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  • Disk Scheduling & Storage MCQs | OS Algorithms | Suraku Academy

Q: 1

Match the following—

List – IList – II
A.    Disk Scheduling1.    Round Robin
B.    Batch Processing2.    Scan
C.    Time Sharing3.    LIFO
D.    Interrupt Processing4.    FIFO

  1. A-3, B-4, C-2, D-1

  2. A-4, B-3, C-2, D-1

  3. A-2, B-4, C-1, D-3

  4. A-3, B-4, C-1, D-2

[ Option C ]

Disk scheduling algorithms manage the order in which disk I/O requests are processed. Common methods are FCFS (FIFO), SSTF, SCAN, LOOK, etc. The SCAN (also called Elevator algorithm) which moves the disk arm across the disk servicing requests in one direction.
In Batch Processing, jobs are processed in the order they arrive or according to some simple sequence. A common scheduling method is FIFO (First In First Out), where the first job to arrive is the first to be processed.
Time-sharing systems use Round Robin Scheduling to give each process a fair share of CPU.
Interrupts are handled in the reverse order of occurrence, often following a LIFO (Last In First Out) scheme where the most recent interrupt is serviced first.

Q: 2 Consider a disk queue with requests for Input/Output to blocks on cylinders in the given order. The disk head is initially at cylinder 53.
98, 183, 37, 122, 14, 124, 65, 67
What will be the number of total head movement for SSTF and SCAN algorithms respectively?

  1. 236, 208

  2. 208, 236

  3. 208, 208

  4. 236, 236

[ Option A ]

Disk scheduling algorithms decide the order in which I/O requests are serviced to minimize disk head movement. Two common algorithms are:
SSTF (Shortest Seek Time First):

  • Selects the request closest to the current head position.
  • Reduces total head movement but can cause starvation for far-away requests.

SCAN (Elevator Algorithm):

  • The disk head moves in one direction, servicing all requests until it reaches the end, then reverses direction.
  • Provides more uniform wait times and avoids starvation.

Q: 3 Which of the following is NOT a characteristic of RAID?

  1. RAID is a set of physical disk drives viewed by the OS as a single logical drive.

  2. Data are distributed across physical drives of an array in a scheme known as striping.

  3. Redundant disk capacity is used to stores parity information, which guaranteeing data recoverability in case of disk failure.

  4. RAID is primarily used to compress and encrypt data across multiple disks.

[ Option D ]

RAID (Redundant Array of Independent Disks) is a storage technology that combines multiple physical disks into a single logical unit to improve performance and reliability (fault tolerance). Key characteristics of RAID include:

  • Data Striping: Distributing data across multiple disks to improve read/write performance.
  • Redundancy/Parity: Using extra disk space to store parity or mirrored data for fault tolerance.
  • Logical View: The OS treats the RAID array as a single logical drive, even though it consists of multiple physical drives.

Q: 4 Which of the following is a disk scheduling policy ensuring selection of the disk I/O request that requires the least movement of the disk arm from its current position?

  1. Shortest-service-time-first (SSTF)

  2. First In First Out (FIFO)

  3. SCAN algorithm

  4. Back and forth over disk

[ Option A ]

In an operating system, multiple processes often request disk I/O operations at the same time. To manage these requests efficiently, the OS uses Disk Scheduling Algorithms, which decide the order in which the disk head (arm) moves to service requests. The main goal is to reduce Seek Time, which is the time taken by the disk arm to move to the track where data is located.

Among the given options, the Shortest-Service-Time-First (SSTF) algorithm selects the disk I/O request that is closest to the current position of the disk arm. This means the arm moves the least possible distance, which minimizes seek time and improves performance.

ALGORITHMDESCRIPTION
FIFO (First In First Out)Serves requests in the order they arrive. Simple but may cause long seek times.
SSTF (Shortest-Service-Time-First)Chooses the request nearest to the current head position. It reduces seek time but may cause starvation.
SCAN (Elevator Algorithm)Moves arm in one direction, serving requests, then reverses. It provides fairness and reduces variance.

Q: 5 Match RAID levels in Column I with their description in Column II:

Column – I Column – II
a.  RAID 2 i.  Bit-interleaved parity
b.  RAID 3ii.  Mirrored
c.  RAID 5iii.  Block-interleaved distributed parity
d.  RAID 1iv.  Redundant via Hamming Code

Choose the correct option:

  1. a – iii, b – i, c – ii, d – iv

  2. a – ii, b – iv, c – iii, d – i

  3. a – iv, b – i, c – iii, d – ii

  4. a – iv, b – iii, c – i, d – ii

[ Option C ]

RAID (Redundant Array of Independent Disks) is a storage technology that combines multiple physical disks into one logical unit for better performance, reliability, or both. Each RAID level has a specific method for distributing and storing data and parity across disks.

RAID LEVELDESCRIPTIONKEY FEATURE
RAID 0Disk Striping, splits data across all disks.No redundancy, highest performance.
RAID 1Mirrored.Data is duplicated on two disks for fault tolerance.
RAID 2Redundant via Hamming Code.Uses error-correcting (Hamming) code for fault tolerance.
RAID 3Bit-interleaved parity.Data is striped at the byte level with a dedicated parity disk.
RAID 5Block-interleaved distributed parity.Data and parity information are distributed across all disks.

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