Important Short Questions and Answers: I/O System in OS

1. Define Caching.

A cache is a region of fast memory that holds copies of data. Access to the cached copy is more efficient than access to the original. Caching and buffering are distinct functions, but sometimes a region of memory can be used for both purposes.

2. Define Spooling.

A spool is a buffer that holds output for a device, such as printer, that cannot accept interleaved data streams. When an application finishes printing, the spooling system queues the corresponding spool file for output to the printer. The spooling system copies the queued spool files to the printer one at a time.

3. What are the various Disk-Scheduling Algorithms?

The various disk-scheduling algorithms are,

First Come First Served Scheduling

Shortest Seek Time First Scheduling

SCAN Scheduling

C-SCAN Scheduling

LOOK Scheduling

4. What is Low-Level Formatting?

Before a disk can store data, it must be divided into sectors that the disk controller can read and write. This process is called low-level formatting or physical formatting. Low-level formatting fills the disk with a special data structure for each sector. The data structure for a sector consists of a header, a data area, and a trailer.

5. What is the use of Boot Block?

For a computer to start running when powered up or rebooted it needs to have an initial program to run. This bootstrap program tends to be simple. It finds the operating system on the disk loads that kernel into memory and jumps to an initial address to begin the operating system execution. The full bootstrap program is stored in a partition called the boot blocks, at fixed location on the disk. A disk that has boot partition is called boot disk or system disk.

6. What is Sector Sparing?

Low-level formatting also sets aside spare sectors not visible to the operating system. The controller can be told to replace each bad sector logically with one of the spare sectors. This scheme is known as sector sparing or forwarding.

7. What are the techniques used for performing

Program I/O. med I/O

Interrupt driven I/O

Direct Memory Access (DMA).

8. Give an example of an application in which data in a file should be accessed in the following order:

Sequentially - Print the content of the file.

Randomly - Print the content of record i. This record can be found using hashing or index techniques

9. What problems could occur if a system allowed a file system to be mounted simultaneously at more than one location?

There would be multiple paths to the same file, which could confuse users or encourage mistakes. (Deleting a file with one path deletes the file in all the other paths.)

10. Why must the bit map for file allocation be kept on mass storage rather than in main memory?

In case of system crash (memory failure), the free-space list would not be lost as it would be if the bit map had been stored in main memory.

11. What criteria should be used in deciding which strategy is best utilized for a particular file?

Contiguous - File is usually accessed sequentially, if file is relatively small.

 Linked - File is usually accessed sequentially, if the file is large.

Indexed - File is usually accessed randomly, if file is large.

12. What is meant by RAID?

"RAID" is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. The different schemes architectures are named by the word RAID followed by a number, as in RAID 0, RAID 1, etc. RAID's various designs involve two key design goals: increase data reliability and/or increase output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array.

13. What is meant by Stable Storage?

Stable storage is a classification of computer data storage technology that guarantees atomicity for any given write operation and allows software to be written that is robust against some hardware and power failures. To be considered atomic, upon reading back a just written-to portion of the disk, the storage subsystem must return either the write data or the data that was on that portion of the disk before the write operation.

14. What is meant by Tertiary Storage?

Tertiary storage or tertiary memory provides a third level of storage. Typically it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands; this data is often copied to secondary storage before use.

15. Write a note on Descriptor?

UNIX processes use descriptors to reference I/O streams. Descriptors are small unsigned integers obtained from the open and socket system calls.. A read or write system call can be applied to a descriptor to transfer data.

The close system call can be used to deallocate any descriptor. Descriptors represent underlying objects supported by the kernel, and are created by system calls specific to the type of object. In 4.4BSD, three kinds of objects can be represented by descriptors: files, pipes, and sockets.

16. Write short notes on Pipes?

A pipe is a linear array of bytes, as is a file, but it is used solely as an I/O stream, and it is unidirectional. It also has no name, and thus cannot be opened with open.Instead, it is created by the pipe system call, which returns two descriptors, one of which accepts input that is sent to the other descriptor reliably, without duplication, and in order. The system also supports a named pipe or FIFO. A FIFO has properties identical to a pipe, except that it appears in the file system; thus, it can be opened using the open system call. Two processes that wish to communicate each open the FIFO: One opens it for reading, the other for writing.