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Storage Devices: Types, Capacity and Access

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Key points

  • Magnetic tape is sequential access; HDD, SSD and CD are direct access
  • HDD access time = seek time + rotational latency + transfer time
  • CD 700 MB, DVD 4.7 GB single layer, Blu-ray 25 GB single layer
  • Memory hierarchy: Register, Cache, RAM, SSD, HDD, optical or tape
  • Defragmentation helps HDD only; SSDs use TRIM
  • RAID 0 has no fault tolerance; RAID 5 usable capacity is (n − 1) x S
On this page
  1. Types of storage
  2. Access methods
  3. Inside a hard disk
  4. Optical disc capacity
  5. Memory hierarchy
  6. Defragmentation and RAID

Types of storage

TypeExamplesNote
MagneticHDD, magnetic tape, floppy diskTape is used for long-term archives
OpticalCD, DVD, Blu-rayRead using a laser
Solid-stateSSD, pen drive, SD cardFlash based, no moving parts
CloudGoogle Drive, OneDrive, DigiLockerStored on remote servers

Access methods

  • Sequential access: data must be read in order from the beginning. Magnetic tape works this way.
  • Direct or random access: any location can be reached directly. HDD, SSD and CD work this way.

Inside a hard disk

A platter is the spinning magnetic disc. A track is a concentric circle, a sector is a part of a track, and a cylinder is the set of tracks at the same position across all platters.

Formula

Access time = Seek time + Rotational latency + Data transfer time

Seek time is the movement of the head to the right track. Rotational latency is the wait for the right sector to arrive under the head. Speed is measured in RPM, such as 5400 or 7200.

Optical disc capacity

DiscTypical capacityLaser
CD700 MBInfrared
DVD single layer4.7 GBRed
DVD dual layer8.5 GBRed
Blu-ray single layer25 GBBlue-violet

Key point

If the question does not say dual layer, use the single-layer figure. DVD means 4.7 GB unless stated otherwise.

Memory hierarchy

Order

Register > Cache > RAM > SSD > HDD > Optical disc or magnetic tape

Speed and cost fall from left to right; capacity rises.

Defragmentation and RAID

Defragmentation rejoins scattered file parts and speeds up an HDD. It is not needed for an SSD, which has no moving head, and unnecessary writes shorten its life. SSDs use TRIM instead.

RAID, Redundant Array of Independent Disks, combines several disks for speed or safety.

LevelTechniqueMinimum disksUsable capacityFault tolerance
RAID 0Striping2n x SNone
RAID 1Mirroring2SOne disk
RAID 5Striping with parity3(n − 1) x SOne disk
RAID 6Double parity4(n − 2) x STwo disks

Exam tip

Despite the word Redundant in the name, RAID 0 offers no redundancy at all. It is pure striping for speed.

Practice questions

Answer all, then check. Explanations appear after checking.

1Which of the following storage devices uses sequential access?
2What is the storage capacity of a standard single-layer DVD?
3RAID 1 protects data by using which technique?
4A data centre configures a RAID 5 array using four hard disks of 2 TB each. What is the usable storage capacity?
5Consider: 1. Regular defragmentation improves the performance of an SSD. 2. An SSD generally reads and writes faster than an HDD. 3. Android is based on the Linux kernel. Which are correct?

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