Transcript Systems Architecture, Fifth Edition

Seagate Cheetah 15K.5, full of perpendicular
goodness with 300 GB space and a mad
15,000 rpm
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
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Chapter Goals (continued)
• Describe factors that determine storage device
performance
• Choose appropriate secondary storage
technologies and devices
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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 data
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Characteristics of Storage Devices
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Speed
Volatility
Access method
Portability
Cost and capacity
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Speed
• Primary storage speed
– Typically faster than secondary storage speed by a
factor of 105 or more
– Expressed in nanoseconds (billionths of a second)
• Secondary storage speed
– Expressed in milliseconds (thousandths of a second)
• Data transfer rate = 1 second/access time (in seconds)
x unit of data transfer (in bytes)
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Volatility
• Primary storage devices are generally volatile
– Cannot reliably hold data for long periods
• Secondary storage devices are generally
nonvolatile
– Hold data without loss over long periods of time
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Access Method
• Serial access (linear)
• Random access (direct access)
• Parallel access (simultaneous)
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Portability
• Removable storage media with standardized
formats (e.g., compact disc and tape storage)
• Typically results in slower access speeds
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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 - expensive (high speed and
combination of parallel/random access methods)
• Capacity of secondary storage devices is greater
than primary storage devices
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Memory-Storage Hierarchy
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Primary Storage Devices
• Critical performance characteristics
– Access speed
– Data transfer unit size
• Must closely match CPU speed and word size to
avoid wait states
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Storing Electrical Signals
• Directly
– By devices such as batteries and capacitors
– Trade off between access speed and volatility
• Indirectly
– Uses energy to alter the state of a device; inverse
process regenerates equivalent electrical signal
• Modern computers use memory implemented with
semiconductors (RAM and NVM)
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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
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Random Access Memory
• To bridge performance gap between memory and
microprocessors
– Read-ahead memory access
– Synchronous read operations
– On-chip memory caches
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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, Erasable
Programmable ROM=EPROM, and
Electronically EPROM = EEPROM)
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Nonvolatile Memory
• Flash RAM (most common NVM)
– Competitive with DRAM in capacity and read
performance
– Relatively slow write speed
– Limited number of write cycles
• NVM technologies under development
– Ferroelectric RAM
– Polymer memory
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Memory Packaging
• Dual in-line packages (DIPs)
– Early RAM and ROM circuits
• Single in-line memory module (SIMM)
– Standard RAM package in late 1980s
• Double in-line memory module (DIMM)
– Newer packaging standard
– A SIMM with independent electrical contacts on
both sides of the module
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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
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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
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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
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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 values
zero and one
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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, temperature
variations
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Magnetic Tape
• Two approaches to recording data
– Linear recording
– Helical scanning
• Several formats and standards (e.g., DDS [DAT],
AIT, Mammoth, DLT, LTO)
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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
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Locating a Block of Data
• Average seek time: (average) time required
to move from one track to another
• [Also called track-to-track seek time]
• Latency: time required for disk to rotate to
beginning of correct sector
• [Also called rotational delay]
• Transfer time: time required to transfer a
block of data to the disk controller buffer
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Disk Access Times
• Avg. Seek time (track-to-track seek time)
– average time to move from one track to another
• Avg. Latency time (rotational delay
– average time to rotate to the beginning of the sector
– Avg. Latency time = ½ * 1/rotational speed
• Transfer time
– 1/(# of sectors * rotational speed)
• Total Time to access a disk block
– Avg. seek time + avg. latency time + avg. transfer time
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Example
• Calculate the average access time for a disk rotating at
7,200 rpm, with Avg. Seek time 5 microseconds, and 500
sectors/track.
• SOL: Average Access Time =(seek) + (rot. Delay) +
(Transfer Time)
• Average Access Time =(5 microseconds) + (½) *
(1/7,200RPM) + 1/(500* 7,200RPM
• =.000005 + ½ * .0001389 (60 sec) + .000000278 (60 sec)
• =.000005+.004167+.0000167=.0041887sec….ANS
• Which performance value is the most significant?
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Disk Block Formats
Single Data Block
Header for Windows disk
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To increase capacity per platter, disk manufacturers
divide tracks into zones and vary the sectors per track
in each zone.
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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 with high capacity
requirements, and where portability in a
standardized format is needed
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Optical storage devices read data by shining laser
beam on the disc.
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Layout: CD-ROM vs. Standard Disk
CD-ROM
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Hard Disk
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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
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CD-ROM
Advantages
Drawbacks
• Standardized format
• High density
• Cheap to manufacture
• Cannot be rewritten
• Capacity limited to 700
MB
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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, and
manufacture small quantities of identical CDs
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Phase-Change Optical Discs
• Enables nondestructive writing to optical storage
media
• Materials change state easily from non-crystalline
(amorphous), to crystalline, and then back again
– Reflective layer is a compound of tellurium,
selenium, and tin
• Example: CD-RW
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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
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Summary
• Storage devices and their underlying technologies
• Characteristics common to all storage devices
• Technology, strengths, and weaknesses of primary and
secondary storage
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Solid State Drive!
•
In 1984, Dr. Fujio Masuoka from Toshiba
invented the first flash memory, which
Intel introduced as a commercial product
four years later. Solid state drives based
on flash memory have a number of unique
advantages over conventional hard drive
technology. Since they lack mechanical
parts, they are much more resistant to
shock, consume far less power, and
release far less heat. They also produce no
sound during operation and respond much
more quickly than a mechanical drive.
The size of a flash drive can also be very
small (relative to a mechanical hard
drive), resulting in a lighter device. All
these advantages beg the question 'Why
are we still using what might seem as an
outdated mechanical hard drive
technology?'
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Fastest Memory
(Aug 2008)
The fastest memory on the market was
showcased by Corsair. The company
displayed the DDR3-2133MHz memory
modules, which have a 1GB capacity
and are capable of performing at
latencies of 8-8-8-24. These
“Dominator” modules are integrated
with the company’s unique Dual-path
Heat eXchange (DHX) technology. The
DHX utilizes two paths for heat
removal – an aluminum heat spreader,
and a special PCB, which has a copper
base that absorbs heat transferred by a
dedicated heat sink. According to the
company, the DHX modules, each of
which includes a total of four heat
sinks, provide an extremely stable,
reliable, and overclockable memory.
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http://thefutureofthings.com/news/1143/a-look-back-atcebit-2008-storage-and-memory.html
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PCI Drive??
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The DDR Drive X1 is a PCI Express based plug in card with 8GB of capacity, and
bootable.
According to sources close to the firm, the drive will have a custom and
upgradeable high performance DMA Engine, an external power jack with a
switching AC adaptor, and will go into a 512 unit initial production run.
Although the firm gave no details of the pricing, a solid state drive at an
affordable price is likely to be a popular item.
http://www.theinquirer.net/en/inquirer/news/2006/01/03/solid-state-ddr-drivegoes-into-prototype
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