secondary storage structure

Secondary storage

structure

Types of storage

Random access storage

Serial access storage – example: Magnetic tapes

Direct access storage – example :Magnetic disks

Kinds of devices

Character device

Block device

Magnetic tapes

Long tape coated with magnetic material

Consists of 9 tracks

TRACKS

Magnetic tapes Read-write head A byte of data is stored laterally Data is stored and accessed as records

TRACKS

Read write head

record

Floppy disk

Two component address: Track number and sector number

TRACK

SECTOR

Floppy diskRead-write head:

Magnetic disk

Comparison between magnetic disks and tapes

Disk attachment Host- attached storageExample: hard disk drives , CD,DVD

Network-attached storage

Storage area network Uses storage protocols instead of network protocols. Connects servers and storage units

Disk Scheduling Algorithms

Seek time and rotational latency: the time necessary to move the disk arm to the desired track, called

the seek time The time necessary for the desired sector to rotate to the disk head,

called the rotational latency. ROTATIONAL LATENCY

First come First serve (FCFS):

The request that comes first are serviced first

4585

14685

108110

592

Total tracks traversed = 45+85+146+85+108+110+59+2= 640

Shortest Seek Time First (SSTF):

The request near the current head position is serviced first

12

3023

59

8424

2

2

Total tracks traversed = 12+2+30+23+84+24+2+59= 236

SCAN algorithm: (Elevator algorithm) The read-write head moves in a particular

direction and services all the requests along that direction

Traverses till the last track and then reverses its direction and services the remaining requests

16

1423

59

65

3124

2

2

Does not change direction at 14th track.Reaches 0th track and then reverses.

Look algorithm

Does not traverse till 0th track

C-SCAN algorithm: (Circular scan)

12

31

2

59

23

24

2

14

16Does not change direction at 183rd track.Reaches 199th track and then reaches 0th track.

C-LOOK algorithm:

12

31

2

59

23

24

169

2 Does not traverse till 199th track. Returns to 14th track (not 0th track) after 183rd track