CALectureWeek4.ppt

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CARC103 – Computer Architecture
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Prescribed Text
Bird, S. D. (2017), Systems Architecture, 7th ed, Cengage Learning
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Systems Architecture,

Seventh Edition
Chapter 5
Data Storage Technology
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Systems Architecture, Seventh Edition

Chapter Objectives
In this chapter, you will learn to:
Describe the distinguishing characteristics of primary and secondary storage
Describe the devices used to implement primary storage
Compare secondary storage alternatives
Describe factors that storage device performance
Choose appropriate secondary storage technologies and devices

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Systems Architecture, Seventh Edition

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FIGURE 5.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Storage Device Components
Storage medium – a device or substance on which data is stored, for example:
Storage circuitry of a flash drive or RAM device
Metallic surface of a magnetic disk
Reflective surface of an optical disc
Read/Write (R/W) mechanism – a device used to access (read) and store (write) data values on a storage medium, for example
Access circuitry of a flash drive or RAM device
Laser and photo-detector in an optical disc drive
Device controller – an interface device that connects the storage device (or its R/W mechanism) to the system bus (more on this topic in Chapter 6)

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Systems Architecture, Seventh Edition

Storage Device Characteristics
Storage devices vary in the following important characteristics:
Speed
Volatility (or lack thereof)
Access method
Portability (or lack thereof)
Cost and capacity
Each storage device/technology has a unique combination of these characteristics
Primary and secondary storage devices have very different characteristic combinations
Primary storage – fast, volatile, parallel access, non-portable, and relatively expensive
Secondary storage – slow, non-volatile, various access methods, may be portable, less expensive

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Systems Architecture, Seventh Edition

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FIGURE 5.2 Primary and secondary storage and their component devices
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Speed
Speed is a complex characteristic:
How quickly can data be found on the storage medium?
Once found, how quickly can it be transferred to/from other computer system components?
If many data items are being read or written at once, do the answers to the above questions differ for the first and last data items?
How much data is (how many bits or bytes are) read or written at “one time”?
Because the issue of speed is complex, there are multiple speed-related measures (metrics)

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Systems Architecture, Seventh Edition

Access Time
Access time is the elapsed time required to complete one read or write operation
Access time is:
The sum of time required to:
“Accept” the read or write command
“Find” the appropriate location on the storage medium
Transfer data to/from the location
Assumed to be the same for reading and writing unless two different times are stated
Constant for some storage devices (e.g., RAM) and variable for others (e.g., disk)
For devices with variable access time, a more specific measure is:
Average access time – average of access times for many different storage device locations

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Systems Architecture, Seventh Edition

Data Transfer Rate
Access time is an important but incomplete measure of storage device speed:
How much data is read or written in a single read/write operation?
Does access time “degrade” with repeated reads and writes?
Storage devices usually accept or provide data in fixed-size units per read/write operation:
Block – a generic term describing the amount of data transferred in one read/write operation – term is most commonly used with magnetic tapes but can be used with any storage device
Sector – the amount of data transferred to/from an optical or magnetic disk in one read/write operation – usually 512 bytes

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Systems Architecture, Seventh Edition

Data Transfer Rate – Continued
Data transfer rate in seconds is computed as:

The result is a number stated in data units per second, for example:
100 megabits per second, or 100 Mbps
10 megabytes per second, or 10 MBps
5000 sectors per second
Example – assume that a primary storage device has a 15 nanosecond access time for a 64-bit data unit:

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Systems Architecture, Seventh Edition

Exercise
Compute data transfer rates for the following devices:

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Storage Device Avg. Access Time Data Xfer Unit Size

RAM 4 nanoseconds 64 bits

Optical disc 100 milliseconds 512 bytes

Magnetic disc 5 milliseconds 512 bytes

Systems Architecture, Seventh Edition

Exercise – Answer
RAM data transfer rate
64 bits per transfer = 8 bytes per transfer
1 second ÷ 4 ns (.000000004 seconds) per transfer = 250,000,000 transfers per second
250,000,000 transfers per second × 8 bytes per transfer = 2,000,000,000 Bps ÷ 10243 ≈ 1.86 GBps

Optical disc transfer rate
1 second ÷ 100 ms (.01 seconds) per transfer = 100 transfers per second
100 transfers per second × 512 bytes per transfer = 51,200 Bps ÷ 1024 ≈ 50 KBps
Fortunately, discs are normally read sequentially and sequential access times are much faster !!

Magnetic disk transfer rate
1 second ÷ 5 ms (.005 seconds) per transfer = 2000 transfers per second
100 transfers per second × 512 bytes per transfer = 1,024,000 Bps ÷ 10242 ≈ 0.98 MBps
Fortunately, defragmentation yields many sequential reads and sequential access times are much faster !!
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Systems Architecture, Seventh Edition

Volatility
A storage device or medium is:
Nonvolatile – if it holds data without loss over long periods of time (typically, years or decades)
Volatile – if it cannot reliably hold data for long periods of time
Volatility is not a binary characteristic, it’s a matter of degree, for example:
RAM is non-volatile in typical use but its storage life can be extended hours or days with battery backup
Flash RAM circuits hold data indefinitely but they “wear out” with repeated use
Data stored on a magnetic disk will last decades if the device is powered off, but the device will normally fail in less than two decades if used continuously
For storage devices with removable storage media (e.g., DVD), the “lifetimes” of both the device and the medium must be considered
For example, you could put a DVD in a time capsule to be opened in 1000 years, but will anyone have a working DVD reader at that time?

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Systems Architecture, Seventh Edition

Access Methods
Access methods fall into three broad classes:
Serial access
Tape, for example
Access locations or organized serially (in a line)
Access to location N requires accessing (or skipping over) locations 1 through N+1
Old technology – avoided whenever possible!
Random access
Disk, for example
Also called direct access
Any location on the storage medium can be accessed in (approximately) the same amount of time
Parallel access
Disk arrays, for example
Multiple locations on the same storage medium can be accessed at the same time
A single device can employ multiple methods!

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Systems Architecture, Seventh Edition

Portability
Storage device portability comes in two forms:
The entire device (medium, R/W mechanism, and maybe the controller) is portable
For example, flash drive, compact flash card, external USB hard drive, many cell phones when connected to the USB port of a laptop or desktop computer
Only the storage medium is removable and portable
For example, CD and DVD
Portability is usually obtained at the expense of speed
For portable devices – slower “external” communication technologies and standards are used
For example, flash drive vs. installed RAM
For removable media – loss of control over environmental conditions necessitates performance compromises
For example, sealed magnetic disc drive vs. CD or DVD

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Systems Architecture, Seventh Edition

Cost and Capacity
“Improvements” in one characteristic while holding the others constant generally increase cost

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Characteristic Cost Implications

Speed Cost increases as speed increases

Volatility Cost increases as volatility decreases (permanence is more expensive than volatility)

Access method Serial is cheapest, direct is more expensive, parallel is more expensive than non-parallel

Portability Portability increases cost, though most portable devices sacrifice other characteristics to minimize the cost increase

Capacity Within limits, cost increases proportionally to capacity

Systems Architecture, Seventh Edition

Memory-Storage Hierarchy
A computer system includes multiple types of storage devices
Each device has a unique combination of characteristics
Each device is optimal (or at least reasonable) for certain purposes
The mix of devices and their capacities is a cost/performance trade-off

FIGURE 5.3 Comparison of storage devices in terms of cost and access speed
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Primary and Secondary Storage
Primary storage holds instructions and data for immediate/frequent CPU access
Speed mismatches between the CPU and primary storage cause wait states
Minimizing wait states dramatically improves CPU and computer system performance
Thus, primary storage devices generally emphasize speed and “faster” access methods at the expense of other characteristics
Secondary storage holds large quantities of data for long time periods
Thus, secondary storage devices tend to emphasize non-volatility and cost at the expense of other characteristics

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Systems Architecture, Seventh Edition

Storing Electrical Signals
Data is represented and processed within the CPU as electrical signals
Thus, to store a CPU’s data we must either:
Store electrical signals directly
Use the electrical signals to generate something else that can be stored
Methods of directly storing electrical signals include:
Batteries – poorly suited to rapid storage/retrieval
Capacitors – faster than batteries, but require frequent recharge unless they’re “big”
Mechanical switches – non-volatile, but slow and unreliable
Transistor-based switches – Very fast but require continuous flow of electricity (i.e., volatile)

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Systems Architecture, Seventh Edition

Flip-Flop Circuit
A flip-flop circuit is an electrical switch that “remembers” its last position (open or closed) as long as power continuously flows though it
Flip-flop circuits are the basic component of SRAM and CPU registers

FIGURE 5.4 A flip-flop circuit composed of two NAND gates: the basic component of SRAM and CPU registers
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Random Access Memory
Random Access Memory (RAM) is a primary storage technology with the following characteristics:
Bits are stored using transistors and/or capacitors
Semiconductor chip
Read and write access time is approximately equal
Combination of random and parallel access
Basic RAM types:
Static RAM (SRAM)
All bit storage uses flip-flop circuits
Very fast but relatively expensive
Dynamic RAM (DRAM)
Uses transistors and capacitors
Capacitors require many refresh cycles per second – reads/writes must wait for a refresh to complete
Slower and less expensive than SRAM

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Systems Architecture, Seventh Edition

Performance Enhancement Techniques
Both RAM types are slower than the CPU
Various “tricks” can be played (individually or in combination) to minimize the difference, including:
Read-ahead memory access
Assume sequential access to memory locations
Prefetch next memory location and have it “waiting by the door”
Synchronous read operations
A variation on read-ahead
Pipelined memory access based on the CPU or bus clock (for example, SDRAM) or a fraction of the clock (for example, DDR SDRAM)
As with CPU pipelining, out-of-sequence accesses can reduce/eliminate the performance gain
On-chip caches
Used in combination with some form of read-ahead access
The “waiting space by the door” is composed of SRAM (the cache)
The “real” storage is DRAM
Sometimes called enhanced DRAM (EDRAM)

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Systems Architecture, Seventh Edition

Non-Volatile Memory
Non-volatile memory:
Any memory device that can hold content without continuous power flow
Before 1990 (and occasionally since), implemented using various forms of read-only memory
ROM technologies (oldest to newest, but all are “old”):
Read-only memory (ROM)
Circuitry has data content “designed in”
Programmable ROM (PROM)
Manufactured blank and written once
Each bit has a one and zero circuit – fry the one that you don’t need!
Erasable PROM (EPROM)
Manufactured blank, written non-destructively
Reset to blank state by exposure to ultraviolet light
Electronically-erasable PROM (EEPROM)
Lose the UV light and do the same thing by sending an appropriate signal
The device contains circuitry to “erase itself”

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Systems Architecture, Seventh Edition

Non-Volatile RAM
Non-volatile RAM (NVRAM):
Any RAM device that can hold content without continuous power flow
The “Holy Grail” before the 1990s
As with many “quests”, success solves old problems but creates new ones
The first and still most widely-used NVRAM is called flash RAM:
Cost and bit density is comparable to DRAM
Reads at DRAM speeds but writes more slowly (how much more slowly is a rapidly changing thing)
Every write is mildly destructive – device begins to fail after many writes (how many is also a changing thing, currently 100,000s to low millions)

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Systems Architecture, Seventh Edition

Non-Volatile RAM – Newer Technologies
Magnetoresistive RAM (MRAM)
Stores bit values with two magnetic elements
One with fixed polarity
Other with polarity that changes when a bit is written
Second magnetic element’s polarity determines whether a current passing between the elements encounters low (a 0 bit) or high (a 1 bit) resistance
Read and write speeds comparable with SRAM
Density comparable with DRAM
Writes aren’t destructive―better longevity than flash RAM
Phase-change memory (PRAM or PCRAM)
Germanium, antimony, and tellurium (GST)
GST switches between amorphous and crystalline states when heated
Amorphous state exhibits low reflectivity (useful in rewritable optical storage media) and high electrical resistance
Crystalline state exhibits high reflectivity and low electrical resistance.
Lower storage density and slower read times than flash RAM
Write time is much faster, and it doesn’t wear out as quickly
Unknown whether either technology will succeed in the marketplace

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Systems Architecture, Seventh Edition

Memory Packaging
Early packages were dual-inline package (DIP) chips installed on expansion cards or directly in motherboard
Replaced in the late 1980s by single in-line memory modules (SIMMs)
DIPS are permanently mounted on a small card
Current packaging is dual in-line memory modules (DIMMs)
SIMMs with (different) electrical contacts on both sides
More contacts required by larger word sizes and bus width

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Systems Architecture, Seventh Edition

Memory Packaging – Continued
Burd, Systems Architecture, seventh edition, Figure 5.5 Copyright © 2015 Course Technology
30-pin SIMM
72-pin SIMM

DDR DIMM

DDR2 DIMM

DDR2 DIMM (laptop)

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Systems Architecture, Seventh Edition

Magnetic Storage Principles
Magnetic storage converts bit values represented as electrical signals into variations in the magnetic field of a specific location on a magnetic storage medium
The storage medium is typically metallic or some other coercible material (i.e., a material that will accept and hold a magnetic charge)

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Systems Architecture, Seventh Edition

Magnetic Read/Write Operations
A magnetic R/W head operates two ways
When writing, current flows left-to-right or right-to-left and the magnetic gap generates a magnetic field with the same polarity as the current flow
When reading, the stored magnetic charge induces an electric current in the direction of the magnetic polarity
In either case, direction of current flow represents a zero or one bit value

FIGURE 5.6 Principles of magnetic data storage
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Magnetic Storage Limitations
Stored magnetic charge must be above a minimum amount required to generate detectable current flow in the R/W head – sometimes called the read threshold
When stored magnetic charge falls below the read threshold, data is effectively “lost”
The stored charge is determined by:
Strength of the “write” magnetic field
Mass of coercible material that holds a bit value
Magnetic properties of the coercible material
Loss of charge due to magnetic leakage, magnetic decay, and loss of coercible material
Magnetic decay is the loss of magnetic charge strength over time
All magnets lose charge over time
Rate of charge loss varies with coercible material and its mass

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Systems Architecture, Seventh Edition

Magnetic Storage Limitations – Continued
Magnetic leakage is the cancellation of magnetic charge in adjacent areas of opposite polarity
Causes relatively rapid loss of charge
Worst when bit storage areas can be adjacent in three dimensions (e.g., magnetic tape)
With most magnetic storage medium the coercible material is a coating:
High-purity metals on disk platters
Iron-oxide or chrome oxide on magnetic tapes
Friction during the read/write process can wear away coercible material – primarily an issue for tapes and floppy disks
Time, physical stress, heat, and humidity can weaken the bond that holds the coercible material to the substrate – especially a problem with tapes dues to stretching

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Systems Architecture, Seventh Edition

Magnetic Storage Limitations – Continued
Areal density (also called recording density or bit density) describes the surface area of a storage medium used to store 1 bit
Areal density is a number stated as bits per area unit
Typical units are bits, bytes, or tracks per inch
Areal density can be increased by halving both dimensions of a bit area, but:
Coercible material mass falls by a factor of 4
The storage medium is more susceptible to data loss from magnetic decay, etc.
Thus, all other things held equal, increasing areal density:
Increases storage capacity
Decreases reliability (increases volatility)

FIGURE 5.7 Areal density is a function of a bit area’s length and width (a); density can be quadrupled by halving the length and width of bit areas (b)
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Magnetic Storage Limitations – Summary
Burd, Systems Architecture, Seventh edition, Table 5.2 Copyright © 2015 Cengage
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Factor Description

Magnetic decay Natural charge decay over time; data must be written at a higher power than the read threshold to avoid data loss.

Magnetic leakage Cancellation of adjacent charges of opposite polarity and migration of charge to nearby areas; data must be written at a higher power than the read threshold to avoid data loss.

Areal density The coercible material per bit decreases as the areal density increases; higher areal density makes stored data more susceptible to loss caused by decay and leakage if all other factors are equal

Media integrity Stability of coercible material and its attachment to the substrate; physical stress and extremes of temperature and humidity must be avoided to prevent loss of coercible material.

Device electronics and mechanics Speed or capacity increases must be coupled with improvements in components that position the read/write heads and the storage medium.

Systems Architecture, Seventh Edition

Magnetic Tape
Coercible material is coated onto a plastic ribbon wound on a spool
A motor turns the spool pulling the tape past a fixed R/W head
Modern tapes are:
Housed in a small plastic case
Wound/unwound at high speed

FIGURE 5.8 Components of a typical cassette or cartridge tape
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Magnetic Disk
Magnetic disk storage uses rotating platters covered with coercible material
Disk terminology:
Platter – 1 disk, usually recorded on both sides
Spindle – One center mounting and attached motor – usually rotating multiple platters
Track – one concentric circle on one platter (the recording surface that passes under a R/W head as the platter rotates once
Cylinder – set of tracks on all recording surfaces the same distance from the edge
Sector – a fraction of track holding 512 or 4096 bytes

Burd, Systems Architecture, seventh edition, Figures 5.9 and 5.10 Copyright © 2016 Cengage
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Systems Architecture, Seventh Edition

Magnetic Disk Performance
Disk performance depends on:
Head-to-head switching time
There is usually only one set of read/write circuitry shared among multiple R/W heads
Switching proceeds in series – if head 1 is currently active and the next read requires head 5 then 4 switching operations are required
Switching time is very fast – typically single digit nanoseconds
Track-to-track seek time
Time required to move R/W heads from their current position to the next track to be read or written
Relatively slow because its mechanical – milliseconds
Variable – moves over larger numbers of tracks require more time – any “specification” is an average
Rotational delay
Time waiting for a desired sector to rotate beneath the R/W head
Relatively slow because its mechanical – milliseconds
Higher RPMs decrease rotational delay
Variable – waits for larger numbers of sectors require more time – any “specification” is an average, usually based on ½ rotation time

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Systems Architecture, Seventh Edition

Average Access Delay
Average access delay – the time required to “move” between two sectors separated by an “average” number of recording surfaces, tracks, and sectors
For example, assume:
7500 RPM
5 platters, 10 recording surfaces and R/W heads
1000 tracks per recording surface

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Systems Architecture, Seventh Edition

Average and Sequential Access Time
Average access time is the sum of average access delay and the time required to read one sector
For example, assume previous example and 250 sectors per track:

Sequential access time – time to read two adjacent sectors on the same track and recording surface
Depends only on rotation speed
The second term in the formula above (3.2 microseconds)

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Systems Architecture, Seventh Edition

Fragmentation
As data contents are created and deleted over time sectors of a single file tend to become scattered across random disk locations
This condition is called fragmentation
Requiring file storage in sequential sectors isn’t feasible – not flexible enough (more on this in Chapter 12)
Since average access time is so much greater than sequential access time
Fragmentation substantially reduces read/write performance
Files are most efficient to read when they’re sequential by:
Sequential sectors in a track
Sequential tracks in a cylinder
Sequential cylinders
Defragmentation is a disk reorganization process that takes scattered sectors of the same file and reorganizes them for maximal read/write efficiency

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Systems Architecture, Seventh Edition

Windows Disk Defragmenter
Burd, Systems Architecture, Seventh edition, Figures 5.11 Copyright © 2015 Course Technology
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Systems Architecture, Seventh Edition

Disk Data Transfer Rates
As with access delay, data transfer rates for disks depend heavily on assumptions about the physical distribution and ordering of read/write locations across disk locations
Optimistic – data is stored sequentially
Pessimistic – data is scattered randomly
Maximum data transfer rate is the fastest rate at which a disk can deliver data to other computer system components:
Assumes no delays other than access delay
Assumes sequential access to physically adjacent sectors
Assumes H2H and T2T seek times are irrelevant
For example, assuming previous specs:

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Systems Architecture, Seventh Edition

Sustained Data Transfer Rate
Sustained data transfer rate – is computed based on an assumed “typical distribution” of data
How is data “typically” distributed?
In the worst case, data is distributed randomly and average access delay is always incurred, for example:

The real sustained data transfer rate is somewhere between the above numbers
Disk manufacturers “play games” with their assumptions to generate specifications that sometimes overstate real-world performance

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Systems Architecture, Seventh Edition

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TABLE 5.3 Hard disk drive performance statistics

Model Use Rotation speed (rpm) Average access time (ms) Maximum sustained DTR (MBps)

ST500LT012 Laptop 5400 12.5 118.5

ST1000DM003 Desktop 7200 8.5 156

ST600MP0005 Server 15,000 2.5 202

Systems Architecture, Seventh Edition

Variable Sector Density
To increase capacity per platter, disk manufacturers divide tracks into zones and vary the sectors per track in each zone

FIGURE 5.12 A platter divided into two zones with more sectors per track in the outer zone
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Solid-State Drives
A solid-state drive (SSD) is a secondary storage device that packages flash memory (or other NVM devices) within a format that mimics a traditional magnetic disk drive:
Magnetic disk format enables interchangeability
SSDs will evolve their own formats and interface standards over time – already happening in high-density servers
The pros and cons compared to magnetic disks are in flux (see next slide)
Neither technology is clearly superior to the other at present, though SSDs continue to make inroads and will likely “improve” more quickly

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Systems Architecture, Seventh Edition

Comparison of Solid-State and Magnetic Disk Drives in 2015
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Device characteristic Solid-state drives Magnetic disk drives

Read speeds For both random and sequential reads, data transfer rates of 250-350 MBps are typical For sequential reads, data transfer rates of 50-100 MBps are typical. For random reads, 10-25 Mbps.

Write speeds Write times depend are typically 80% of read times, longer (slower) with inexpensive devices Write times are typically 5% to 15% slower than read times.

Volatility Operational life depends on use and NVM technology. SLC flash RAM wears out after 100,000 or more write operations but that’s mitigated with wear leveling. MLC flash RAM wears out after 10,000 write operations. Typical device lifetime is up to 10 years for desktops, less for servers. Typical operational life is 10 to 20 years, with an unlimited number of accesses.

Power consumption No motors or servos. Power required by chips decreases as fabrication size shrinks. Motors and servos consume significant power.

Portability Lack of moving parts provides inherent portability with little or no performance penalty. Use of moving parts limits the performance of portable drives compared with non-portable drives.

Capacity and cost Maximum  capacity of 2 TB per drive. Cost ranges from $1.50 to $4 per gigabyte, depending on capacity, interface, and NVM technology. Maximum capacity is 6 TB per drive. Cost ranges from 10 cents  to $1.50 per gigabyte, depending on capacity, interface, and performance.

Systems Architecture, Seventh Edition

Optical Disk Storage
Optical disc storage uses a platter with optical rather than magnetic properties
Similarities to magnetic disk
Both use spinning platters
Both organize data into sectors and tracks
Physical geometry and performance calculations are similar
Differences from magnetic disk
Data recorded as variations in reflectivity rather than magnetic charge/polarity
Read/write head uses lasers and photodetectors
Single platter (1 or 2 recording surfaces) is the norm
Removable platters are the norm
Slower RPMs are the norm
Writing is much slower than reading

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Systems Architecture, Seventh Edition

Optical Disk – Read Operation Basics
Variations in reflectivity encode bits
For reading:
A laser bounces off the surface
A photodetector at a complementary angle detects a high (a) or low (b) amount of reflected light

FIGURE 5.13 Optical disc read operations for a 1 bit (a) and a 0 bit (b)
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Optical Disk Format Variations
Various optical disc technologies have different answers to the questions:
Are storage media manufactured with predefined data content that …

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