Secondary Storage Rohit Khokher

1. Secondary Storage RohitKhokher

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 Floppy Disk Hard Disk CD-ROM WORM Disk Disk Pack Winchester Disk ZIP 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. 7 Track;6-bit BCD; Even parity (all tracks must have even number of 1) A frame Track/Channel numbers Parity bit zone Numeric 6-bit BCD for letter A (Table 4.2 page 36 Even parity (all tracks must have even number of 1) Error Checking

6. 9 Track;8-bit EBCDIC; Even parity (all tracks must have even number of 1) EBCDIC for the letter A Track/Channel numbers A frame Error Checking Odd parity (all tracks must have odd number of 1) 8-bit EBCDIC for letter A (Table 4.3 page 8

7. Terminologies Record: Group of characters separated by blank Inter record gap (IRG) Blocks: Records are grouped into blocks Blocking factor: # of records perblocks Inter Block gap (IBG) 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 Generally 8 bit EBCDIC code is used to record the data items. Surface: 2 surfaces per disk. Tracks:000-199. 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 surfacex # 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 #) A platter coated with iron oxide: A magnetizable material

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

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: 100-300 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 Smaller Capacity, faster access time and higher cost Larger Capacity, slower access time and lower cost Main Memory Hard Disk Magnetic tape, Floppy, … Mass Storage Memory Hierarchy