UGC NET Paper 2 Computer Science – Operating Systems PYQ: File Systems & Disk Scheduling

Indexed/grouped allocation (of disk blocks to a file) is useful because:
(A) It supports both sequential and direct access.
(B) The entire data block is available for data (no pointers embedded in it).
(C) It does not require a lot of space for keeping pointers.
(D) It has no external fragmentation.

A moving-head disk has 200 tracks, numbered 0 to 199. The head has just finished a request at track 125 and is currently at track 143 (so it is moving in the direction of increasing track numbers). The pending request queue, in FIFO arrival order, is: 86, 147, 91, 177, 94, 150, 102, 175, 130. What is the total head movement (in cylinders) to service all these requests using the SCAN algorithm?

A disk head is currently at cylinder 50. The pending requests, in the order they arrived, are for cylinders: 4, 34, 10, 7, 19, 73, 2, 15, 6, 20. Using the Shortest-Seek-Time-First (SSTF) disk scheduling policy, and given that it takes 1 ms to move the head between two adjacent cylinders, what is the total time needed to service all the requests?

Consider an operating system capable of loading and executing a single sequential user process at a time. The disk head scheduling algorithm used is first come first served (FCFS). If FCFS is replaced by shortest seek time first (SSTF) and the vendor claims 50% better benchmark results, what is the expected improvement in the I/O performance of user programs?

What is called journaling in a Linux operating system?

In a file allocation system, the following allocation schemes are used: (A) Contiguous allocation, (B) Indexed allocation, (C) Linked allocation. Which allocation scheme(s) will NOT suffer from external fragmentation?

Suppose a Linux file system has a block size of 2K bytes, a disk address is 32 bits, and an i-node contains the disk addresses of the first 12 direct blocks of a file, one single-indirect block, and one double-indirect block. Approximately, what is the largest file that can be represented by an i-node?

Consider a disk system having 60 cylinders. Disk requests are received by the disk drive for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. Assuming the disk head is currently at cylinder 20, what is the time taken to satisfy all the requests if it takes 2 milliseconds to move from one cylinder to an adjacent one and the Shortest Seek Time First (SSTF) algorithm is used?

A disk queue has requests for I/O on cylinders 98, 183, 37, 122, 14, 124, 65, 67, with the disk head currently at cylinder 53. Using the shortest seek time first (SSTF) disk scheduling algorithm, what is the total head movement (in cylinders) needed to service all requests?

Consider the following statements about UNIX file permissions:
(A) UNIX provides three permission types — Read, Write and Execute.
(B) UNIX provides three permission sets — Owner, Group and Others.
Which of the above statement(s) is/are true?

In the ________ disk scheduling algorithm, the disk head moves from one end of the disk to the other, servicing requests along the way. When it reaches the other end, it immediately returns to the beginning of the disk without servicing any requests on the return trip.

Suppose there are six files F1, F2, F3, F4, F5, F6 with corresponding sizes 150 KB, 225 KB, 75 KB, 60 KB, 275 KB and 65 KB respectively. The files are to be stored on a sequential device in such a way that optimizes (average) access time. In what order should the files be stored?

Match the following for the UNIX file system:
List I: (A) Boot block (B) Super block (C) Inode block (D) Data block
List II: (I) Contains information about the file system — free block list, free inode list, etc. (II) Contains the operating system files as well as program and data files created by users. (III) Contains the boot program and partition table. (IV) Contains a table entry for every file in the file system, storing file attributes.

Which of the following statements about the UNIX file system is NOT correct?

In UNIX, files are protected using a 9-bit permission mode ("rights bits"). Consider the statements: (I) A mode of 641 (octal) means the owner can read and write the file, other members of the owner's group can only read it, and all other users can only execute it. (II) A mode of 100 (octal) allows the owner to execute the file, but prohibits all other access. Which of the following is correct?

A disk head is initially positioned at track 32. The disk I/O request queue, in arrival order, is: 98, 37, 14, 124, 65, 67. Using the FCFS disk-scheduling algorithm, find the total number of head movements (in tracks) required to service all requests.