Linux with Operating System Concepts

Device
Maximum 
Transfer Rate
Common Size
Common Cost
Common Usage
Hard disk
1 Gbit/s
1 TB
$100–150
Store the operating system, 
application software, and 
most user data
Optical disk 
(DVD)
400 Mbit/s
5 GB
$1
Store multimedia data and 
software, available for 
portability
Magnetic tape
140 Mbit/s
5 TB
Varies based on size, 
up to $500
Store backup and archival 
data
SSD
Up to 600 Mbit/s 256 MB (up to 
2 TB)
Varies based on size, 
up to $700 for 2 TB, 
$200 for 256 GB
Used in place of or in 
addition to hard disk, 
often supports storage 
area networks (SAN)
USB (3.0)
400 Mbit/s
Up to 32 GB
$20–40
Store data files for 
portability
Floppy disk
1 Mbit/s
1.5 MB
$.50
Obsolete

---

Navigating the Linux File System
◾
109
directions as being positive or negative (representing the 1s and 0s of binary). Hard disk 
technology continues to improve in terms of both transfer rate and storage capacity.
Information stored on disk is broken into blocks. Each block is of a fixed size. This 
allows a file to be distributed in blocks across the surface of the disk and even across mul-
tiple surfaces. The advantages of this are twofold. First, because all blocks are the same 
size, deleting a file is handled by returning blocks back to be reused. We will see when we 
explore tape storage that it is unlikely that deleted file space can be reused. Second, as the 
disk drive rotates the disk very rapidly, placing disk file content in contiguous locations 
would not be efficient because transfer of data takes some time and so the disk will have 
rotated beyond the next disk block location when the next block is ready for reading/writ-
ing. The drive would have to wait for another revolution before access could resume.
Disk access time consists of three distinct parts: seek time, rotational delay (or rota-
tional latency), and transfer time. Seek time is the time it takes for the read/write head to 
move across the surface of the disk from its current location to the intended disk track. 
Rotational delay is the time it takes for the desired sector to rotate underneath the waiting 
read/write head. Transfer time is the time it takes to perform the magnetization onto the 
disk’s surface and move that information as binary data to memory, or the time it takes 
to receive the binary information from memory and write each bit as a new magnetic 
moment onto the disk. Transfer time can be as rapid as 1 Gb/s although most hard disk 
drives seldom reach this rate. Seek time is dependent on where the read/write head is now 
and where it has to move to and the speed of the read/write head’s arm. Rotational latency 
is dependent on which sector is currently under/over the read/write head, which sector is 
being sought, and how rapidly the disk is spinning. Also, since sectors take up more space 
Sectors
Read/write
head arm
swings in an
arc to reach
any disk track
Tracks
Disk surface
broken into
tracks and
sectors
Spindle spins all disk
surfaces, read/write head
waits until proper sector
rotates underneath it
Read/write
arm actuator
4 platter disk drive
All read/write heads
move togther on
one arm
8 read/write heads
1 above and
1 below each surface
Drive spindle
FIGURE 3.9 
Hard disk technology: access, read/write heads, spindle. (From Richard Fox, 2013,