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Understanding Operating Systems Understanding Operating Systems

Understanding Operating Systems - PowerPoint Presentation

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Seventh Edition Chapter 7 Device Management Learning Objectives After completing this chapter you should be able to describe Features of dedicated shared and virtual devices Concepts of blocking and buffering and how they improve IO performance ID: 409491

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Slide1

Understanding Operating SystemsSeventh Edition

Chapter 7Device ManagementSlide2

Learning Objectives

After completing this chapter, you should be able to describe:Features of dedicated, shared, and virtual devicesConcepts of blocking and buffering, and how they improve I/O performanceRoles of seek time, search time, and transfer time in calculating access time

Differences in access times in several types of devices

2

Understanding Operating Systems, 7eSlide3

Learning Objectives (cont'd.)

Strengths and weaknesses of common seek strategies and how they compareLevels of RAID and what sets each apart

Understanding Operating Systems, 7e

3Slide4

Types of Devices

Three categories: dedicated, shared, and virtualDedicated device Assigned to one job at a timeFor entire time that job is active (or until released)

Examples: tape drives, printers, and plotters

Disadvantage

Must be allocated for duration of job’s execution

Inefficient if device is not used 100 percent of time

4

Understanding Operating Systems, 7eSlide5

Types of Devices (cont'd.)

Shared deviceAssigned to several processesExample: direct access storage device (DASD)Processes share DASD simultaneously

Requests interleaved

Device manager supervision

Controls interleaving

Predetermined policies determine conflict resolution

Understanding Operating Systems, 7e

5Slide6

Types of Devices (cont'd.)

Virtual deviceDedicated and shared device combinationDedicated device transformed into shared deviceExample: printer

Converted by spooling program

Spooling: speeds up slow dedicated I/O devices

Universal serial bus (USB) controller

Interface between operating system, device drivers, applications, and devices attached via USB hostAssigns bandwidth to each device: priority-based

High, medium, or low priority

Understanding Operating Systems, 7e

6Slide7

Management of I/O Requests

I/O traffic controllerWatches status of devices, control units, channelsThree main tasksDetermine if path availableIf more than one path available, determine which one to select

If

paths all busy, determine when one is available

Maintains database containing each unit’s status and

connections

Understanding Operating Systems, 7e

7Slide8

Management of I/O Requests (cont'd.)

I/O schedulerSame job as process scheduler (Chapter 4)Allocates devices, control units, and channelsIf requests greater than

available paths

Decides which request to satisfy first:

based on different criteria

In many systems I/O requests not preemptedFor some systems

Allow preemption with I/O request subdivided

Allow preferential treatment for high-priority requests

Understanding Operating Systems, 7e

8Slide9

Management of I/O Requests (cont'd.)

I/O device handlerPerforms actual data transferProcesses device interruptsHandles error conditionsProvides

detailed scheduling algorithms

Device dependent

Each I/O device type has its own device handler algorithm

Understanding Operating Systems, 7e

9Slide10

Understanding Operating Systems, 7e

(figure 7.1)

Each control block contains the information it needs to manage the channels, control units, and devices in the I/O subsystem.

© Cengage Learning 2014

10Slide11

I/O Devices in the Cloud

Local operating system’s role in accessing remote I/O devicesEssentially the same role performed accessing local devicesCloud provides access to many more devices

Understanding Operating Systems, 7e

11Slide12

Sequential Access Storage Media

Magnetic tapeEarly computer systems: routine secondary storage Records stored seriallyRecord length determined by application programRecord identified by position on tape

Record access

Tape rotates passing under read/write head: only when access requested for read or write

Time-consuming process

Understanding Operating Systems, 7e

12Slide13

Sequential Access Storage Media (cont'd.)

Tape density: characters recorded per inchDepends upon storage method (individual or blocked records)

Understanding Operating Systems, 7e

(figure 7.2)

Nine-track magnetic tape with three characters recorded using odd parity. A 1/2-inch wide reel of tape, typically used to back up a mainframe computer, can store thousands of characters, or bytes, per inch.

© Cengage Learning 2014

13Slide14

Sequential Access Storage Media (cont'd.)

Interrecord gap (IRG)½ inch gap inserted between each recordSame size regardless of sizes of records it separates

Blocking: group records into blocks

Transfer rate: (tape density) x (transport speed)

Interblock gap (IBG)

½ inch gap inserted between each blockMore efficient than individual records and IRG

Optimal block size

Entire block fits in buffer

Understanding Operating Systems, 7e

14Slide15

Sequential Access Storage Media (cont'd.)

Understanding Operating Systems, 7e

(figure 7.3)

IRGs in magnetic tape. Each record requires only 1/10 inch of tape. When 10 records are stored individually on magnetic

tape, they are separated by IRGs, which adds up to 4.5 inches of tape. This totals 5.5 inches of tape.

© Cengage Learning 2014

(figure 7.4)

Two blocks of records stored on magnetic tape, each preceded by an IBG of 1/2 inch. Each block holds 10 records, each of which is still 1/10 inch. The block, however, is 1 inch, for a total of 1.5 inches.

© Cengage Learning 2014

15Slide16

Sequential Access Storage Media (cont'd.)

Blocking advantagesFewer I/O operations neededLess wasted tape spaceBlocking disadvantages

Overhead and software routines needed for blocking, deblocking, and record

keeping

Buffer space wasted

When only one logical record

needed

Understanding Operating Systems, 7e

16Slide17

Sequential Access Storage Media (cont'd.)

Access timePoor for routine secondary storage except files with very high (90 to 100 percent) sequential activity

Understanding Operating Systems, 7e

(table 7.1)

Access times for

2400-foot magnetic

tape with

a tape transport

speed

of 200

ips

.

© Cengage Learning 2014

17Slide18

Direct Access Storage Devices

Directly read or write to specific disk areaRandom access storage devicesThree categoriesMagnetic disks

Optical discs

Solid state (flash) memory

Access time variance

Not as wide as magnetic tapeRecord location directly affects access

time

Understanding Operating Systems, 7e

18Slide19

Magnetic Disk Storage

Computer hard drivesSingle platter or stack of magnetic platters

Understanding Operating Systems, 7e

(figure 7.5)

A disk pack is a stack of magnetic platters. The read/write heads move between each pair of surfaces, and all of the heads are moved in unison by the arm.

© Cengage Learning 2014

19Slide20

Magnetic Disk Storage (cont’d.)

Two recording surfaces (top and bottom)Each surface formattedConcentric tracks: numbered from track 0 on outside to highest track number in centerRead/write heads move in unison: virtual cylinder

Accessing a record: system needs three things

Cylinder number

Surface number

Sector number

Understanding Operating Systems, 7e

20Slide21

Access Times

File access time factorsSeek time (slowest)Time to position read/write head on trackDoes not apply to fixed read/write head devices

Search time

Rotational delay

Time to rotate DASD

Rotate until desired record under read/write headTransfer time (fastest)

Time to

transfer

data

Secondary storage to main memory transfer

Understanding Operating Systems, 7e

21Slide22

Fixed-Head Magnetic Drives

Record access requires two itemsTrack number and record numberTotal access time = search time + transfer timeDASDs rotate continuouslyThree basic positions for requested record

In relation to read/write head position

DASD has little access variance

Good candidates: low activity files, random access

Blocking minimizes access time

Understanding Operating Systems, 7e

22Slide23

Understanding Operating Systems, 7e

(figure 7.8)

As a disk rotates, Record 1 may be near the read/write head and ready to be scanned, as seen in (a); in the farthest position just past the head, (c); or somewhere in between, as in the average case, (b).

© Cengage Learning 2014

(table 7.2)

Access times for

a fixed-head

disk drive

at 16.8 ms/revolution.

© Cengage Learning 2014

23Slide24

Movable-Head Magnetic Drives

Access time = seek time + search time + transfer timeSearch time and transfer time calculationSame as fixed-head DASD

Blocking: good way to minimize access time

Understanding Operating Systems, 7e

(table 7.3)

Typical access times for a movable-head drive, such as a typical hard drive.

© Cengage Learning 2014

24Slide25

Device Handler Seek Strategies

Predetermined device handlerDetermines device processing orderGoal: minimize seek time Types

First-come, first-served (FCFS);

shortest seek time first (SSTF)

; SCAN (including LOOK, N-Step SCAN, C-SCAN, and C-LOOK)

Scheduling algorithm goals

Minimize arm movement, mean response time, and variance in response time

25

Understanding Operating Systems, 7eSlide26

Device Handler Seek Strategies (cont'd.)

First-come, first-served (FCFS)On average: does not meet the three seek strategy goals Disadvantage: extreme arm movement

Understanding Operating Systems, 7e

(figure 7.9)

Typical access times for a movable-head drive, such as a typical hard drive.

© Cengage Learning 2014

26Slide27

Device Handler Seek Strategies (cont'd.)

Shortest seek time first (SSTF)Requests with track closest to one being served Minimizes overall seek timePostpones traveling to out of

way

tracks

Understanding Operating Systems, 7e

(figure 7.10)

Using the SSTF algorithm, with all track requests on the wait queue, arm movement is reduced by almost one third while satisfying the same requests shown in Figure 7.9 (using the FCFS algorithm).

© Cengage Learning 2014

27Slide28

Device Handler Seek Strategies (cont'd.)

SCANDirectional bitIndicates if arm moving toward/away from disk centerAlgorithm moves arm methodicallyFrom outer to inner track: services every request in its path

When innermost track reached: reverses direction and moves toward outer tracks

Services every request in its path

Understanding Operating Systems, 7e

28Slide29

Device Handler Seek Strategies (cont'd.)

LOOK (elevator algorithm) Arm does not go to either edgeUnless requests exist

Eliminates indefinite postponement

Understanding Operating Systems, 7e

(figure 7.11)

The LOOK algorithm makes the arm move systematically from the first requested track at one edge of the disk to the last requested track at the other edge. In this example, all track requests are on the wait queue.

© Cengage Learning 2014

29Slide30

Device Handler Seek Strategies (cont'd.)

N-Step SCANHolds all requests until arm starts on way backNew requests grouped together for next sweepC-SCAN (Circular SCAN)

Arm picks up requests on path during inward sweep

Provides more uniform wait time

C-LOOK

Inward sweep stops at last high-numbered track request

No last track access unless required

Understanding Operating Systems, 7e

30Slide31

Device Handler Seek Strategies (cont'd.)

Best strategyFCFS best with light loadsService time unacceptably long under high loadsSSTF best with moderate loads

Localization problem under heavy loads

SCAN best with light to moderate loads

Eliminates indefinite postponement

Throughput and mean service times SSTF similarities

C-SCAN best with moderate to heavy loads

Very small service time variances

Understanding Operating Systems, 7e

31Slide32

Search Strategies: Rotational Ordering

Rotational orderingOptimizes search timesOrders requests once read/write heads positionedRead/write head movement time Hardware

dependent

Reduces time wasted

Due to rotational delay

Request arrangementFirst sector requested on second track: next number higher than one just served

Understanding Operating Systems, 7e

32Slide33

Search Strategies: Rotational Ordering (cont'd.)

Understanding Operating Systems, 7e

(figure 7.12)

Example of a virtual cylinder with five sectors for each of its five tracks.

© Cengage Learning 2014

33Slide34

Understanding Operating Systems, 7e

(table 7.4)

It takes 36 ms to fill the eight requests on the movable-head cylinder shown in Figure 7.12. For this example, seek time is 5 ms/track, search time is 1 ms/sector, and data transfer is 1 ms.

© Cengage Learning 2014

34Slide35

Understanding Operating Systems, 7e

(table 7.5)

It takes 28 ms to fill the same eight requests shown in Table 7.5 after the requests are ordered to minimize search time, reducing it from 13 ms to 5 ms.

© Cengage Learning 2014

35Slide36

Optical Disc Storage

Design featuresSingle spiralling trackSame-sized sectors: from center to disc rimSpins at constant

linear

velocity (CLV

)More sectors and more discdata than magnetic disk

Understanding Operating Systems, 7e

(figure 7.13)

On an optical disc, the sectors (not all sectors are shown here) are of the same size throughout the disc. The disc drive changes speed to compensate, but it spins at a constant linear velocity (CLV).

© Cengage Learning 2014

36Slide37

Optical Disc Storage (cont'd.)

Two important performance measuresSustained data-transfer rateSpeed to read massive data amounts from disc

Measured in megabytes per second (Mbps)

Crucial for applications requiring sequential access

Average access time

Average time to move head to specific disc locationExpressed in milliseconds (ms)

Third feature

Cache size

(hardware)

Buffer to transfer data blocks from disc

Understanding Operating Systems, 7e

37Slide38

CD and DVD Technology

CDData recorded as zeros and onesPits: indentationsLands: flat areasReads with low-power laser

Light strikes land: reflects to photodetector

Light striking a pit: scattered and absorbed

Photodetector converts light intensity into digital signal

Understanding Operating Systems, 7e

38Slide39

CD and DVD Technology (cont'd.)

CD-R (compact disk recordable) technologyRequires expensive disk controllerRecords data using write-once techniqueData cannot be erased or modified

Disk

Contains several layers

Gold reflective layer and dye layer

Records with high-power laserPermanent marks on dye layerCD cannot be erased after data recorded

Data read on standard CD drive (low-power beam)

Understanding Operating Systems, 7e

39Slide40

CD and DVD Technology (cont'd.)

CD-RW and DVD-RW: rewritable discsData written, changed, and erasedUses phase change technologyAmorphous and crystalline phase states

Record data: beam heats up disc

State changes from crystalline to amorphous

Erase data: low-energy beam to heat up pits

Loosens alloy to return to original crystalline state

Understanding Operating Systems, 7e

40Slide41

CD and DVD Technology (cont'd.)

DVDs: compared to CDsSimilar in design, shape, and sizeDiffers in data capacityDual-layer, single-sided DVD holds 13 CDsSingle-layer, single-sided DVD holds 8.6 GB (MPEG video compression)

Differs in laser wavelength

Uses red laser (smaller pits, tighter spiral)

Understanding Operating Systems, 7e

41Slide42

Blu-Ray Disc Technology

Same physical size as DVD/CDSmaller pitsMore tightly wound tracksUse of blue-violet laser allows multiple layersFormats: BD-ROM (read only), BD-R (recordable), and BD-RE (rewritable)

Understanding Operating Systems, 7e

42Slide43

Solid State StorageImplements Fowler-Nordheim tunneling phenomenon

Stores electrons in a floating gate transistorElectrons remain even after power is turned off

Understanding Operating Systems, 7e

43Slide44

Flash Memory Storage

Electrically erasable, programmable, and read-only memory (EEPROM)Nonvolatile and removable Emulates random accessDifference: data stored securely (even if removed)Write data: electric charge sent through floating gate

Erase data: strong electrical field (flash) applied

Understanding Operating Systems, 7e

44Slide45

Solid State Drives (SSDs)

Fast but currently pricey storage devicesTypical device functions in smaller physical space than magnetic drivesWork electronically: no moving partsRequire less power; silent; relatively lightweightDisadvantages

Catastrophic crashes: no warning messages

Data transfer rates: degrade over time

Hybrid drive

Combines SSD and hard drive technology

Understanding Operating Systems, 7e

45Slide46

Components of the I/O Subsystem

I/O channelsProgrammable units Positioned between CPU and control unitSynchronize device speedsCPU (fast) with I/O device (slow)

Manage concurrent processing

CPU and I/O device requests

Allow overlap

CPU and I/O operations

Understanding Operating Systems, 7e

46Slide47

Components of the I/O Subsystem (cont'd.)

I/O channel programSpecifies action performed by devicesControls data transmission between main memory and control unitsI/O control unit: receives and interprets signalDisk controller (disk drive interface)

Links disk drive and system bus

I/O subsystem configuration

Multiple paths increase flexibility and reliability

Understanding Operating Systems, 7e

47Slide48

Components of the I/O Subsystem (cont'd.)

Understanding Operating Systems, 7e

(figure 7.17)

Typical I/O

subsystem configuration.

© Cengage Learning 2014

48Slide49

Components of the I/O Subsystem (cont'd.)

Understanding Operating Systems, 7e

(figure 7.18)

I/O subsystem configuration with multiple paths, which increase both flexibility and reliability. With two additional paths, shown with dashed lines, if Control Unit 2 malfunctions, then Tape 2 can still be accessed via Control Unit 3.

© Cengage Learning 2014

49Slide50

Communication Among Devices

Problems to resolveKnow which components are busy/freeSolved by structuring interaction between unitsAccommodate requests during heavy I/O

traffic

Handled by buffering records and queuing requests

Accommodate speed disparity between CPU and I/O devices

Handled by buffering records and queuing requests

Understanding Operating Systems, 7e

50Slide51

Communication Among Devices (cont'd.)

I/O subsystem units finish independently of othersCPU processes data while I/O performedSuccess requires device completion knowledgeHardware flag tested by CPUChannel status word (CSW) contains flag

Three bits in flag represent I/O system component (channel, control unit, device)

Changes zero to one (free to busy)

Flag tested with

polling and

interrupt

s

Interrupts are more efficient way to test flag

Understanding Operating Systems, 7e

51Slide52

Communication Among Devices (cont'd.)

Direct memory access (DMA)Allows control unit main memory access directlyTransfers data without the intervention of CPU

Used for high-speed devices (disk)

Buffers

Temporary storage areas in main memory, channels, control units

Improves

data movement synchronization

Between relatively slow I/O devices and very fast CPU

Double buffering:

record processing by CPU while another is read or written by channel

Understanding Operating Systems, 7e

52Slide53

Communication Among Devices (cont'd.)

Understanding Operating Systems, 7e

(figure 7.19)

Example of double

buffering: (a) the CPU is reading from Buffer 1 as Buffer 2 is being filled; (b) once Buffer 2 is filled, it can be read quickly by the CPU while Buffer 1 is being filled again.

© Cengage Learning 2014

53Slide54

RAID

Physical disk drive set viewed as single logical unitPreferable over few large-capacity disk drivesImproved I/O performanceImproved data recovery Disk failure event

Introduces redundancy

Helps with hardware failure recovery

Significant factors

in RAID level selectionCost, speed, system’s applications

Increase

s

hardware costs

Understanding Operating Systems, 7e

54Slide55

RAID (cont'd.)

Understanding Operating Systems, 7e

(figure 7.20)

Data being transferred in parallel from a Level 0 RAID configuration to a large-capacity disk. The software in the controller ensures that the strips are stored in correct order.

© Cengage Learning 2014

55Slide56

Understanding Operating Systems, 7e

(table 7.7)

The seven standard levels of RAID provide various degrees of error correction. Cost, speed, and the system’s applications are significant factors to consider when choosing a level.

© Cengage Learning 2014

56Slide57

Level Zero

Uses data striping (not considered true RAID)No parity and error correctionsNo error correction/redundancy/recoveryBenefitsDevices appear as one logical unit

Best for large data quantity: non-critical data

Understanding Operating Systems, 7e

(figure 7.21)

RAID Level 0 with four disks in the array. Strips 1, 2, 3, and 4 make up a stripe. Strips 5, 6, 7, and 8 make up another stripe, and so on.

© Cengage Learning 2014

57Slide58

Level One

Uses data striping (considered true RAID)Mirrored configuration (backup)Duplicate set of all data (expensive)Provides redundancy and improved reliability

Understanding Operating Systems, 7e

(figure 7.22)

RAID Level 1 with three disks in the main array and three corresponding disks in the backup array, the mirrored array.

© Cengage Learning 2014

58Slide59

Level Two

Uses small stripes (considered true RAID)Hamming code: error detection and correctionExpensive and complexSize of strip determines number of array disks

Understanding Operating Systems, 7e

(figure 7.23)

RAID Level 2. Seven disks are needed in the array to store a 4-bit data item, one for each bit and three for the parity bits. Each disk stores either a bit or a parity bit based on the Hamming code used for redundancy.

© Cengage Learning 2014

59Slide60

Level Three

Modification of Level 2Requires one disk for redundancyOne parity bit for each strip

Understanding Operating Systems, 7e

(figure 7.24)

RAID Level 3. A 4-bit data item is stored in the first four disks of the array. The fifth disk is used to store the parity for the stored data item.

© Cengage Learning 2014

60Slide61

Level Four

Same strip scheme as Levels 0 and 1Computes parity for each stripStores parities in corresponding stripHas designated parity disk

Understanding Operating Systems, 7e

(figure 7.25)

RAID Level 4. The array contains four disks: the first three are used to store data strips, and the fourth is used to store the parity of those strips.

© Cengage Learning 2014

61Slide62

Level Five

Modification of Level 4Distributes parity strips across disksAvoids Level 4 bottleneckDisadvantageComplicated to regenerate data from failed device

Understanding Operating Systems, 7e

(figure 7.26)

RAID Level 5 with

four disks

. Notice

how the

parity strips

are distributed among the

disks.

© Cengage Learning 2014

62Slide63

Level Six

Provides extra degree of error protection/correctionTwo different parity calculations (double parity)Same as level four/five and independent algorithmParities stored on separate disk across array

Stored in corresponding data strip

Advantage: data restoration even if two disks fail

Understanding Operating Systems, 7e

(figure 7.27)

RAID Level 6.

Notice how

parity strips

and data

check (DC)

strips are

distributed

across the

disks.

© Cengage Learning 2014

63Slide64

Nested RAID Levels

Combines multiple RAID levels (complex)

Understanding Operating Systems, 7e

(figure 7.28)

A RAID Level 10 system.

© Cengage Learning 2014

64Slide65

Nested RAID Levels (cont'd.)

Understanding Operating Systems, 7e

(table 7.8)

Some common

nested RAID

configurations

, always

indicated with

two numbers

that signify

the combination

of levels.

For example

, neither Level

01 nor

Level 10 is the

same as

Level 1.

© Cengage Learning 2014

65Slide66

ConclusionDevice Manager

Manages every system device effectively as possibleDevices Vary in speed and sharability degreesDirect access and sequential access

Magnetic media: one or many read/write heads

Heads in a fixed position (optimum speed)

Move across surface (optimum storage space)

Optical media: disk speed adjustedData recorded/retrieved correctly

Understanding Operating Systems, 7e

66Slide67

Conclusion (cont'd.)

Flash memory: device manager tracks USB devicesAssures data sent/received correctlyI/O subsystem success dependenceCommunication linking

channels, control units, and devices

Seek strategies: advantages and disadvantages (summarized in Table 7.9)

Understanding Operating Systems, 7e

67Slide68

Understanding Operating Systems, 7e

(table 7.9)

Comparison of hard

disk drive

seek

strategies discussed

in this chapter.

© Cengage Learning 2014

68