Topic in Operating Systems
Secondary storage is non-volatile storage, such as hard disks and solid-state drives, that keeps programs and data permanently, unlike volatile main memory. Questions on hard disks break access time into seek time, rotational latency and transfer time, and many are worked disk-scheduling problems: given a request queue and head position, compute total head movement for FCFS, SSTF, SCAN, C-SCAN, LOOK or C-LOOK. RAID levels 0, 1, 5, 6 and 10 are compared for redundancy and capacity. You may also meet low-level formatting, partitions, boot blocks, bad-block handling, swap space, and SSD concepts like wear leveling, TRIM and the flash translation layer.
Below are 30 practice questions from a pool of 210 Secondary Storage MCQs, one of 12 topics in Operating Systems. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. Secondary storage refers to:
Secondary storage consists of non-volatile storage devices (hard drives, SSDs, optical disks) that retain data even when power is off, providing permanent storage for programs and data.
Q2. A hard disk drive (HDD) stores data on:
A hard disk drive stores data on rotating magnetic platters. Data is read/written by a read/write head that moves across the surface of the spinning platters.
Q3. Seek time in a disk drive is:
Seek time is the time required for the disk arm to move the read/write head to the track (cylinder) containing the desired data. It is typically the largest component of disk access time.
Q4. The SSTF (Shortest Seek Time First) disk scheduling algorithm:
SSTF selects the request with the minimum seek time from the current head position. It reduces total head movement but may cause starvation of requests far from the head.
Q5. Rotational latency is:
Rotational latency is the time waiting for the desired sector to rotate under the read/write head after the head has been positioned on the correct track. On average, it is half of one full rotation time.
Q6. A disk partition is:
A disk partition is a logically independent section of a physical disk. Each partition can be formatted with its own file system and acts as a separate logical disk.
Q7. FCFS disk scheduling processes disk I/O requests:
FCFS (First-Come, First-Served) disk scheduling services disk I/O requests in the order they arrive. It is simple but can result in long seek times due to random head movements.
Q8. Disk formatting prepares a disk for use by:
Disk formatting creates the necessary data structures (file system metadata, directory structures, free space tracking) on the disk so the OS can store and retrieve files.
Q9. A Solid State Drive (SSD) stores data using:
SSDs use flash memory (NAND) to store data electronically. They have no moving parts, resulting in faster access times, lower power consumption, and greater durability compared to HDDs.
Q10. Disk access time is the sum of:
Total disk access time = seek time (moving head to correct track) + rotational latency (waiting for sector to come under head) + data transfer time (reading/writing the data).
Q11. The SCAN (elevator) disk scheduling algorithm:
SCAN moves the disk arm in one direction, servicing all requests along the way until it reaches the end of the disk, then reverses direction. It is called the elevator algorithm due to its similarity to elevator movement.
Q12. The C-SCAN (Circular SCAN) algorithm differs from SCAN in that:
C-SCAN treats the cylinders as circular. When the head reaches one end, it immediately returns to the beginning without servicing requests during the return trip, providing more uniform wait times.
Q13. The LOOK disk scheduling algorithm differs from SCAN in that:
LOOK is a practical version of SCAN that reverses direction when there are no more requests ahead in the current direction, rather than always traveling to the physical end of the disk.
Q14. Why is disk scheduling less important for SSDs compared to HDDs?
SSDs have no mechanical head movement, so there is no seek time or rotational latency. Access time is nearly uniform regardless of data location, making traditional disk scheduling algorithms less relevant.
Q15. RAID 0 provides:
RAID 0 stripes data across multiple disks for improved read/write performance but provides no redundancy. If any disk fails, all data is lost.
Q16. RAID 1 provides:
RAID 1 mirrors data by keeping identical copies on two or more disks. It provides high data reliability at the cost of double the storage, and read performance can be improved by reading from both disks.
Q17. RAID 5 uses:
RAID 5 uses block-level striping with distributed parity spread across all disks. It can tolerate one disk failure, offers good read performance, and has efficient storage utilization (only one disk worth of parity).
Q18. Low-level (physical) formatting creates:
Low-level formatting divides the disk surface into sectors that the controller can read and write. It writes sector headers, data areas, and error-correcting codes for each sector. This is done at the factory.
Q19. What is the TRIM command in the context of SSDs?
TRIM informs the SSD controller which blocks contain deleted data and can be erased. Without TRIM, the SSD must erase-before-write during future operations, degrading performance. TRIM enables proactive garbage collection.
Q20. What is wear leveling in SSDs and why is it necessary?
Flash memory cells have a limited number of write/erase cycles. Wear leveling distributes these operations evenly across all cells to prevent some cells from wearing out prematurely, extending the SSD's overall lifespan.
Q21. The boot block on a disk contains:
The boot block (boot sector) contains the bootstrap program that is loaded and executed when the system starts up. It initializes the system and begins loading the operating system.
Q22. A swap space on a disk is used for:
Swap space is a dedicated area on disk used by the virtual memory system to store pages that have been swapped out of main memory, extending the available memory beyond physical RAM.
Q23. What is the difference between RAID 5 and RAID 6?
RAID 6 extends RAID 5 by adding a second parity block, enabling the array to survive two simultaneous disk failures. This provides higher reliability at the cost of additional storage overhead and slightly slower writes.
Q24. What is disk striping and how does it improve performance?
Disk striping splits data across multiple disks at the bit, byte, or block level. This allows parallel I/O operations - multiple disks can read/write simultaneously, multiplying the effective throughput.
Q25. What is the I/O scheduler's role in Linux and how do different schedulers (CFQ, deadline, noop) differ?
CFQ (Completely Fair Queuing) allocates I/O bandwidth fairly. Deadline scheduler ensures requests are served within a time deadline. Noop performs simple FIFO with merging, ideal for SSDs where seek optimization is unnecessary.
Q26. What is the purpose of a disk controller's command queue (NCQ/TCQ)?
Native Command Queuing (NCQ) allows the disk to accept multiple commands and reorder their execution internally to minimize head movement and rotational latency, improving overall throughput.
Q27. What is the write amplification problem in SSDs?
Write amplification occurs because SSDs must erase entire blocks before writing. Garbage collection may require reading, erasing, and rewriting valid data from partially-used blocks, causing more physical writes than the logical data would suggest.
Q28. What is a hot spare disk in RAID configurations?
A hot spare is an inactive disk installed in a RAID array. When an active disk fails, the hot spare is automatically activated and data is rebuilt onto it, minimizing the window of vulnerability.
Q29. What is the difference between NVMe and SATA interfaces for SSDs?
NVMe (Non-Volatile Memory Express) is a protocol designed for SSDs using PCIe lanes, offering much higher bandwidth (up to 7 GB/s vs 600 MB/s), lower latency, and deeper command queues compared to the SATA interface.
Q30. What is the significance of the 4K sector size (Advanced Format) in modern disk drives?
Advanced Format uses 4K sectors instead of traditional 512-byte sectors. Larger sectors reduce the ratio of overhead (headers, ECC) to data, improve error correction capability, and better align with OS block sizes.
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