arm?– Sum all possible seek distancesfrom all possible tracks / # possible» Assumes average seek distance is random– Disk industry standard benchmarkCS252/PattersonLec 5.111/31/01Data Rate: Inner vs. Outer Tracks• To keep things simple, orginally kept same number ofsectors per track– Since outer track longer, lower bits per inch• Competition ⇒⇒ decided to keep BPI the same for alltracks (“constant bit density”)⇒⇒ More capacity per disk⇒⇒ More of sectors per track towards edge⇒⇒ Since disk spins at constant speed,outer tracks have faster data rate• Bandwidth outer track 1.7X inner track!– Inner track highest density, outer track lowest, so not reallyconstant– 2.1X length of track outer / inner, 1.7X bits outer / innerCS252/PattersonLec 5.121/31/01Devices: Magnetic DisksSectorTrackCylinderHeadPlatter• Purpose:– Long-term, nonvolatile storage– Large, inexpensive, slow level inthe storage hierarchy• Characteristics:– Seek Time (~8 ms avg)» positional latency» rotational latency• Transfer rate– 10-40 MByte/sec– Blocks• Capacity– Gigabytes– Quadruples every 2 years (aerodynamics)7200 RPM = 120 RPS => 8 ms per rev ave rot. latency = 4 ms128 sectors per track => 0.25 ms per sector1 KB per sector => 16 MB / sResponse time = Queue + Controller + Seek + Rot + XferService timePage 3CS252/PattersonLec 5.131/31/01Disk Performance Model /Trends• Capacity+ 100%/year (2X / 1.0 yrs)• Transfer rate (BW)+ 40%/year (2X / 2.0 yrs)• Rotation + Seek time– 8%/ year (1/2 in 10 yrs)• MB/$> 100%/year (2X / 1.0 yrs)Fewer chips + areal densityCS252/PattersonLec 5.141/31/01State of the Art: Barracuda 180– 181.6 GB, 3.5 inch disk– 12 platters, 24 surfaces– 24,247 cylinders– 7,200 RPM; (4.2 ms avg.latency)– 7.4/8.2 ms avg. seek(r/w)– 64 to 35 MB/s (internal)– 0.1 ms controller time– 10.3 watts (idle)source: www.seagate.comLatency = Queuing Time + Controller time +Seek Time + Rotation Time + Size / Bandwidthper accessper byte{+SectorTrackCylinderHeadPlatterArmTrack BufferCS252/PattersonLec 5.151/31/01Disk Performance Example (will fix later)• Calculate time to read 64 KB (128 sectors) forBarracuda 180 X using advertised performance;sector is on outer trackDisk latency = average seek time + averagerotational delay + transfer time + controlleroverhead = 7.4 ms + 0.5 * 1/(7200 RPM)+ 64 KB / (65 MB/s) + 0.1 ms = 7.4 ms + 0.5 /(7200 RPM/(60000ms/M))+ 64 KB / (65 KB/ms) + 0.1 ms = 7.4 + 4.2 + 1.0 + 0.1 ms = 12.7 msCS252/PattersonLec 5.161/31/01CS 252 Administrivia• We have a TA! Yu-jia Jin: [email protected]• Please send 1-2 paragraph summary of papers tohim BEFORE CLASS Friday– J.GRAY, Turing Award Lecture: "What Next? A dozen remainingIT problems"; We will discuss Friday– Should have already turned in» G. MOORE, "Cramming More Components onto IntegratedCircuits"» J. S. LIPTAY, "Structural Aspects of the System/360Model 85, Part II: The Cache"• Please fill out Third Edition chapter surveys for 1,5, 6; 1,5 before Friday, 6 by next Wednesday– Link from 252 Web page (click on survey)• Project suggestions are on web site; start looking–
View Full Document
Unlocking...