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Secondary Storage Rohit Khokher. Limitations of Primary Storage Limited Capacity: It cannot be used to store the large volumes of data items. Volatility:

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Presentation on theme: "Secondary Storage Rohit Khokher. Limitations of Primary Storage Limited Capacity: It cannot be used to store the large volumes of data items. Volatility:"— Presentation transcript:

1 Secondary Storage Rohit Khokher

2 Limitations of Primary Storage Limited Capacity: It cannot be used to store the large volumes of data items. Volatility: Looses data when electric power is turned off or interrupted. Data Portability: It cannot be used to port the data from one place to other. These limitations lead to the development of auxiliary memory or secondary storage devices.

3 Secondary Storage Devices Sequential Access Devices Direct Access Devices Magnetic Tape Magnetic Disk Optical Disk Hard Disk Floppy Disk CD-ROMWORM Disk ZIP Disk Disk Pack Winchester Disk

4 Magnetic Tape The first truly mass auxiliary storage device was the magnetic tape drive Tape drives have a major problem; can you describe it?

5 ABCDEFGHIJKLMNOP Numeric zone Parity bit Track/Channel numbers 7 Track;6-bit BCD; Even parity (all tracks must have even number of 1) A frame 6-bit BCD for letter A (Table 4.2 page 36 Even parity (all tracks must have even number of 1) Error Checking

6 ABCDEFGHIJKLMNOP Track/Channel numbers 9 Track;8-bit EBCDIC; Even parity (all tracks must have even number of 1) A frame 8-bit EBCDIC for letter A (Table 4.3 page 8 Odd parity (all tracks must have odd number of 1)Error Checking 3s digit 2s digit Added Zone Zone Parity bit Zone Units digit 4s digit EBCDIC for the letter A

7 Terminologies Inter record gap (IRG) Blocking factor: # of records perblocks Inter Block gap (IBG) Blocks: Records are grouped into blocks Record: Group of characters separated by blank Storage capacity of tape = Data recording density x Length Data transfer rate (bytes/second to main memory Tape drive Tape controller Tape Types (Half-inch tape reel, Half-inch tape cartridge, Quarter-inch streamer tape and 4mm Digital Audio Tape (DAT)

8 Advantages & Limitations of Magnetic Tape Advantages – Virtually unlimited storage capacity. – Low cost. – Easy to handle and store. – Easy to port. Limitations – The are not suitably for frequent data access. – Must be operated in dustless environment. – Must be operated in controlled environment. – Must have proper identification labels

9 Uses of Magnetic Tape Good for applications that require sequential data processing. Data backup. Data archiving. Physically large data transfer Software distribution

10 Magnetic Disks A platter coated with iron oxide: A magnetizable material Generally 8 bit EBCDIC code is used to record the data items. Surface: 2 surfaces per disk. Tracks: Sectors: 8 sectors per track. Disk pack: 200 disks. Cylinder: Each track from all recording surfaces in a disk pack form a cylinder. Storage capacity = # of surfaces × # of tracks per surface x # of sectors per track x # of bytes per sector. Access mechanism: Read/write head moves to an address by specifying the (disk #, track #, sector #, byte #)

11 Magnetic Disks Seek Time: Time to move the R/W head to the desired track. Latency or Rotational Delay Time: Time to rotate the disk to the desired sector of a track. Average Latency Time? Transfer Rate: Rate at which data to be written/read. Transfer rate = n x r /60 where n is the # of bytes per track and r is the revolution per second. Disk Formatting: Formatting prepares a disk to be used by different disk drives. Formatting creates file allocation table (FAT) that specifies the track and sector of the data items. Important: Magnetic Disks are direct or random access device because data items can be accessed by specifying its address. Disk drives with interchangeable disks. Disk drives with fixed disks. Disk Controller: Controls the movement of R/W heads, and disk rotation.

12 Magnetic Disk Floppy Disk (1.4 MB) Hard Disk Zip Bernoulli Disk (Portable/Fixed) Disk Packs Winchester Disk Magnetic Disk Type

13 Advantages & Limitations Advantages – Direct Access. – Simultaneous sharing. – On-line/Off-line access. – Unlimited disk may be added. – Low cost. – Portable – Fast Limitations – Inefficient for sequential data processing. – Sharing make data unsecured – Disk failure. – Cost of magnetic tape is lower – Must be operated in dustless environment

14 Uses of Magnetic Drives Random access applications Shared on-line secondary storage. Data backup. Data archiving Data and software transportation. Software distribution.

15 OPTICAL DISK In the optic readable CD-ROM, the data storage consists of millions of indentations burnt into light reflecting silver surface. The burnt dents reflect less light than the shiny surface.

16 Tracks The CD-ROM has only one track, a spiral winding its way from the center to the outer edge. The spiral track is ideal for reading large block of serial data.

17 Access MEchanism A weak laser beam is sent to the disk through a two-way mirror and the sensor registers the difference in light reflection from the burnt and shiny areas as zeros and ones. Access time: millisecond. CD-ROM (Compact disk read only memory). WORM (Write once read memory) R/W CD (Read/Write compact disk)

18 Advantages & Limitations Advantages – Low cost – Ideal for reading large volumes of data. – No mechanical R/W heads. – Long life data storage. – Easy to handle, store and port. Limitations – Slower than magnetic disk. – Complicated drive than magnetic disk. Uses of optical disk – Distribution of large amount of data and Mulltimedia.

19 Mass Storage Disk Array: It uses a set of magnetic disks. Automated tape library: Uses a set of magnetic tapes. CD-ROM jukebox: Uses a set of CDROM

20 Cache Memory Main Memory Hard Disk Magnetic tape, Floppy, … Mass Storage Smaller Capacity, faster access time and higher cost Larger Capacity, slower access time and lower cost Memory Hierarchy


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