1. Introduction to Computing
Lecture-3-Storage Devices
Muhammad Hafeez
Department of Computer Science
GC University Lahore

2. Today’s Agenda Storage Devices Working of Storage Devices
Working of Hard Disk Drives
How Files are stored

3. Storage Devices
In Modern computer billions trillions bytes of data is stored
Secondary Memory or Auxiliary Memory supports in storage of large amount of data/ information for future use.
It is non-volatile but slow memory.
Data Reading/ Writing takes time
Data do no loss when power turns off
Inexpensive than Primary Memory

4. Types of Storage Devices

5. Types of Input Devices Sequential Access Devices
Data is accessed in linear/ sequential manner
Slower but cheap
Example: Magnetic Tape
Random/ Direct Access Devices
Data is accessed in constant/ random/ equal time
Most Widely used devices for secondary storage
Example: Hard Disk Drives, Optical Disks, Memory Storage Devices

6. Magnetic Disks A thin, circular plate/ platter of metal or plastic.
Both sides are coated with magnetic material (iron oxide)
Use Standard code to store data
Several of these platters are protected in sealed, contaminated specially designed envelope
Data is stored as tiny magnetized and non-magnetized spots (both for 1 and 0)

7. Tracks and Sectors
HDD is divided number of invisible concentric circles called tracks
Outmost track is 0 and tracks are numbers
Tracks on a platter range from 40 to several thousands depending on capacity of disk.

8. Tracks

9. Sectors A Track is further divided into pi shaped sectors
8 or more sectors per track
Sectors contain 512 bytes
HDDs are designed to Read/Write a complete sector at a time

10. Sectors

11. Sectors

12. Sectors and Tracks Usually sectors are measured per track
Hence, a 200 tracks and 8 sectors disk has total 200x8 sectors
Each sector is uniquely numbered
Data address on disk is a physical address composed of sector number, track number and surface number
The scheme is called CHS (Cylinder-Head Sector) addressing. The same is called disk geometry.

13. Sectors and Tracks

14. Number of Sectors/ Track Varies to Increase Storage
Innermost track has 8 sectors per track
Next has 9 sectors per track
Outer group of track has 10 sectors per track

15. Disk Formatting
Formatting is the process of dividing the disk into tracks and sectors
Track: A narrow recorded band that forms a full circle on the surface of disk
Sectors: Tracks are broken into arc-shaped area called sectors
A sector typically stores 512 bytes of data
Sometimes hard disk during formatting mark some sectors as bad sectors cause they are damaged.

16. Disk Formatting Low Level Formatting: OS-Level Formatting:
Disk drives lay down a magnetic pattern on the disk surface
Enable the disk drives to define and store data defined by disk drives of the computer
OS-Level Formatting:
Create File Access Table (FAT) that is a table for sectors and tracks of data
FAT grows with time
Scans and marks bad sectors

17. Sectors and Tracks

18. Platters Platters rotate at high speed
Usually 5400 to revolutions per minute (rpm)

19. Hard Disk Contamination

20. What is meaning of Volatile

21. Storage Capacity

22. Storage Capacity of Disk
Storage capacity of disk = Number of recording surfaces x Numbers of tracks per surface x Numbers of Sectors per track x Numbers of bytes per sector

23. Disk Access Time
Access Time: It is the time computer makes a request of data from Disk to Primary Storage and The Time this request is completed.
Disk Access Time Depends upon:
1. Seek Time
2. Latency
3. Transfer Rate

24. Disk Access Time Seek Time Latency
As the disk unit receive the Read/Write command, the time to place Read/Write head on desired track is called seek time.
Latency
It is the time to spin the desired sector under the Read/Write Head once the disk head is placed on desired track.

25. Disk Access Time Transfer Rate:
It is the time data is Read/Written from/to disk.
As the transfer rate is negligible as compared to seek time and latency
Disk Access Time = Average Seek Time x Average Latency

26. Disk Controllers
A special purpose chip/ electronic circuit that controls transfer of data to/from disk to System Bus
PC vendors advertise types of disk interfaces supported by the controller on their motherboard
In addition to USB that can function as external hard drive interface, four other types of interface for external drives are common

27. Disk Controllers Interfaces
EIDE: (Enhanced Integrated Drive Electronics) use parallel signals to transfer data to HDDs, SSDs, RAID, Optical Disks
SCSI: (Small Computer System Interface) use parallel signals, can support 8 to 15 devices
SAS: (Serial-attached SCSI) use serial signals to transfer data, more fast and reduced interference, can support more devices, lower cost
SATA: (Serial Advanced Technology Attachment) More thinner and flexible cable, less interference, fast, connect more devices

28. Cloud Storage
Provide storage on internet for readily accessible data/ information
Google OneDrive
Dropbox

29. Enterprise Storage
RAID (Redundant Array of Independent Disks)

30. Enterprise Storage
NAS (Network Attached Storage) A Server

31. Enterprise Storage
SAN (Storage Area Network) A Network

32. Questions
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