ENGS 116 Lecture 171 Introduction to I/O Vincent Berk November 19, 2008 Reading for today: Sections 4.4 – 4.9 Reading for Monday: Sections 6.1 – 6.4 Reading for next Monday: Sections 6.5 – 6.9 Homework for Friday: 5.4, 5.6, 5.10, 4.1, 4.17

ENGS 116 Lecture 172 The Big Picture: Where are we now? Control Datapath Memory Input Output Processor

ENGS 116 Lecture 173 I/O Systems

ENGS 116 Lecture 174 Storage System Issues Historical Context of Storage I/O Secondary and Tertiary Storage Devices Storage I/O Performance Measures A Little Queueing Theory Processor Interface Issues I/O Buses Redundant Arrays of Inexpensive Disks (RAID) I/O Benchmarks

ENGS 116 Lecture 1755 Motivation: Who Cares About I/O? CPU Performance: 50% to 100% per year I/O system performance limited by mechanical delays < 5% per year (IO per sec or MB per sec) Amdahl's Law: system speed-up limited by the slowest 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 CPU Diminishing value of faster CPUs

ENGS 116 Lecture 1766 Today: Processing power doubles every 18 months Today: Memory size doubles every 18 months (4X/3 yrs) Today: Disk capacity doubles every 18 months Disk positioning rate (seek + rotate) doubles every ten years! Technology Trends The I/O GAP

ENGS 116 Lecture 177 Storage 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 Interesting twist: Apple I-pod –Data utilities high capacity, hierarchically managed storage

ENGS 116 Lecture 178 Historical Perspective 1956 IBM Ramac — early 1970s Winchester –Developed for mainframe computers, proprietary interfaces –Steady shrink in form factor: 27 in. to 14 in. 1970s developments –5.25-inch floppy disk form factor –early emergence of industry standard disk interfaces ST506, SASI, SMD, ESDI Early 1980s –PCs and first generation workstations Mid 1980s –Client/server computing –Centralized storage on file server accelerates disk downsizing: 8 inch to 5.25 inch –Mass market disk drives become a reality industry standards: SCSI, IDE, SATA 5.25-inch drives for standalone PCs, end of proprietary interfaces

ENGS 116 Lecture 179 Disk History Data density Mbit/sq. in. Capacity of Unit Shown Megabytes 1973: 1. 7 Mbit/sq. in 140 MBytes 1979: 7. 7 Mbit/sq. in 2,300 MBytes Source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even more data into even smaller spaces”

ENGS 116 Lecture 1710 Historical Perspective Late 1980s/Early 1990s: –Laptops, notebooks, (palmtops) –3.5 inch, 2.5 inch, 1.8 inch formfactors –Formfactor plus capacity drives market, not so much performance Recently Bandwidth improving at 40%/ year –Challenged by DRAM, flash RAM in PCMCIA cards still expensive, Intel promises but doesn’t deliver unattractive MBytes per cubic inch –Optical disk fails on performance (e.g., NEXT) but finds niche (CD ROM)

ENGS 116 Lecture 1711 Disk History 1989: 63 Mbit/sq. in 60,000 MBytes 1997: 1450 Mbit/sq. in 2300 MBytes Source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even more data into even smaller spaces” 1997: 3090 Mbit/sq. in 8100 MBytes

ENGS 116 Lecture 1712 ENGS 116 Lecture 17 Alternative Data Storage Technologies: Early 1990s Data CapBPITPIBPI*TPITransfer Access Technology(MB)(Million)(KByte/s) Time Conventional Tape: Cartridge (.25") minutes IBM 3490 (.5") seconds Helical Scan Tape: Video (8mm) secs DAT (4mm) secs D-3 (1/2")20,00015 secs? Magnetic & Optical Disk: Hard Disk (5.25") ms IBM 3390 (10.5") ms Sony MO (5.25") ms

ENGS 116 Lecture 1713 ENGS 116 Lecture 17 Devices: Magnetic Disks Purpose: –Long-term, nonvolatile storage –Large, inexpensive, slow level in the storage hierarchy Characteristics: –Seek Time (~ 8 ms avg) positional latency rotational latency Transfer rate –About a sector per ms (10-40 MB/s) –Blocks Capacity –Gigabytes –Quadruples every 3 years 7200 RPM = 120 RPS  8 ms per rev avg. rot. latency = 4 ms 128 sectors per track  ms per sector 1 KB per sector  16 MB / s Response time = Queue + Controller + Seek + Rot + Transfer Service time Sector Track Cylinder Head Platter

ENGS 116 Lecture 1714 ENGS 116 Lecture 17 Disk Device Terminology Disk Latency = Queueing Time + Controller Time + Seek Time + Rotation Time + Transfer Time Order-of-magnitude times for 4K byte transfers: Seek: 8 ms or less Rotate: rpm Transfer: rpm Platter Outer Track Inner Track Sector Head Arm Actuator

ENGS 116 Lecture 1715 ENGS 116 Lecture 17 Disk Device Terminology

ENGS 116 Lecture 1716 Tape vs. Disk Longitudinal tape uses same technology as hard disk; tracks its density improvements Disk head flies above surface, tape head lies on surface Inherent cost-performance based on geometries: fixed rotating platters with gaps (random access, limited area, 1 media / reader) vs. removable long strips wound on spool (sequential access, "unlimited" length, multiple / reader) Modern technology: Helical Scan (VCR, Camcorder, DAT) Spins head at angle to tape to improve density

ENGS 116 Lecture 1717 R-DAT Technology 90° Wrap Angle Drum Direction of Tape Track Rotary Drum Four Head Recording Tracks Recorded ± 20° w/o guard band Read After Write Verify Helical Recording Scheme 2000 RPM R R W W

ENGS 116 Lecture 1718 Example: R-DAT Technology Rotating (vs. Stationary) head (Digital Audio Tape) Highest areal recording density commercially available High density due to: –high coercivity metal tape –helical scan recording method –narrow, gapless (overlapping) recording tracks 10X improvement capacity & transfer rate by 1999 –faster tape and drum speeds –greater track overlap No significant changes since late 90s

ENGS 116 Lecture 1719 R-DAT Technology Block Track (2900 Data Bytes) Frame (2 Tracks) Group (22 Frames + Optional Group ECC, 128 K bytes) 65% of Track is Data Area 70% Data Bytes 30% Bytes Parity Plus Reed-Solomon Codes Track Finding Area (Servo) Subcode Area (Index) Margin Area Theoretical Bit Error Rates: w/o group ECC: one in w/ group ECC: one in Tape Track Frame DDS ANSI Standard (HP, SONY)

ENGS 116 Lecture 1720 ENGS 116 Lecture 17 Current Drawbacks to Tape Tape wears out: –Helical 100s of passes to 1000s for longitudinal Head wears out: –2000 hours for helical Both must be accounted for in economic/reliability model Long rewind, eject, load, spin-up times; not inherent, just no need in marketplace (so far) Designed for archival storage Capacities: 700 GB per tape, but used in big libraries (150 PB)

ENGS 116 Lecture 1721 Modern Day Tape specs. Sun StorageTek SL 8500 tape library –Up to 150,000 TB of archival storage –With 2,048 drives: TB/hr –60 minutes to read entire library –11 seconds to find and load any type in library

ENGS 116 Lecture 1722 Optical Disks: CD-ROM vs. DVD Both 12 cm in diameter (most-common size for CD-ROM) CD-ROM –Maximum 80 minutes of music –700 MB of information DVD (Digital Versatile/Video Disk) –4.7 GB of information on one of its two sides — enough for 133- minute movie –With 2 layers on each of its two sides, will hold up to 17 gigabytes –Uses MPEG-2 file compression standard Blue Ray DVD (higher density) –Up to 200 GB of capacity per disc

ENGS 116 Lecture 1723 Disk I/OPerformance Response time = Queue + Device Service time Proc Queue IOCDevice Metrics: Response Time Throughput

ENGS 116 Lecture 1724 Response Time vs. Productivity Interactive environments: Each interaction or transaction has 3 parts: –Entry Time: time for user to enter command –System Response Time: time between user entry & system replies –Think Time: Time from response until user begins next command 1st transaction 2nd transaction What happens to transaction time as system response time shrinks from 1.0 sec to 0.3 sec? –With keyboard: 4.0 sec entry, 9.4 sec think time –With graphics: 0.25 sec entry, 1.6 sec think time

ENGS 116 Lecture 1725 Time graphics 1.0s graphics 0.3s conventional 1.0s conventional 0.3s entryrespthink Response Time & Productivity 0.7 sec off response saves 4.9 sec (34%) and 2.0 sec (70%) total time per transaction => greater productivity Another study: everyone gets more done with faster response, but novice with fast response = expert with slow

ENGS 116 Lecture 1726 Response Time & Productivity

ENGS 116 Lecture 1727 Disk Time Example Disk parameters: –Transfer size is 8K bytes –Advertised average seek is 12 ms –Disk spins at 7200 RPM –Transfer rate is 4 MB/sec Controller overhead is 2 ms Assume that disk is idle so no queueing delay What is average disk access time for a sector? –Avg seek + avg rotational delay + transfer time + controller overhead –12 ms + 0.5/(7200 RPM/60) + 8 KB/4 MB/s + 2 ms – = 20 ms Advertised seek time assumes no locality: typically 1/4 to 1/3 advertised seek time: 20 ms => 12 ms

ENGS 116 Lecture 1728 The following simplified diagram shows two potential ways of numbering the sectors of data on a disk (only two tracks are shown and each track has eight sectors). Assuming that typical reads are contiguous (e.g., all 16 sectors are read in order), which way of numbering the sectors will be likely to result in higher performance? Why?

ENGS 116 Lecture 1729 Processor Interface Issues Interconnections –Busses Processor Interface –Interrupts –Memory mapped I/O I/O Control Structures –Polling –Interrupts –DMA –I/O controllers –I/O processors Capacity, Access Time, Bandwidth

ENGS 116 Lecture 1730 Busses Bus: a shared communication link between subsystems Disadvantage: a communication bottleneck, possibly limiting the maximum I/O throughput Bus speed is limited by physical factors Two generic types of busses –I/O –CPU-memory Bus transaction: sending address & receiving or sending data

ENGS 116 Lecture 1731 Bus Options OptionHigh performanceLow cost Bus widthSeparate addressMultiplex address & data lines& data lines Data widthWider is fasterNarrower is cheaper (e.g., 32 bits)(e.g., 8 bits) Transfer sizeMultiple words hasSingle-word transfer less bus overheadis simpler Bus mastersMultipleSingle master (requires arbitration)(no arbitration) Split Yes — separate No — continuous transaction?Request and Replyconnection is cheaper packets for higher and has lower latency bandwidth (needs multiple masters) ClockingSynchronousAsynchronous

ENGS 116 Lecture 1732 Interface Peripheral Memory I/O Interface CPU Separate I/O instructions (in, out) 40 Mbytes/sec optimistically 10 MIP processor completely saturates the bus! VME bus Multibus-II Nubus Common memory & I/O bus CPU Memory Interface Peripheral Independent I/O bus Memory bus

ENGS 116 Lecture 1733 I/O devices Backplane bus Memory Processor a. c. b. Memory Processor Processor-memory bus Bus adapter I/O bus Bus adapter I/O bus Bus adapter I/O bus Memory Processor Bus adapter Processor-memory bus I/O bus Backplane bus