Operating System Concepts
Chapter 10 { Mass-Storage Structure
Based on the 9th Edition of:
Abraham Silberschatz, Peter B. Galvin and Jreg Gagne:. Operating System
Concepts
Department of Information Technology, College of Business, Law & Governance
Overview Disk Structure Disk SCheduling Disk Management
Learning Objectives
To describe the physical structure of secondary storage
devices and its effects on the uses of the devices
To explain the performance characteristics of mass-storage
devices
To evaluate disk scheduling algorithms
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Outline
1 Overview of Mass-Storage Structure
2 Disk Structure
3 Disk Scheduling
4 Disk Management
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Overview Disk Structure Disk SCheduling Disk Management
Overview of Mass-Storage Structure
Magnetic disks provide bulk of secondary storage of modern
computers
Drives rotate at 60 to 250 times per second
Transfer rate is rate at which data flow between drive and
computer
Positioning time (random-access time) is time to move disk
arm to desired cylinder (seek time) and time for desired sector
to rotate under the disk head (rotational latency)
Head crash results from disk head making contact with the
disk surface |That’s bad
Disks can be removable
Drive attached to computer via I/O bus
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Overview of Mass-Storage Structure
Moving-head Disk Mechanism
track t
sector s
spindle
cylinder c
platter
arm
read-write
head
arm assembly
rotation
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Overview of Mass-Storage Structure
Hard Disks
Platters range from .85″ to 14″ (historically) |Commonly
3.5″, 2.5″, and 1.8″
Range from 30GB to 3TB per drive
Performance
Transfer Rate
theoretical 6 GB/sec; Effective (real) 1GB/sec
Seek time from 3ms to 12ms |9ms common for desktop
drives
Average seek time measured or calculated based on 1/3 of
tracks
Latency based on spindle speed, i.e., 1=(RPM=60) = 60=RPM
Average latency = 1=2 latency
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Overview of Mass-Storage Structure
Magnetic Tape
Was early secondary-storage medium
Relatively permanent and holds large quantities of data
Access time slow. Random access ≈ 1000 times slower than
disk
Mainly used for backup, storage of infrequently-used data,
transfer medium between systems
Once data under head, transfer rates comparable to disk; i.e.,
140MB/sec and greater
200GB to 1.5TB typical storage
Common technologies are LTO-f3,4,5g and T10000
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Disk Structure
Disk drives are addressed as large 1-dimensional arrays of
logical blocks, where the logical block is the smallest unit of
transfer
The 1-dimensional array of logical blocks is mapped into the
sectors of the disk sequentially
Sector 0 is the first sector of the first track on the outermost
cylinder
Mapping proceeds in order through that track, then the rest of
the tracks in that cylinder, and then through the rest of the
cylinders from outermost to innermost
Logical to physical address should be easy
Non-constant # of sectors per track via constant angular
velocity
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Overview Disk Structure Disk SCheduling Disk Management
Storage Area Network (SAN)
Common in large storage environments
Multiple hosts attached to multiple storage arrays – flexible
LAN/WAN
storage
array
storage
array
data-processing
center
web content
provider
server
client
client
client
server
tape
library
SAN
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Storage Area Network (SAN)
Storage Area Network (SAN) is one or more storage arrays
|Connected to one or more Fibre Channel switches
Hosts also attach to the switches
Easy to add or remove storage, add new host and allocate it
storage |Over low-latency Fibre Channel fabric
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Storage Area Network (SAN)
Network-attached storage (NAS) is storage made available
over a network rather than over a local connection (such as a
bus). Remotely attaching to file systems
NAS
client
NAS
client
LAN/WAN client
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Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 The surface of a magnetic disk platter is divided into .
A. sectors
B. arms
C. tracks
D. cylinders
Answer:
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Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 The surface of a magnetic disk platter is divided into .
A. sectors
B. arms
C. tracks
D. cylinders
Answer: C
Chapter 10 { Mass-Storage Structure Operating System Concepts 12
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 The surface of a magnetic disk platter is divided into .
A. sectors
B. arms
C. tracks
D. cylinders
Answer: C
2 What are the two components of positioning time?
A. seek time + rotational latency
B. transfer time + transfer rate
C. effective transfer rate – transfer rate
D. cylinder positioning time + disk arm positioning time
Answer:
Chapter 10 { Mass-Storage Structure Operating System Concepts 12
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 The surface of a magnetic disk platter is divided into .
A. sectors
B. arms
C. tracks
D. cylinders
Answer: C
2 What are the two components of positioning time?
A. seek time + rotational latency
B. transfer time + transfer rate
C. effective transfer rate – transfer rate
D. cylinder positioning time + disk arm positioning time
Answer: A
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Disk Scheduling
The operating system is responsible for using hardware
efficiently |for the disk drives, this means having a fast
access time and disk bandwidth
Minimize seek time
Seek time ≈ seek distance
Disk bandwidth is the total number of bytes transferred,
divided by the total time between the first request for service
and the completion of the last transfer
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
There are many sources of disk I/O request
OS
System processes
Users processes
I/O request includes input or output mode, disk address,
memory address, number of sectors to transfer
OS maintains queue of requests, per disk or device
Idle disk can immediately work on I/O request, busy disk
means work must queue
Optimization algorithms only make sense when a queue exists
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
Note that drive controllers have small buffers and can manage
a queue of I/O requests (of varying depth)
Several algorithms exist to schedule the servicing of disk I/O
requests
The analysis is true for one or many platters
We illustrate scheduling algorithms with a request queue
(0-199)
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
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Disk Scheduling
First-Come-First-Served (FCFS) Algorithm
Illustration shows total head movement of 640 cylinders
0 14 37 536567 98 122124 183199
queue 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
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Disk Scheduling
Shortest Seek Time First (SSTF) Algorithm
SSTF selects the request with the minimum seek time from
the current head position |may cause starvation
Illustration shows total head movement of 236 cylinders
0 14 37 536567 98 122124 183199
queue 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
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Disk Scheduling
SCAN Algorithm
The disk arm starts at one end of the disk, and moves toward
the other end, servicing requests until it gets to the other end
of the disk, where the head movement is reversed and
servicing continues.
SCAN algorithm Sometimes called the elevator algorithm
Illustration shows (see the next slide) total head movement of
208 cylinders
But note that if requests are uniformly dense, largest density
at other end of disk and those wait the longest
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
SCAN Algorithm (Cont.)
0 14 37 536567 98 122124 183199
queue 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
C-SCAN Algorithm
Provides a more uniform wait time than SCAN
The head moves from one end of the disk to the other,
servicing requests as it goes
When it reaches the other end, however, it immediately returns
to the beginning of the disk, without servicing any requests on
the return trip
Treats the cylinders as a circular list that wraps around from
the last cylinder to the first one
Total number of cylinders?
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
C-SCAN Algorithm
0 14 37 53 65 67 98 122124 183199
queue = 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
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Disk Scheduling
C-LOOK Algorithm
LOOK a version of SCAN, C-LOOK a version of C-SCAN
Arm only goes as far as the last request in each direction,
then reverses direction immediately, without first going all the
way to the end of the disk
Total number of cylinders?
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
C-LOOK Algorithm
0 14 37 536567 98 122124 183199
queue = 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
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Overview Disk Structure Disk SCheduling Disk Management
Disk Scheduling
Selecting a Disk-Scheduling Algorithm
SSTF is common and has a natural appeal
SCAN and C-SCAN perform better for systems that place a
heavy load on the disk |Less starvation
Performance depends on the number and types of requests
The disk-scheduling algorithm should be written as a separate
module of the operating system, allowing it to be replaced
with a different algorithm if necessary
Either SSTF or LOOK is a reasonable choice for the default
algorithm
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Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the SCAN scheduling algorithm, what is the order that
the requests are serviced, assuming the disk head is at cylinder
88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer:
Chapter 10 { Mass-Storage Structure Operating System Concepts 25
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the SCAN scheduling algorithm, what is the order that
the requests are serviced, assuming the disk head is at cylinder
88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: B
Chapter 10 { Mass-Storage Structure Operating System Concepts 25
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the SCAN scheduling algorithm, what is the order that
the requests are serviced, assuming the disk head is at cylinder
88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: B
2 True or False { In general, LOOK disk head scheduling will
involve less movement of the disk heads than SCAN disk head
scheduling.
Answer:
Chapter 10 { Mass-Storage Structure Operating System Concepts 25
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the SCAN scheduling algorithm, what is the order that
the requests are serviced, assuming the disk head is at cylinder
88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: B
2 True or False { In general, LOOK disk head scheduling will
involve less movement of the disk heads than SCAN disk head
scheduling.
Answer: True
Chapter 10 { Mass-Storage Structure Operating System Concepts 25
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the C-SCAN scheduling algorithm, what is the order
that the requests are serviced, assuming the disk head is at
cylinder 88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer:
Chapter 10 { Mass-Storage Structure Operating System Concepts 26
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the C-SCAN scheduling algorithm, what is the order
that the requests are serviced, assuming the disk head is at
cylinder 88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: D
Chapter 10 { Mass-Storage Structure Operating System Concepts 26
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the C-SCAN scheduling algorithm, what is the order
that the requests are serviced, assuming the disk head is at
cylinder 88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: D
2 True or False { LOOK disk head scheduling offers no
practical benefit over SCAN disk head scheduling.
Answer:
Chapter 10 { Mass-Storage Structure Operating System Concepts 26
Overview Disk Structure Disk SCheduling Disk Management
Quick Quiz
1 Consider a disk queue holding requests to the following
cylinders in the listed order: 116, 22, 3, 11, 75, 185, 100, 87.
Using the C-SCAN scheduling algorithm, what is the order
that the requests are serviced, assuming the disk head is at
cylinder 88 and moving upward through the cylinders?
A. 116 – 22 – 3 – 11 – 75 – 185 – 100 – 87
B. 100 – 116 – 185 – 87 – 75 – 22 – 11 – 3
C. 87 – 75 – 100 – 116 – 185 – 22 – 11 – 3
D. 100 – 116 – 185 – 3 – 11 – 22 – 75 – 87
Answer: D
2 True or False { LOOK disk head scheduling offers no
practical benefit over SCAN disk head scheduling.
Answer: False
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Overview Disk Structure Disk SCheduling Disk Management
Disk Management
Low-level formatting, or physical formatting { Dividing a disk
into sectors that the disk controller can read and write
Each sector can hold header information, plus data, plus error
correction code (ECC)
Usually 512 bytes of data but can be selectable
To use a disk to hold files, the operating system still needs to
record its own data structures on the disk
Partition the disk into one or more groups of cylinders, each
treated as a logical disk
Logical formatting or making a file system
To increase efficiency most file systems group blocks into
clusters
Disk I/O done in blocks
File I/O done in clusters
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Overview Disk Structure Disk SCheduling Disk Management
Disk Management
Raw disk access for apps that want to do their own block
management, keep OS out of the way (databases for example)
Boot block initializes system
The bootstrap is stored in ROM
Bootstrap loader program stored in boot blocks of boot
partition
Methods such as sector sparing used to handle bad blocks
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Overview Disk Structure Disk SCheduling Disk Management
Disk Management
Booting from a disk in Windows
MBR
partition 1
partition 2
partition 3
partition 4
boot
code
partition
table
boot partition
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Overview Disk Structure Disk SCheduling Disk Management
End of Chapter 10
Chapter 10 { Mass-Storage Structure Operating System Concepts 30