Chapter 5 Data Storage Technology Chapter 5 Data

Chapter 5 Data Storage Technology

Chapter 5 Data Storage Technology Chapter Outline Storage Device Characteristics Primary Storage Devices CPU Memory Access Magnetic Storage Optical Mass Storage Devices

Chapter Goals • Describe the distinguishing characteristics of primary and secondary storage • Describe the devices used to implement primary storage • Describe memory allocation schemes • Compare and contrast secondary storage technology alternatives • Describe factors that determine storage device performance • Choose appropriate secondary storage technologies and devices

Data Storage Technology

Storage Device Characteristics Breakdown Of Topics • • • Speed Volatility Access Methods Serial Access Random Access Parallel Access Portability Cost and Capacity Memory Storage Hierarchy

Storage Devices • Consist of a read/write mechanism and a storage medium – Device controller provides interface • Primary storage devices – Support immediate execution of programs • Secondary storage devices – Provide long-term storage of programs and

Primary and Secondary Storage Devices

Characteristics of Storage Devices • Speed • Volatility • Access method • Portability • Cost and capacity

Speed • Primary Storage – Typically much faster than secondary storage – Expressed in nanoseconds • billionths of a second = 10 -6 second • Secondary Storage – Expressed in milliseconds • thousandths of a second = 10 -3 second • Data Transfer Rate = amount of data (bytes) / access time (seconds) = answer in “bytes per second”

Volatility Primary storage devices are usually volatile. (Cannot reliably hold data for long periods of time) • Volatile memory requires constant power to maintain the stored information. • The most cost-effective primary storage technologies are volatile. • Non-volatile technologies have been widely used for primary storage in the past and may again be in the future. Secondary storage devices are nonvolatile

Access Method • Random Access (Direct Access) – any storage location can be accessed in the same amount of time – well suited for primary storage • Sequential Access – accessing a storage location takes a varying amount of time, depending on which storage location was accessed last – delays include • seek (position access device correctly) • cycle (wait for desired location in a revolving medium to appear at the access device)

Portability • Removable storage media with standardized formats – compact disc – tape storage • Typically results in slower access speeds

Cost and Capacity • Cost increases: – With improved speed, volatility, or portability – As access method moves from serial to random to parallel access method • Primary storage is expensive – high speed and combination of parallel/random access methods • Capacity of secondary storage devices is

Storage Device Characteristics and Their Relationship to Cost

Memory. Storage Hierarchy

Primary Storage Devices Breakdown Of Topics • Storing Electrical Signals • Random Access Memory • Nonvolatile Memory • Memory Packaging

Primary Storage Devices • Critical performance characteristics – Access speed – Data transfer unit size • Must closely match CPU speed and word size to avoid wait states

Storing Electrical Signals • Directly – by devices such as batteries and capacitors – trade-off between access speed and volatility • Indirectly – write: use energy to alter the state of a device – read: an inverse process produces an equivalent electrical signal • Modern computers use memory implemented

Random Access Memory Characteristics • Microchip implementation using semiconductors • Ability to read and write with equal speed • Random access to stored bytes, words, or larger data units Basic types • Static RAM (SRAM) – uses transistors • Dynamic RAM (DRAM) – uses transistors and capacitors

The Cross-Coupled Gate Time X Y Z ~Z t 0 1 1 0 t 1 0 0 1 t+1 1 0 t+1 1 1 0 1

Random Access Memory Bridging The Performance Gap Between Memory And Microprocessors • Read-ahead memory access • Synchronous read operations • On-chip memory caches

Core Memory

Core Memory

Nonvolatile Memory • Random access memory with long-term or permanent data retention • Usually relegated to specialized roles and secondary storage; slower write speeds and limited number of rewrites • Generations of devices – ROM – EPROM – EEPROM – etc.

Nonvolatile Memory • Flash RAM (most common NVM) – Competitive with DRAM in capacity and read performance – Relatively slow write speed – Limited number of write cycles – Based on EEPROM technology • NVM technologies under development – Ferroelectric RAM – Polymer memory

Memory Packaging • Dual in-line packages (DIPs) – Early RAM and ROM circuits • Single in-line memory module (SIMM) – Standard RAM package in late 1980 s • Double in-line memory module (DIMM) – Newer packaging standard – A SIMM with independent electrical

Memory Packaging

Memory Packaging SIMM memory module DIMM memory module SODIMM memory module

Memory Packaging

CPU Memory Access Breakdown Of Topics • Physical Memory Organization • Memory Allocation and Addressing

CPU Memory Access • Critical design issues for primary storage devices and processors – Physical organization of memory – Organization of programs and data within memory – Method(s) of referencing specific memory locations

Physical Memory Organization • Physical memory – Actual number of memory bytes that physically are installed in the machine • Most and least significant bytes • Big endian and little endian • Addressable memory – Highest numbered storage byte that can be represented

A Linear Address Space

Memory Allocation and Addressing • Memory allocation – Assignment of specific memory addresses to system software, application programs, and data • Absolute addressing • Indirect addressing (relative addressing) – Offset register

Memory Allocation A Simple Scheme

Memory Allocation for Multiple Programs

Memory. Storage Hierarchy

Magnetic Storage • Exploits duality of magnetism and electricity – Converts electrical signals into magnetic charges – Captures magnetic charge on a storage medium – Later regenerates electrical current from stored magnetic charge • Polarity of magnetic charge represents bit

Principles of Magnetic Storage

Data Loss in Magnetic Storage

Areal Density

Magnetic Tape • Ribbon of plastic with a coercible (usually metallic oxide) surface coating • Mounts in a tape drive for reading and writing • Relatively slow serial access • Compounds magnetic leakage; wraps upon itself • Susceptible to stretching, friction,

Cassette or Cartridge Magnetic Tape

Magnetic Tape • Two approaches to recording data – Linear recording – Helical scanning • Several formats and standards – DDS [DAT] – AIT – Mammoth – DLT – LTO – etc.

Parallel Tracks vs. Helical Scanning

Technology Focus Quarter Inch Committee (QIC) Year Cartridge Size (Inches) QIC-80 1988 4 x 6 . 08 28 14, 700 QIC-120 1991 4 x 6 0. 125 15 10, 000 QIC-525 1992 4 x 6 0. 525 26 20, 000 QIC-2100 1993 4 x 6 2. 1 30 50, 800 QIC-3095 1995 3. 25 x 2. 5 4 72 67, 733 QIC-3220 1997 3. 25 x 2. 5 10 108 106, 400 Format Capacity Tracks (GB) Recording Density (bpi)

Magnetic Disk • Flat, circular platter with metallic coating that is rotated beneath read/write heads • Random access device; read/write head can be moved to any location on the platter • Hard disks and floppy disks • Cost performance leader for general-purpose on-line secondary storage

Components of a Magnetic Disk Drive

Track and Sector Organization on a Disk Platter Surface

Magnetic Disk Access Time • Head-to-head switching time • Track-to-track seek time • Rotational delay • Most important performance numbers – Average access time – Sequential access time – Sustained data transfer rate

Hard Disk Drive Performance Statistics Average Access Time is most strongly affected by Rotational Speed

Track Zones

Optical Mass Storage Devices • Store bit values as variations in light reflection • Higher areal density and longer data life than magnetic storage • Standardized and relatively inexpensive • Uses – read-only storage with low performance requirements – applications requiring high capacity,

Optical Disc Read Operations One Bit Zero Bit

Optical and Magneto-optical Storage Technology

CD-ROM • Read-only; data permanently embedded in durable polycarbonate disc • Bit values represented as flat areas (lands) and concave dents (pits) in the reflective layer • Data recorded in single continuous track that spirals outward from center of disc • Popular medium for distributing software and large data sets

CD-ROM Advantages Drawbacks • Standardized format • Cannot be rewritten • High density • Capacity limited to • Cheap to 700 MB manufacture

CD-R • Uses a laser that can be switched between high and low power and a laser-sensitive dye embedded in the disc • Relatively cheap • Common uses – create music CDs on home computers – back up data from other storage devices – create archives of large data sets – manufacture small quantities of identical CDs

Magneto-Optical • Uses a laser and reflected light to sense bit values • Technology peaked in the mid 1990 s • Advantages over CD-RW – faster access speed – higher capacity

Phase-Change Optical Discs • Enables nondestructive writing to optical storage media • Materials change state easily from noncrystalline (amorphous), to crystalline, and then back again – Reflective layer is a compound of tellurium, selenium, and tin • Example: CD-RW

DVD • Improves on CD and CD-RW technology – Increased track and bit density: smaller wavelength lasers and more precise mechanical control – Improved error correction – Multiple recording sites and layers

Summary • Storage devices and their underlying technologies • Characteristics common to all storage devices • Technology, strengths, and weaknesses of primary and secondary storage

Chapter Goals • Describe the distinguishing characteristics of primary and secondary storage • Describe the devices used to implement primary storage • Describe memory allocation schemes • Compare and contrast secondary storage technology alternatives • Describe factors that determine storage device performance • Choose appropriate secondary storage technologies and devices