Secondary Storage Devices -Exam Notes & MCQs

Secondary Storage Devices | Exam Notes & MCQs

📌 Sequential vs Direct Access

Sequential Access	Direct / Random Access
Data accessed in stored order (e.g., Magnetic Tape)	Any location directly, ≈ equal access time
Access time varies by position	e.g., Magnetic Disk, Optical Disk, SSD

Key Point

Magnetic tape is classic sequential; HDD/SSD are direct access.

🎞️ Magnetic Tape (Sequential)

IRG – Inter-Record Gap, IBG – Inter-Block Gap (blocking factor = records/block)
Capacity = Density (bpi) × Length | Transfer rate = Density × Tape Speed
Types: ½-inch reel, ½-inch cartridge, ¼-inch streamer (QIC), 4mm DAT (helical scan, DDS)
✅ Advantages: unlimited capacity, low cost, portable & reusable
⚠️ Limitations: no random access, dust/temp sensitive

💿 Magnetic Disk (Direct Access)

Organization: Platters → Tracks (concentric) → Sectors (512 bytes) → Cylinders. CHS addressing (Cylinder-Head-Sector).

Access time = Seek time + Latency + Transfer time

Seek time – position R/W head on track
Latency – rotational delay (average = half rotation)
Types: Zip/Bernoulli, Disk pack (interchangeable), Winchester (fixed, sealed, higher density)
📈 Advantages: random access, shared on-line storage, good for multi-user
⚠️ Disadvantages: sequential processing less efficient, crash risk, dust-free needed

Cylinder concept

Corresponding tracks across surfaces → reduces head movement.

🔆 Optical Disks (Laser)

Track pattern: single spiral (not concentric) → sectors equal length → variable rotation speed.
Read mechanism: Land (strong reflection=1), Pit (weak=0).
Types: CD-ROM (read only, ISO9660), WORM/CD-R (write once), CD-RW (~100 cycles), DVD (4.7GB/8.5GB, MPEG-2)
⚡ Access time: slower (100–300 ms) vs magnetic disks.
✅ Very low cost per bit, 30+ years archival life, reliable, portable.
⚠️ Slower access, sensitive to scratches/dust.

⚡ Solid State Drives & Flash Memory

🔹 HDD
Mechanical, spinning platters, cheaper per GB, up to 10TB, more power & noise.

🔸 SSD
No moving parts, flash memory, 3-4x faster, silent, low power, reliable (MTBF ~2M hrs), but costlier.

Flash Drive (Pen Drive): USB, EEPROM, 1GB–256GB. Memory Card (SD/MMC): 8MB–128GB, used in cameras/phones.

Hybrid configurations: Dual-drive (OS on SSD + user on HDD) | SSD as cache (boot + frequent files).

SSD advantages

Faster boot, shock resistant, no vibration.

🗄️ Mass Storage, Backup & HSS

Mass Storage: Disk Array (RAID – Redundant Array of Inexpensive Disks), Automated Tape Library, CD Jukebox.

Backup types: Full backup (all files) | Incremental backup (only new/changed files). Backup policy defines periodicity, media, off-site storage.

Storage Hierarchy (pyramid): Cache → Main memory → SSD → HDD → Near-line (tape library) → Off-line. HSM software auto-moves frequent data upward.

Online: always ready (HDD/SSD) | Near-line: auto without human (jukebox) | Off-line: requires human (tape, pen drive).

❓ Top MCQs for Exams

1. Which of the following is a sequential access device?

A) Magnetic disk B) Optical disk C) Magnetic tape D) SSD

✅ Answer: C) Magnetic tape

2. The time required to position read/write head on correct track is called?

A) Latency B) Seek time C) Transfer rate D) Access time

✅ Answer: B) Seek time

3. Smallest unit of data access by a disk drive?

A) Byte B) Block C) Sector D) Track

✅ Answer: C) Sector (typically 512 bytes)

4. Which optical disk allows rewriting multiple times?

A) CD-ROM B) WORM C) CD-RW D) DVD-ROM

✅ Answer: C) CD-RW

5. Fastest data access speed among secondary storage?

A) Magnetic tape B) HDD C) SSD D) Optical disk

✅ Answer: C) SSD

6. What does RAID stand for?

A) Random Array of Independent Disks B) Redundant Array of Inexpensive Disks

✅ Answer: B) Redundant Array of Inexpensive Disks

7. In magnetic tape, IBG stands for?

A) Inter-Block Gap B) Inter-Record Gap C) Internal Buffer Gate

✅ Answer: A) Inter-Block Gap (separates blocks)

8. Which disk uses a single spiral track?

A) Magnetic disk B) Optical disk C) Hard disk D) Floppy

✅ Answer: B) Optical disk

9. Backup that copies only new/changed files since last backup?

A) Full backup B) Incremental backup C) Differential

✅ Answer: B) Incremental backup

10. Which storage requires human intervention?

A) Online B) Near-line C) Off-line D) Cache

✅ Answer: C) Off-line storage

11. High density helical scan is used in which magnetic tape?

A) ½-inch reel B) ¼-inch streamer C) 4mm DAT D) Cartridge

✅ Answer: C) 4mm DAT

12. Standard CD-ROM capacity typically?

A) 4.7 GB B) 650 MB C) 256 GB D) 10 TB

✅ Answer: B) 650 MB (approx 700MB for newer)

13. Which memory is volatile?

A) SSD B) Magnetic tape C) Primary storage (RAM) D) Optical disk

✅ Answer: C) Primary storage

14. In CHS addressing, 'C' denotes?

A) Cache B) Cylinder C) Cluster D) Controller

✅ Answer: B) Cylinder

15. Highest cost per bit storage?

A) Magnetic tape B) HDD C) Optical D) Cache memory

✅ Answer: D) Cache memory (part of hierarchy)

⚙️ HDD vs SSD (Quick Recap)

Attribute	HDD	SSD
Capacity	Up to 10TB+	Up to 4TB
Speed	Slower	3-4x faster
Power	More	Less
Noise/Vibration	Present	None
Reliability (MTBF)	~1.5M hrs	~2.0M hrs
Cost per GB	Low	Higher

Hybrid systems

Use SSD for OS + cache, HDD for bulk storage.

📊 Storage Hierarchy

Top → Bottom: Cache → Main Memory → SSD → HDD → Near-line (tape library) → Off-line (tape/optical/pen drive)

Online: HDD/SSD (direct control)
Near-line: auto access (jukebox)
Off-line: human intervention, portable.

HSM (Hierarchical Storage Management) auto-moves data across tiers.

📎 Key Points to Remember

Magnetic tape → sequential, IRG/IBG, backup/archival
Magnetic disk → direct access, sectors, seek+latency
Optical disk → spiral track, land/pit, CD/DVD
SSD → flash, no moving parts, faster, costlier
Full backup vs incremental backup policy
RAID, tape library, jukebox → mass storage

💾 Secondary Storage Devices