Presented by: Manjeet Singh

Secondary Storage Devices

• Magnetic media• Tape• Disks

• Optical Media• Compact Discs

• CD-R, WORM (Write Once, Read Many)• CD-RW

• DVD• DVD-R• DVD-RW

Classification og secondary storage devicesSecondary storage devices allow us to store the

information and program permanently.Ther are two methods of accessing data from the secondary storage devices

SEQUENTIAL:Sequential access means the computer system must search the storage devices from the beginning until it finds the required piece of data .This data access method is less expensive than other method because it uses magnetic tape,which is cheaper than disk.The disadvantage of sequential organisation is that searching for data is slow.

Direct: computer can go directly to the information that the user wants .There is no restriction on the order of reading or writing indirect access method .

Magnetic Tape

• What is magnetic tape?• Thin layer of material capable of storing a magnetic signal

• Usually contains Iron Oxide

• Protected by backing layer called the “Substrate”

Examples of Magnetic Media

• Some you are probably familiar with:• Cassette tapes• VHS video tape• Computer Tape• 8-track• DAT

Magnetic Tape Uses Today

• Today, tape based systems are usually used for backup purposes only.

• Tape still provides the most cost effective method of storing larger amounts of backup information

• Reliable as long as temperature is kept low and humidity conditions are kept right.

• Although hard disks are increasingly used as a backup mechanism, tapes are portable.

• Backups can be taken off-site for greater risk prevention

Advantages of magnetic tapeMagnetic tape holds high data recording

density.They have unlimited storage capacityas

manny tapes can be used to store large amount of data.

These steps are easily transportable because they are compact in size and light weight.They are also easy to handle and store.

Disadvantages of magnetic tapeMagnetic tape should be kept in a dust free

environment any away from corrosive gases or chemicals as they can cause tape reading errors.

They are not flexible when file updating requires records deletion.

Data transmission is slow as compared to magnetic disk.

Magnetic diskMagnetic disk are the most widely used and

popular storage medium for direct access secondary storage.They offer high storage capacity and have the capability to access the stored data directly.The surface of a disk is divided into tracks and sectors .Multiple disks are maintained and used together to create large disk storage system.

Looking at a surface

Surface of disk showing tracks and sectors

sector

tracks

Cylinders

SEEK — controller advances read/write head to proper track

LATENCY — waits for proper sector to rotate under head

READ/WRITE — disk head scans the sector for read or write

Direct Access

Types of magnetic disk

Hard Disks

• Today, most people use Hard Disks for secondary storage

• The basic technology used in hard disks is similar to that of magnetic tape

• Magnetic material is layered onto a high-precision aluminum disk• The disk head can move to any point on the platter almost instantly compared to tape• With tape, the head touches the tape. With disks, the head never touches the platter• Tape moves at approx 5 cm/s. Disk platters move at up to 7500 cm/s (272 km/h!)

Hard Disks

• To increase capacity, a hard disk will usually contain several platters

Hard Disks

• The heads never touch the platters, but they are very close. This makes hard disks susceptible to mechanical shock.

Storing Information on a Hard Disk

• Each platter is broken up into tracks and sectors• Tracks are concentric circles on the disk• Each track is broken up into a series of sectors

Track (yellow ring)

Sector (yellow ring between the lines)

Floppy Disks

• Floppy disks are similar to hard disks

• Because the medium is “floppy”, the disks cannot operate at the same speeds as a hard disk.

• Floppy disks are older technology which haven’t received a lot of attention since the late 1980s.

• Their capacity hasn’t increased much

• Many computers today do not even come with floppy disk drives anymore.

Optical disk• Optical disks are very much like hard disks

• Hard disks store information using magnetic material

• Bits are stored by changing the magnetic properties of the magnetic material• Bits are read by picking up the tiny magnetic field with a read head

• Optical disks store information as pits in a physical medium

• A laser is used to determine if a pit is present or not.

Optical Disc StorageDesign difference

Magnetic disk Concentric tracks of

sectors Spins at constant

angular velocity (CAV)

Wastes storage space but fast data retrieval

CD ROM

• CD Roms use the same technology as audio Compact Discs.

• A master disc is created.• Copies of the disc are created through a pressing process• The discs are aluminum sandwiched between plastic• CDs are single sided.

Label

Acrylic

Aluminum

Plastic

Advantages of optical diskOptical disk posses large capacity to store data or

information in the form of multimedia , graphics and video files.

They can store more data in less amount of space as compared to magnetic tape and floppy or zip disks.

Optical disk hold more data recording densityThe magnetic fields do not affect optical diskAn optical disk is much tougher than tapes and

floppy diskDue to their small size and light weight .These disk

are easily portable.

Disadvantage of optical diskOptical disk is comparatively costlier than

storage devices, floppy or zip disk .It is not as easy to copy an optical disk as ,it

is to a floppy disk.They possess slow data access speed as

compared to magnetic disk.Optical disk is more complicated than floppy

disk or magnetic disk.

Magneto-Optical StorageCombines magnetic and optical disk

technologyMagnetic disk comparison

Reads and writes similarlyMagneto-optical (MO) disks store several GBAccess rate

Faster than floppy Slower than hard drive

Hardier than optical discs