secondary storage structure
TRANSCRIPT
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
