understanding operating systems 1 chapter seven : device management system devices sequential access...

Understanding Operating Systems

1

Chapter Seven : Device Management

• System Devices

• Sequential Access Storage Media

• Direct Access Storage Devices

• Components of I/O Subsystem

• Communication Among Devices

• Management of I/O Requests

Paper Storage Media

Magnetic Tape Storage

Magnetic Disk Storage

Optical Disc Storage


2

Device Management Functions

• Track status of each device.

• Use preset policies to determine which process will get a device and for how long.

• Allocate the devices.

• Deallocate the devices at 2 levels:– At process level when I/O command has been executed & device

is temporarily released

– At job level when job is finished & device is permanently released.


3

System Devices

• Differences among system’s peripheral devices are a function of characteristics of devices, and how well they’re managed by the Device Manager.

• Most important differences among devices

– Speeds – Degree of sharability.

• By minimizing variances among devices, a system’s overall efficiency can be dramatically improved.


4

Dedicated Devices

• Assigned to only one job at a time and serve that job for entire time it’s active. – tape drives, printers, and plotters, demand this kind of

allocation scheme, because it would be impossible to share.

• Disadvantage -- must be allocated to a single user for duration of a job’s execution.

– Can be quite inefficient, especially when device isn’t used 100 % of time.


5

Shared Devices

• Assigned to several processes.

– disk pack (or other direct access storage device) can be shared by several processes at same time by interleaving their requests.

• Actual storage managed by File System.

• All conflicts must be resolved based on predetermined policies to decide which request will be handled first.


6

Virtual Devices

• Combination of dedicated devices that have been transformed into shared devices.

– Printers, faxes are converted into sharable devices through a spooling program that reroutes all print requests to a disk.

– Output sent to printer for printing only when all of a job’s output is complete and printer is ready to print out entire document.

– Because disks are sharable devices, this technique can convert one printer into several “virtual” printers, thus improving both its performance and use.


7

Sequential Access Storage Media

• Magnetic tape used for secondary storage on early computer systems; now used for routine archiving & storing back-up data.

• Records on magnetic tapes are stored serially, one after other.

• Each record can be of any length. – Length is usually determined by the application program.

• Each record can be identified by its position on the tape.

• To access a single record, tape is mounted & “fast-forwarded” from its beginning until locate desired position.


8

Magnetic Tapes

• Data is recorded on 8 parallel tracks that run length of tape.

• Ninth track holds parity bit used for routine error checking.

• Number of characters that can be recorded per inch is determined by density of tape (800, 1600 or 6250 bpi).

Parity

••••

• ••

• ••••

Characters


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Storing Records on Magnetic Tapes

• Can store records individually or grouped into blocks. – If individually, each record is separated by a space to indicate its

starting and ending places.

– If blocks, then entire block is preceded by a space and followed by a space, but individual records are stored sequentially within block.

• Interrecord gap (IRG) is gap between records about 1/2 inch long regardless of the sizes of the records it separates.

• Interblock gap (IBG) the gap between blocks of records; still 1/2 inch long.


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Pros & Cons of Blocking

• Fewer I/O operations are needed because a single READ command can move an entire block (physical record that includes several logical records) into main memory.

• Less tape is wasted because size of physical record exceeds size of gap.

• Overhead and software routines are needed for blocking, de-blocking, and record keeping.

• Buffer space may be wasted if you need only one logical record but must read an entire block to get it.


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Transfer Rates & Speeds

• Block size set to take advantage of transfer rate.

• Transfer rate -- density of the tape, multiplied by the tape transport speed (speed of the tape)

transfer rate = density * transport speed

• If transport speed is 200 inches per second, at 1600 bpi, a total of 320,000 bytes can be transferred in one second, – Theoretically optimal size of a block is 320,000 bytes.

– Buffer must be equivalent.


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Direct Access Storage Devices (Random Access Storage Devices)

• Direct access storage devices (DASDs)-- any devices that can directly read or write to a specific place on a disk.

• Two major categories:

• DASD with fixed read/write heads

• DASD with movable read/write heads.

• Although variance in DASD access times isn’t as wide as with magnetic tape, location of specific record still has a direct effect on amount of time required to access it.


13

Fixed-Head Drums

• Magnetically recordable drums.

• Resembles a giant coffee can covered with magnetic film and formatted so the tracks run around it.

• Data is recorded serially on each track by the read/write head positioned over it.

• Fixed-head drums were very fast but also very expensive, and they did not hold as much data as other DASDs.


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Fixed Head Disks

• Fixed-head disks -- each disk looks like a phonograph album.

• Covered with magnetic film that has been formatted, usually on both sides, into concentric circles.

• Each circle is a track. Data is recorded serially on each track by the fixed read/write head positioned over it.

• One head for each track.

Rotation


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Movable-Head Disks

• Movable-head disks have only a few read/write heads that move from track to track to cover entire surface of drum. – Least expensive device has only 1 read/write head for entire drum

– More conventional design has several read/write heads that move together.

• One read/write head that floats over the surface of the disk.

• Disks can be individual units (used with many PCs) or part of a disk pack (a stack of disks).


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Cylinders

• It’s slower to fill a disk pack surface-by-surface than to fill it up track-by-track.

• If fill Track 0 of all surfaces, got virtual cylinder of data.– Are as many cylinders as there are tracks.

– Cylinders are as tall as the disk pack.

• To access any given record, system needs:– Cylinder number, so arm can move read/write heads to it.

– Surface number, so proper read/write head is activated.

– Record number, so read/write head know when to begin reading or writing.


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Sectors

• Each track is divided into a fixed number of fixed sized sectors [clusters]

• A sector is the finest level of access to DASD data.


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Optical Disc Storage (CD-ROM)

• Optical disc drives uses a laser beam to read and write to multi-layered discs.

• Optical disc drives work in a manner similar to a magnetic disk drive.– Head on an arm that moves forward and backward across the disc.

• Uses a high-intensity laser beam to burn pits (indentations) and lands (flat areas) in disc to represent ones and zeros, respectively. Based on reflectivity.


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Optical Disc Storage (CD-ROM)


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Concentric Tracks vs. Spiraling Tracks

• Magnetic disk consists of concentric tracks of sectors and it spins at a constant speed (constant angular velocity). – Because sectors at outside of disk spin faster past

read/write head than inner sectors, outside sectors are much larger than sectors located near center of disk.

• An optical disc consists of a single spiraling track of same-sized sectors running from center to rim of disc. – Allows many more sectors & much more data to fit on

optical disc compared to magnetic disk of same size.


21

CD-ROM Technology

• CD-ROM -- first commonly used optical storage DASD.

• Stores very large databases, reference works, complex games, large software packages, system documentation, and user training material.

• CD-ROM jukeboxes (autochangers or libraries) are capable of handling multiple discs and networked to distribute multimedia and reference works to distant user.


22

CD-Recordable Technology (CD-R)

• CD-R drives record data on optical discs using a write-once technique.

• WORM (write once, read many).

• Only a finite amount of data can be recorded on each disc and, once data is written, it can’t be erased or modified.

• It has an extremely long shelf life.


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CD-Recordable Technology (CD-R)


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CD-Rewritable Technology (CD-RW)

• CD-RW drives can read a standard CD-ROM, CD-R and CD-RW discs.

• CD-RW discs can be written and rewritten many times by focusing a low-energy laser beam on surface, heating media just enough to erase pits that store data and restoring recordable media to its original state.

• Useful for storing large quantities of data and for sound, graphics, and multimedia applications.


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• In order to achieve these effects in the recording layer, the CD-Rewritable recorder use three different laser powers:

the highest laser power, which is called "Write Power", creates a non-crystalline (absorptive) state on the recording layer

the middle power, also known as "Erase Power", melts the recording layer and converts it to a reflective crystalline state

the lowest power, which is "Read Power", does not alter the state of the recording layer, so it can be used for reading the data.

http://www.pctechguide.com/09cdr-rw.htm#Formats


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Digital Video Disc (DVD) Techonolgy

• DVD uses infrared laser to read disc (holds equivalent of 13 CD-ROM discs).

• By using compression technologies, has more than enough space to hold a 2-hour of movie with enhanced audio.

– Single layered DVDs can hold 4.7 GB

– Double-layered disc can hold 8.5 GB on each side of the disc. DVDs are used to store music, movies, and multimedia applications.

• DVD-RAM is a writable technology that uses a red laser to read, modify, and write data to DVD discs.


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Digital Video Disc (DVD) Techonolgy


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Three Factors Contribute To Time Required To Access a File

• Seek time -- time required to position the read/write head on the proper track. (Doesn’t apply to devices with fixed read/write heads.)– Slowest of the three factors

• Search time (rotational delay) -- time it takes to rotate DASD until requested record is under read/write head.

• Transfer time -- when data is actually transferred from secondary storage to main memory.– Fastest.


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Access Time For Fixed-Head Devices

• Fixed-head devices can access a record by knowing its track number and record number.

• Total amount of time required to access data depends on:– Rotational speed is constant within each device (although it varies

from device to device)

– Position of record relative to position of the read/write head.

search time (rotational delay)

+ transfer time (data transfer)

access time


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Access Time For Movable-Head Devices

• Movable-head DASDs adds time required to move arm into position over the proper track (seek time).

seek time (arm movement)

search time (rotational delay)

+ transfer time (data transfer)

access time

• Seek time is the longest and several strategies have been developed to minimize it.


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Components of the I/O Subsystem

CPU

Channel 1

Control Unit 1

Channel 2

Control Unit 2

Control Unit 3

Control Unit 4

Disk 1

Disk 2

Disk 3

Tape 1

Tape 2

Tape 3

Tape 4

Disk 4

Disk 5


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I/O Subsystem : I/O Channel

• I/O Channel -- keeps up with I/O requests from CPU and pass them down the line to appropriate control unit. – Programmable units placed between CPU and control unit.

– Synchronize fast speed of CPU with slow speed of the I/O device.

– Make it possible to overlap I/O operations with processor operations so the CPU and I/O can process concurrently.

• Use channel programs that specifies action to be performed by devices & controls transmission of data between main memory & control units.

• Entire path must be available when an I/O command is initiated.


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I/O Subsystem : I/O Control Unit

• I/O control unit interprets signal sent by channel.– One signal for each function.

• At start of I/O command, info passed from CPU to channel:– I/O command (READ, WRITE, REWIND, etc.)

– Channel number

– Address of physical record to be transferred (from or to secondary storage)

– Starting address of a memory buffer from which or into which record is to be transferred


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Device Manager Must

• Know which components are busy and which are free.

• Be able to accommodate requests that come in during heavy I/O traffic.

• Accommodate disparity of speeds between CPU and I/O devices.Handled by “buffering”

records & queueing requests

Solved by structuring interaction between

units


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Communication Among Devices

• Each unit in I/O subsystem can finish its operation independently from others.

• CPU is free to process data while I/O is being performed, which allows for concurrent processing and I/O.

• Success of operation depends on system’s ability to know when device has completed operation. – Uses a hardware flag that must be tested by CPU.


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Hardware Flag Used To Communicate When A Device Has Completed An Operation

• Composed made up of three bits.

– Each bit represents a component of I/O subsystem.

– One each for channel, control unit, and device.

• Resides in the Channel Status Word (CSW)

– In a predefined location in main memory and contains info indicating status of channel.

• Each bit is changed from zero to one to indicate that unit has changed from free to busy.


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Testing the Flag : Polling or Interrupts

• Polling uses a special machine instruction to test flag.– CPU periodically tests the channel status bit (in CSW).

• Major disadvantage with this scheme is determining how often the flag should be polled.

– If polling is done too frequently, CPU wastes time testing flag just to find out that channel is still busy.

– If polling is done too seldom, channel could sit idle for long periods of time.


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Interrupts

• Use of interrupts is a more efficient way to test flag.

• Hardware mechanism does test as part of every machine instruction executed by CPU.

• If channel is busy flag is set so that execution of current sequence of instructions is automatically interrupted.

• Control is transferred to interrupt handler, which resides in a predefined location in memory.

• Some sophisticated systems are equipped with hardware that can distinguish between several types of interrupts.


40

Direct Memory Access (DMA)

• I/O technique that allows a control unit to access main memory directly.

• Once reading or writing begins, remainder of data can be transferred to and from memory without CPU intervention.

• To activate this process CPU sends enough info to control unit to initiate transfer of data

• Then CPU goes to another task while control unit completes transfer independently.

• This mode of data transfer is used for high-speed devices such as disks.


41

Buffers

• Buffers are temporary storage areas residing in convenient locations throughout system: main memory, channels, and control units.

• Used extensively to better synchronize movement of data between relatively slow I/O devices & very fast CPU.

• Double buffering --2 buffers are present in main memory, channels, and control units. – While one record is being processed by CPU another can be read

or written by channel


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I/O Software

• Device Independence.• Handle errors at the lowest level.• Handle blocking/interrupts• Layered


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I/O Software

• User Application– issues the I/O request

• Device Independent O/S– accepts application request

– handles permissions, allocation

• Device Drivers– translates O/S commands into device specific commands

– translates to cylinder, track, sector

• I/O Device Handlers– manage the behavior of the device.


44

I/O Device Handler

• I/O device handler processes the I/O interrupts, handles error conditions, and provides detailed scheduling algorithms, which are extremely device dependent.

• Each type of I/O device has own device handler algorithm.– first come first served (FCFS)

– shortest seek time first (SSTF)

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

• Every scheduling algorithm should :– Minimize arm movement

– Minimize mean response time

– Minimize variance in response time


45

First Come First Served (FCFS) Device Scheduling Algorithm

• Simplest device-scheduling algorithm:

• Easy to program and essentially fair to users.

• On average, it is the slowest.

• Remember, seek time is most time-consuming of functions performed here, so any algorithm that can minimize it is preferable to FCFS.


46

Shortest Seek Time First (SSTF) Device Scheduling Algorithm

• Uses same underlying philosophy as shortest job next where shortest jobs are processed first & longer jobs wait.

• Request with track closest to one being served (that is, one with shortest distance to travel) is next to be satisfied.

• Minimizes overall seek time.

• Favors easy-to-reach requests and postpones traveling to those that are out of way.


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SCAN Device Scheduling Algorithm

• SCAN uses a directional bit to indicate whether the arm is moving toward the center of the disk or away from it.

• Algorithm moves arm methodically from outer to inner track servicing every request in its path.

• When it reaches innermost track it reverses direction and moves toward outer tracks, again servicing every request in its path.


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LOOK (Elevator Algorithm) : A Variation of SCAN

• Arm doesn’t necessarily go all the way to either edge unless there are requests there.

• “Looks” ahead for a request before going to service it.

• Eliminates possibility of indefinite postponement of requests in out-of-the-way places—at either edge of disk.

• As requests arrive each is incorporated in its proper place in queue and serviced when the arm reaches that track.


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Other Variations of SCAN

• N-Step SCAN -- holds all requests until arm starts on way back. New requests are grouped together for next sweep.

• C-SCAN (Circular SCAN) -- arm picks up requests on its path during inward sweep. – When innermost track has been reached returns to outermost track

and starts servicing requests that arrived during last inward sweep. – Provides a more uniform wait time.

• C-LOOK (optimization of C-SCAN) --sweep inward stops at last high-numbered track request, so arm doesn’t move all the way to last track unless it’s required to do so. – Arm doesn’t necessarily return to the lowest-numbered track; it

returns only to the lowest-numbered track that’s requested.


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Which Device Scheduling Algorithm?

• FCFS works well with light loads, but as soon as load grows, service time becomes unacceptably long.

• SSTF is quite popular and intuitively appealing. It works well with moderate loads but has problem of localization under heavy loads.

• SCAN works well with light to moderate loads and eliminates problem of indefinite postponement. SCAN is similar to SSTF in throughput and mean service times.

• C-SCAN works well with moderate to heavy loads and has a very small variance in service times.


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Redundant Array of Inexpensive Disks (RAID)

• RAID is a set of physical disk drives that is viewed as a single logical unit by OS.

• RAID assumes several smaller-capacity disk drives preferable to few large-capacity disk drives because, by distributing data among several smaller disks, system can simultaneously access requested data from multiple drives.

• System shows improved I/O performance and improved data recovery in event of disk failure.


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RAID -2

• RAID introduces much-needed concept of redundancy to help systems recover from hardware failure.

• Also requires more disk drives which increase hardware costs.

• http://www.acnc.com/04_01_00.html

http://www.acnc.com/04_01_00.html




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Six standard levels of RAID fall into 4 categories. Each offers a unique combination of advantages.

Level Category Description I/O RequestRate

DataTransferRate

0 Data Striping Nonredundant Excellent Excellent1 Mirroring Mirrored Good/Fair Fair/Fair2 Parallel Access Redundant Poor Excellent3 Parallel Access Bit-interleaved parity Poor Excellent4 Independent

AccessBlock-interleaved parity Excellent/ Fair Fair/Poor

5 IndependentAccess

Block-interleaveddistributed parity

Excellent/ Fair Fair/Poor

understanding operating systems 1 chapter seven : device management system devices sequential access...

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