Lecture No 11 Storage Devices. Storage Involves Two Processes Writing data Reading data.

Lecture No 11

Storage Devices

Storage Involves Two Processes

Writing data

Reading data

Storage Media

The materials on which data is stored.

The physical material on which data is stored is called a storage medium.

E.g. Diskette is a storage medium.

Storage Devices

The hardware that writes data to or reads data from a storage medium is called a storage device.

E.g. A floppy disk drive is a storage device.

Two Categories ofStorage Technology

Magnetic storage

Optical storage

Magnetic Storage

Diskettes

Hard disks

High-capacity floppy disks

Magnetic tape

Optical Storage Compact Disk Read-Only Memory (CD-ROM)

CD-Recordable (CD-R)CD-Rewritable (CD-RW)

Digital Video Disk Read-Only Memory (DVD-ROM)

DVD Recordable (DVD-R/DVD), DVD Rewritable (DVD-RW)

Photo CD

Magnetism Allows Data Storage

Hard disks, diskettes, high-capacity floppy disks and

tapes have a magnetic coating on their surface that

enables each medium to store data.

Magnetic Storage DevicesHow Magnetic Storage Works

A magnetic disk's medium contains iron particles, which can be polarized when given a magnetic charge in one of two directions.

Each particle's direction represents a 1 (on) or 0 (off), representing each bit of data that the CPU can recognize.

A disk drive uses read/write heads containing electromagnets to create magnetic charges on the medium.

Random particles(no data stored)

Current flow(write operation)

Organized particles(represent data)

Medium

Write head

As the mediumrotates, the headwrites the data

Preparing Disks for Storage

Before a magnetic disk can be used, it must be formatted—a process that maps the disk's surface and determines how data will be stored.

During formatting, the drive creates circular tracks around the disk's surface, then divides each track into sectors.

Formatted Disks Have Three Sections

Main sections, called Tracks

Track subsections, called Sectors

Groups of sectors, called Clusters

Logical Format

A logical format is the labeling of tracks and

sectors.

The Logical Format has Four Disk Areas

Master boot record File allocation table Root folder Data area

Master Boot Record (MBR)

This program determines whether the disk contains the basic components of an operating system necessary to run successfully.

Boot sector:

It stores the master boot record (MBR), a small program that runs when you first start (boot) the computer.

File Allocation Table (FAT)

A log created during the logical formatting process

that records the location of each file and status of

each sector on the disk.

Root Folder

The top folder or directory in the disk’s folder hierarchy.

Enables the user to store data on the disk in a logical way.

Data Area

Place where data or program files are actually

stored.

3.5-inch Diskettes (Floppy Disks)

Diskettes are used to transfer files between computers as a backup medium.

Spin rate: 300 revolutions per minute (rpm)

Disk Density=Total no of Sectors * Total no of bytes that each sector can hold.

Storage Capacity of HD disks is 1.44 MB

Hard Disks

Spin rate: from 3,600 to 15,000 rpm

Storage capacity ranges from several hundred MB to more than 200 GB

High rotational speed allows more data to be recorded. Why????

Tape Drives

Commonly used for hard disk backup.

Sequential Access Drives

PC Cards

Used to connect new components like memory and expanding storage capacity to a computer.

Up to 512MB memory.

Two Ways to Measure Storage Device Performance

Average Access Time

Data Transfer Rate

Average Access Time

Amount of time the device takes to move its read/write heads to any spot on the medium.

Measurement should be an Average Time.

Important measure of performance for storage devices and memory.

Access Time Measured in Two Ways

Storage devices: milliseconds (ms)

Memory devices: nanoseconds (ns)

Examples of Average Access Time

Floppy Disk: 100 ms

Hard disk: 6 to 12 ms

High-performance hard disk: 4 or 5 ms

CD-ROM: 80 to 800 ms

Tape drives: slowest at a few seconds to a few minutes

Data Transfer Rate

Amount of time it takes for one device to transfer data to another device.

Measured in units of data per second Note: MBps= Megabytes/sec, Mbps =megabits/sec

8bits = 1 byte

Hard drives are the fastestCD-ROM’s, slow

Floppies, the slowest

Optimizing Disk Performance

When the PC’s performance slows down, disk maintenance, or disk optimization,

can speed things up again

File or Data Compression

Technology for making files smaller to free up storage space.

File compression enables to store more data on a disk effectively increasing the disk’s capacity.

File Compression is performed by a software.

Utilities such as Windows' DriveSpace enable you to compress the entire contents of your hard disk.

File Compression Software

PKZIP and WinZip: Windows-based PC’s

StuffIT: Macintosh

Ways to Optimize Disk Performance

Delete temporary files

Uninstall little-used software programs

Run a disk scanner to look for and fix errors

Defragment the disk.

Drive-Interface Standards Another factor to determine how quickly a drive can

read and write data is the Type Of Controller that driver uses.

Storage Devices need a controller to act as an interface between the Drive and CPU.

Personal Computers use one of two drive-interface standards for built-in disk drives: EIDE or SCSI.

Other type of interface (USB & FireWire) to attach additional disk drives and other devices to a computer.

Drive-Interface Standards

EIDE -- Enhanced Integrated Drive Electronics

SCSI – Small Computer System Interface

USB – Universal Serial Bus

FireWire – IEEE 1394

EIDE

Most new computer systems use EIDE drive-interface standard.

Latest version supports data transfer rates of 66MBps.

Variants: Fast IDE, ATA, Fast ATA, Ultra-ATA, ATA 66 etc.

SCSI Earlier developed to connect third-party peripheral

devices to mainframe computers.

Allows high data transfer rates than EIDE.

Supports 80MBps.

Normally found in Servers, Workstations etc.

USB & FireWire

All sort of peripheral devices can be attached to a computer through USB or FireWire port.

Support High Data transfer rates.