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

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

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

Types of Storage Devices

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

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)

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.

Tracks

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

Sectors

Sectors

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.

Sectors and Tracks

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

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.

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

Sectors and Tracks

Platters Platters rotate at high speed Usually 5400 to 15000 revolutions per minute (rpm)

Hard Disk Contamination

What is meaning of Volatile

Storage Capacity

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

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

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.

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

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

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

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

Enterprise Storage RAID (Redundant Array of Independent Disks)

Enterprise Storage NAS (Network Attached Storage) A Server

Enterprise Storage SAN (Storage Area Network) A Network

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