Secondary Storage

Key Concepts:

1.    Introduction

2.    Storage

3.    Floppy Disks

a)    Traditional Floppy Disk

b)    High Capacity Floppy Disks

(1)  Zip disks

(2)  HiFD disks

(3)  SuperDisk disks

4.    Hard Disks

a)    Internal hard disk

b)    Hard-disk Cartridge or External Hard Disk

5.    Optical Disks

a)    Compact Discs

(1)  CD-ROM

(2)  CD-R

(3)  CD-RW

(4)  Photo CD

b)    Digital Versatile Discs

c)     DataPlay

6.    Other Types

a)    Solid-State

b)    Magnetic Tape

Introduction

§   Secondary storage devices are used to save, to back up, and even to transport files consisting of data or programs from one location or computer to another.

§   The need for storage continues to grow due to higher demands of users to store more digital media such as videos, music, and images.

§   Data is stored on secondary storage in digital or machine code, so it doesn’t need to be translated from the 1’s & 0’s when it is sent to the CPU for processing.

Storage

·        RAM (Random Access Memory) is called primary storage since it is used directly by the CPU for processing data and program instructions.

·        RAM is volatile or temporary storage (once the power is turned off, the contents are lost).

·        Secondary storage provides permanent or non-volatile storage.

·        Secondary storage devices read and write the data onto the storage medium.

·        Reading is the process of retrieving/accessing the data.

·        Writing is the process of storing/saving the data.

·        Important characteristics of secondary storage include:

o   Media or medium: the physical material that holds the data

o   Capacity: measures how much the media can store, typically measured in MB, GB, and TB

o   Storage devices: hardware that reads (and often writes) to storage media

o   Access speed (access time): measures the amount of time to read and/or write to the storage medium

Floppy Disks

§   Floppy disks (floppies, diskettes, disks or flexible disks) are portable, removable storage media.

§   The use flat circular pieces of Mylar plastic coated with a magnetic material to store data.

§   Floppy Disk Drives (FDD) store/retrieve data by magnetizing spots according to an encoding scheme such as ASCII, EBCDIC, or Unicode.

§   To write data, current is sent through a coil in the head as the media rotates. The head's magnetic field aligns the magnetic particles directly below the head on the media. When the current is reversed the particles align in the opposite direction encoding the data digitally. To read data, the magnetic particles in the media induce a tiny voltage in the head coil as they pass under it. This small signal is amplified and sent to the floppy disk controller, which converts the streams of pulses from the media into data, checks it for errors, and sends it to the host computer system.

Traditional Floppy Disk

§   Traditional disk is the 1.44 MB 3 ½” disk, introduced in the 1980’s

§   2HD disks are “two sided, high density”

§   Density refers to how tightly the bits can be packed on the medium

§   A Shutter slides to provide access to the plastic medium

§   Labels can be applied to the external surface of the disk to identify the contents

§   A Write-Protection notch can be moved to protect the disk from accidentally writing over it.

§   Floppies store the data in a series of Tracks and Sectors – each sector can store up to 512 bytes or characters.

High Capacity Floppy Disks

High Capacity Floppy Disks (floppy-disk cartridges) have capacities of much higher than traditional floppies.  Three leading types include:

§   Zip Disks (sold by Iomega) have 100, 250, or 750 MB capacities and connect to the PC via USB cable.  The disks are slightly thicker than traditional floppies, so they require special disk drives.

§   HiFD disks (from Sony Corporation) have 200 MB or 720 MB capacities.  The main advantage is the drives can also read traditional 1.44 MB floppies.

§   SuperDisks (from Imation) have a 120 MB or 240 MB capacity, and the drives can also read traditional 1.44 MB floppies.

Hard Disks

§   Hard disks save files by altering magnetic charges of the disk’s surface

§   Hard disks use a thicker, rigid metallic platter for the base medium

• Data on the disk surface is recorded on tracks which form concentric circles on the disk.
• Each disk has a set number of tracks. Depending on type of disk drive, there may be from 100 to over 10,000 tracks on the disk.
• Tracks with the same number on each disk surface or platter form a cylinder.
• The disks spin at a fixed speed, typically at 3600 rpm (IDE) or 7500 -15,000 rpm(rotations per minute)

 

• Tracks on a disk are organized into sectors.
• To get to a particular piece of data on the disk a track number and a sector number are needed.
• Data is read/written when the required sector on the track rotates into position under the read/write head.
• The time taken to position the head over the correct track/cylinder is called the seek time.
• The time needed for the sector to arrive  is called the latency time.
• Creating the magnetic tracks on a previously blank disk is called formatting the disk.
• Formatting destroys any data that might have been on the disk previously.
• Typical data access times for modern hard disks (i.e. latency + seek) are about 10-15 milliseconds

Internal Hard Disk

§   Also known as a fixed disk

§   Located inside the system unit or chassis

§   Typically mapped as the “C:” drive

§   Advantages are speed and capacity: a 100 GB HD can hold as much as 70,000 traditional 1.44 MB floppies = (100 * 2^30) / 1,440,000

§   Access speeds are measured in milliseconds (ms) e.g. 10 ms

§   Disk rotation speeds are measured in RPM (rotations per minute) e.g. 5,400 RPM

Hard-Disk Cartridges (External Hard Disk)

§   Also known as removable hard disks

§   Limited only by the number of cartridges you use

§   Cartridges typically hold 10-40 GB of storage

§   PC Card Hard disks are credit card sized hard-disk cartridges

§   Examples include IBM’s Microdrive and Toshiba’s MK5002 drives which hold around 5 GB and are typically used on laptops

     Optical Disks

§   Optical disks can hold close to 17 GB of data – enough to store over several million typewritten pages or a medium sized library on a single disk.

§   Optical disks use reflected light rather than magnetized spots.

§   Binary 1’s and 0’s are represented by flat areas called “lands” and bumpy areas called “pits”

§   Unlike hard disks that have concentric tracks, optical disks have a single spiral track that is divided into equally sized sectors for storing data.

§   The most common sized optical disk is 4 ½ inches, and typically stored in a plastic “jewel box”

Compact Disc (CD)

§   One of the most widely used optical formats

§   Typically store 650 MB to 1 GB (1,000 MB) on one side of a CD

§   Rotational speed determines how fast data can be transferred to the CPU

§   24X (24 speed) CD can transfer data at 3.6 MB per second

§   48X (48 speed) CD can transfer data at 7.2 MB per second

c)     CD-ROM

§   Compact Disc – Read Only Memory is similar to a commercial music CD

§   RO means it can not be written over by the user

§   Typically used to deliver large databases, references, or software applications

d)    CD-R

§   Compact Disc – Recordable: write once, read many

§   CD burners typically use these to archive data or record music

e)     CD-RW

§   Compact Disc – ReWritable: write many, read many

§   Used to create and edit multimedia presentations

§   Typically cost a little more than CD-R

Digital Versatile/Video Disc (DVD)

§   A newer format that is replacing CD optical disks

§   DVD’s can store 4.7 GB to 17 GB on a single disk

f)      DVD-ROM

§   Written at manufacturing plant, read many

§   Typically used for video distribution

g)    DVD-R DVD+R

§   Write once, read many

§   Tend to cost more than CD writable disks

§   Used for archiving data and writing video files

h)    DVD-RW DVD+RW DVD-RAM

§   Write many, read many

§   Still working on setting a standard format

DataPlay

An optical write once, read many format like CD-R

§   A smaller disk size (about the size of a quarter)

§   Capacity only 500 MB

§   Music industry liked it because the format is harder to copy

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 Other Types of Secondary Storage

Solid-state storage

§   These devices have no moving parts, so they are fast and reliable

§   Tends to have less capacity, and costs more per byte

§   Flash memory cards are used in notebook computers and digital cameras

§   Key chain hard drives (aka key chain flash memory devices) typically connect through a USB port, and can store up to 1 GB for easy, portable storage

Magnetic Tape

• Data is stored on tracks on the tape - most tapes have 9 tracks.
• Bytes of data (i.e. 8 bits) are stored across the width of the tape.
• The 9th bit is a parity bit. It helps to detect storage errors. Two types of parity can be used.
• With even parity, the parity bit is set so that the group of 9 bits has an even number of `1' bits.
• With odd parity, the parity bit is set so that the group of 9 bits has an odd number of `1' bits.
• Data is stored at different densities (e.g. 1600 and 6250 bpi, where ``bpi'' means ``bytes per inch'').
• There are several varieties of magnetic tape:

- 14" reels of 1/2" wide tape (2400' long)

- Cartridges of 1/4" tape

- Cassettes (very similar to audio cassettes)

- Digital Audio Tape (DAT).  

§   Tapes only provide sequential access, where disk system provide either sequential or direct access

§   Advantage with tape is virtually unlimited storage (just add another tape), it’s reliable, and it’s inexpensive per MB stored.

§   Disadvantage is it’s somewhat slow, and limited to sequential access

§   Often used to back up disk storage, especially for networked systems