Chapter 5
Secondary Storages
國立聯合大學 電子工程學系
蕭裕弘
Chapter Goals
 說明電腦系統常用的儲存設備的
特性
 介紹兩種常見的磁碟系統與其作
業方式
 介紹數種不同的光碟系統與其作
業方式
 介紹 flash memory 與相關產品
 介紹磁帶儲存系統與其作業方式
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1. Introduction
 Different types and different purposes of computer storages:

Primary access from the CPU or secondary (indirect) access by the CPU,
which was based primarily on speed of access to the memory.

Volatile or non-volatile, which is based on the technology (magnetic vs.
electrical, etc.)

Read-only memory, WORM, or read-write, again based on technology.

Random-Access or Sequential-Access, which is based on the mechanism of
reading the memory.

Block or File access.

Media categories, e.g., semiconductor storage, optical storage, magnetooptical storage, and magnetic storage, which is mainly disk-based, rather
than memory-based.
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Primary vs. Secondary Storage
 In traditional parlance, primary storage contains data that
are actively being used (for example, the programs
currently being run and the data they are operating on).

It is typically high-speed, relatively small.

It is often (but not always) volatile.

It is sometimes referred to as "Main Memory."
 Secondary storage, also known as peripheral storage or
auxiliary storage, is where the computer stores
information that is not necessarily in current use.

It is typically slower and higher-capacity than primary
storage.

It is almost always non-volatile.
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Random vs. Sequential Access
 Random-access media has the property of
accessing any portion at any time.
Semiconductor memory and magnetic disk are
examples of this type of storage.
 Sequential-access media by contrast must be
read in sequence regardless of the desired
content. Magnetic tape and certain types of
flash memory have this property.
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Block vs. File Access
 In disk storage, these are the two primary
access methods:

Block access means that the disk is divided
into normally equal-sized blocks which are
accessed at random by the operating system.
 Physical representation

File access contains an abstraction of files
and directories which can be used to refer to
storage content.
 Logical representation
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2. Magnetic Disk Systems
 Magnetic disks are the most widely used storage media on today’s
computers.



Writing Heads
Reading Heads
Storage Media (e.g., computer disks)
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Floppy Disks
 8” – 800 KB (max.)
 5.25” – 1.2 MB (max.)
 3.5” – 1.44 MB (max.)
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Floppy Disk Drives
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Tracks, Sectors, and Clusters
 Tracks

Narrow rings which the read/write head
encodes with 0s and 1s when it writes
data and programs to the disk. Each track
has a number.
 In order to work on a particular computer
system, floppy disks must be formatted.

Formatting divides the disk surface into
pie-shaped sectors, typical 512 bytes per
sector.
 On many PC systems, the part of a track
that crosses a fixed number of contiguous
sectors (typically, 2 ~ 8) forms a unit called
cluster or disk block.

A cluster is the smallest addressable area
on a disk.
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Hard Disks
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Cylinders
 On a hard disk, a cylinder is made of all the tracks of
the same number from all the metal disks that make
up the "hard disk".
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Disk Performance - 1
 There are three primary factors that determine hard drive performance:

Seek time



Rotational latency



A measure of the speed with which the drive can position its read/write
heads over any particular data track.
The standard way to measure seek time is to time a large number of disk
accesses to random locations, subtract the latency (see below) and take the
mean.
The time that elapses between the moment when the read/write head settles
over the desired data track and the moment when the first byte of the
required data appears under the head.
On average, latency is always equal to one half of the rotational period.
Internal data transfer rate

The speed with which the drive's internal read channel can transfer data
from the magnetic media.
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Disk Performance - 2
 Subsidiary performance factors include:

Access time


The external data rate


The sum of the seek time and the
latency.
The speed with which the drive can
transfer data from its buffer to the
host computer system.
Command overhead

The time it takes the drive
electronics to interpret instructions
from the host computer and issue
commands to the read/write
mechanism. In modern drives it is
negligible.
國立聯合大學電子工程學系 – 計算機概論 – 蕭裕弘
In some text books, disk
access time is the sum of
the seek time, the rotational
latency, and the data
movement time (the time
required for the system to
read the data from the disk
and transfer it to the RAM,
or to transfer the data to be
written to the disk from
RAM and then store it on
the disk).
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Disk Standards
 PC hard disk systems can use several different interface
standards which determine performance characteristics:

The density with which data can be packed onto the disk.

The speed of disk access.

How large the disk can be.

The way the disk drive interface with other hardware.
 Three standards dominate the market:

Advanced Technology Attachment/Integrated Drive Electronics
(ATA/IDE)

Small computer system interface (SCSI)

SATA
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Disk Standards – ATA/IDE
 Common versions of ATA/IDE:
Version
Max. Transfer
Rate
Version
Max. Transfer
Rate
ATA
4 MBytes/Sec
ATA/66
66 Mbytes/Sec
ATA-2
16 MBytes/Sec
ATA/100
100 Mbytes/Sec
Ultra-ATA
33 Mbytes/Sec
ATA/133
133 Mbytes/Sec
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Disk Standards – SCSI
 The mainstream implementations of SCSI (in chronological
order) are as follows, using common parlance:

SCSI-1: 5 MB/Sec max.

SCSI-2: 20 MB/Sec max.

SCSI-3: 40 MB/Sec max.

Ultra-2 SCSI: 80 MB/Sec max.

Ultra-3 SCSI: 160 MB/Sec max.

Ultra-320 SCSI : 320 MB/Sec max.

Ultra-640 SCSI : 640 MB/Sec max.
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SCSI vs. EIDE
 SCSI offers more performance than EIDE but at a price.
 Termination is more complex but expansion not too
difficult.
 Having more than 4 (or in some cases 2) IDE drives can
be complicated, with wide SCSI you can have up to 15
per adapter. Some SCSI host adapters have several
channels thereby multiplying the number of possible
drives even further.
 For SCSI you have to dedicate one IRQ per host adapter
SCSI
EIDE
which can control up to 15 drives. With EIDE you need
one IRQ for each channel (which can connect up to 2
disks, master and slave) which can cause conflict.
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Disk Standards – SATA
 SATA has at least three main advantages
over its predecessor, namely speed,
cable management and hot-swappability.
 Initially Serial ATA was released at 150
megabytes per second but it is designed
to scale up quite substantially from there.
 Serial ATA II will double throughput to
300 MB/s and then 600 MB/s is planned
for around 2007.
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Disk Partitioning
 Disk partitioning is
the creation of
logical divisions
upon a hard disk
drive that allows one
to apply operating
system-specific
logical formatting.
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RAID
 A Redundant Array of Independent Disks (RAID) is a system of using
multiple hard drives for sharing or replicating data among the drives.
 The benefit of RAID is increased data integrity, fault-tolerance and/or
performance, over using drives singularly.
Disk mirroring
Disk striping
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3. Optical Discs
 An optical disc is a flat, circular, plastic disc
coated with material on which bits may be
stored in the form of highly reflective areas
and significantly less reflective areas, from
which the stored data may be read when
illuminated with a narrow-beam source, such
as a laser diode.

Compact disc

DVD
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Compact Discs
 A compact disc (or CD) is an optical disc used for storing
digital data. It was originally invented for digital audio
and is also used as a data storage device, a CD-ROM.
 The compact disc was developed in 1979 by Philips and
Sony.
 A 120mm disc can store about 74 minutes of music or
about 650 megabytes of data.

Discs that can store about 700 megabytes (80 minutes of
music) have become more common however.
 The mini-CD is 80mm in diameter, holds about 140MB
of data or 21 minutes of audio, and has the exact same
data format as the larger one.
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CD-ROM 的規格
 資料傳輸速度

1X: 150 Kbytes/Sec

52X: 52 * 150 Kbytes/Sec = 7,800 Kbytes/Sec
 面板播放

可直接由光碟機面板播放音樂光碟
 進片方式

吸片式、拖盤式 (輔助卡榫)
 連接介面

IDE/ATAPI 、SCSI
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Recordability of CD
 Compact discs cannot be easily recorded, as they are
manufactured by etching a glass plate and using that plate to
press metal.
 There are also CD-recordable discs, which can be recorded
by a laser beam using a CD-R writer, and can be played on
most compact disc players.

CD-R recordings are permanent and cannot be recorded
more than once, so the process is also called "burning" a CD.
 CD-RW is a medium that allows multiple recordings on the
same disc over and over again.

A CD-RW does not have as great a difference in the
reflectivity of lands and bumps as a pressed CD or a CD-R,
so many CD audio players cannot read CD-RW discs.
W Speed*R/W Speed*R Speed
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DVD
 DVD is an optical disc storage media format that is used
for playback of movies with high video and sound quality
and for storing data.
 A DVD can contain:

DVD-Video (containing movies (video and sound))

DVD-Audio (containing high-definition sound)

DVD-Data (containing data)
 The disc medium can be:

DVD-ROM (read only, manufactured by a press)

DVD+R/RW (R=Recordable once, RW = ReWritable)

DVD-R/RW

DVD-RAM (random access rewritable)
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DVD Discs
 The disc may have one or two sides, and one or two layers of
data per side; the number of sides and layers determines the
disc capacity:

DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38
gibibytes (GiB)

DVD-9: single sided, double layer, 8.5 GB (7.95 GiB)

DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)

DVD-14: double sided, double layer on one side, single layer on
other, 13.2 GB (12.3 GiB)

DVD-18: double sided, double layer on both sides, 17.1 GB (15.9
GiB)
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Magneto-Optical (M-O) Discs
 Magneto-Optical disc is an optical disc format
that uses a combination of optical and magnetic
technologies.
 The sizes of discs are usually 3.5 inch or 5.25
inch, and disk capacities are usually one of
128MB/230MB/540MB/640MB/1.3GB/2.6GB.
 A special Magneto-optical drive is required to
read these discs.
 During writing, the magneto-optical disc needs
to be erased, written, and then verified, which is
a time-consuming process. Eventually, a
technology called direct overwrite improved
perfomance by eliminating the erase step.
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4. Flash Memory Cards
 Flash memory is a form of EEPROM that allows multiple
memory locations to be erased or written in one
programming operation.


Normal EEPROM only allows one location at a time to
be erased or written, meaning that flash can operate at
higher effective speeds when the systems using it read
and write to different locations at the same time.
All types of flash memory and EEPROM wear out after
a certain number of erase operations.
 Types of flash memory cards:






Memory Stick (MS)
CompactFlash (CF)
SmartMedia (SM)
Secure Digital (SD)
Multi Media Card (MMC)
xD-Picture (xD)
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Flash Memory Drivers
 Flash memory drivers are referred to as
solid-state drives, because they have no
moving parts.


More resistant to shock and vibration than
conventional drivers.
They also have a longer expected life than
magnetic drivers.
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5. Magnetic Tape Systems
 Magnetic tape is an information storage
medium consisting of a magnetizable coating
on a thin plastic strip.
 Magnetic tape was first used to record data in
1951 on the UNIVAC I.
 Tape remains a viable alternative to disk due
to its higher bit density and lower cost per bit.
Tape has historically offered enough
advantage in these two areas above disk
storage to make it a viable product.
 Because of tape’s sequential-access property,
it is used primarily today to back up the
contents of other storage systems.
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