Define the term access time for a hard disk. Explain with the help of an example, how access time and storage capacity for magnetic disks are calculated. How is the storage capacity for a magnetic tape calculated? What are the differences in access mechanism for magnetic tapes and magnetic disks?
Answers
Answer:
Difference Between Magnetic Tape and Magnetic Disk
January 17, 2017 Leave a Comment
Magnetic Tape Vs Magnetic DiskMagnetic tape and magnetic disk both stores the data magnetically. The surface of a magnetic tape and the surface of a magnetic disk are covered with a magnetic material which helps in storing the information magnetically. Both are non-volatile storage. Despite these similarities both differs in many aspects from their appearance to their working, their cost and much more.
The basic difference between magnetic tape and magnetic disk is that magnetic tape is used for backups whereas, magnetic disk are used as secondary storage. Let us discuss some more differences between magnetic tape and magnetic disk with the help of comparison chart shown below.
Content: Magnetic Tape Vs Magnetic Disk
Comparison Chart
Definition
Key Differences
Conclusion
Comparison Chart
BASIS FOR COMPARISON MAGNETIC TAPE MAGNETIC DISK
Basic Used for backup, and storage of less frequently used information. Used as a secondary storage.
Physical Plastic thin, long, narrow strip coated with magnetic material. Several platters arranged above each other to form a cylinder, each platter has a read-write head.
Use Idle for sequential access. Idle for random access.
Access Slower in data accessing. Fast in data accessing.
Update Once data is fed, it can't be updated. Data can be updated.
Data loss If the tape is damaged, the data is lost. In a case of a head crash, the data is lost.
Storage Typically stores from 20 GB to 200 GB. From Several hundred GB to Terabytes.
Expense Magnetic tapes are less expensive. Magnetic disk is more expensive.
Definition of Magnetic Tape
Magnetic Tapes were introduced in 1928, earlier used as a secondary storage medium. Magnetic tape is a thin long narrow plastic strip coated with the magnetizable substance. The tape is wounded over a spool, and it is wounded or unwounded past a read-write head to read from or write data to the tape.Magnetic TapeMagnetic Tapes are nonvolatile in nature and hence it holds the large quantity of data permanently. The magnetic tapes store the data sequentially. The random access to magnetic tapes takes more time than magnetic disk because the magnetic tape has to perform forward and rewind operation to locate a correct spot.
Once the read-write head in magnetic tape is positioned correctly, it writes the data with the same speed as that of the magnetic disk. The data transfer speed of the magnetic tape is similar to the magnetic disk. As random access to magnetic tapes is comparatively slower, it was not very useful for secondary storage. Now, the magnetic tapes are used for backups, for storing the data that is less frequently used.
Magnetic tapes are also used in supercomputer centres for holding the large volume of data that is used for scientific research.
Definition of Magnetic Disk
In modern computers, Magnetic Disk is used for secondary storage. Like Magnetic tape, the magnetic disk is also a non-volatile so, its stores the data permanently. The magnetic disk has several flat circular shaped platters which appear like a CD.The diameter of each platter ranges from 1.8 to 5.25 inches.
Explanation:
Hard Disk Access time is the total elapsed time between the initiation of a particular request for data and receipt of the first bit of that data. Direct access devices (Hard Disk Drive) require varying times to position a disk head over a particular record.Execution of a magnetic disk operation involves:
Wait time: The process waits to be granted device access.
Wait for device: Time the request spend in wait queue.
Wait for channel: Time until a shared 1/0 channel is available.
Positioning time: Time hardware need to position the head. Sometimes called the random access time: Consists of the time to move the disk arm to the desired cylinder, called the seek time Ts (in milliseconds), The time for the desired sector to rotate to the disk head, called the rotational latency (in milliseconds). Rotational speed, r, of 5000 to 10000 rpm.
Transfer rate is the rate at which data flow between the drive and the computer (megabytes of data per second). Transfer time: to transfer b bytes, with N bytes per track
Magnetic Tapes and Magnetic Disks are the types of magnetic memory. Both are called non-volatile storage and used for store data.
Magnetic tape contains thin plastic ribbon is used for storing data. It is a sequential access memory. So the data reas/write speed is slower. It is mainly used for data backups.
Magnetic Disk contains circular disk made of metal or plastic. Both side of disk is usually used for storing data. The disk is coated by magnetic oxide. The disk is divided into multiple concentric circles known as track and tracks are divided into sectors in which data are stored.
