Chapter Seven : Device Management

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 (such as tape drives,
  disk drives, printers, plotters, and terminals).
• 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 systems peripheral devices are
  a function of characteristics of devices, and how
  well theyre managed by the Device Manager.
• Most important differences among devices
• Speeds
• Degree of sharability.
• By minimizing variances among devices, a systems
  overall efficiency can be dramatically improved.

4
Dedicated Devices

• Assigned to only one job at a time and serve that
  job for entire time its active.
• E.g., tape drives, printers, and plotters, demand
  this kind of allocation scheme, because it would
  be awkward to share.
• Disadvantage -- must be allocated to a single
  user for duration of a jobs execution.
• Can be quite inefficient, especially when device
  isnt used 100 of time.

5
Shared Devices

• Assigned to several processes.
• E.g., disk pack (or other direct access storage
  device) can be shared by several processes at
  same time by interleaving their requests.
• Interleaving must be carefully controlled by
  Device Manager.
• 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.
• E.g, printers 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 jobs 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 (e.g., 1600
  or 6250 bpi).

Parity
Characters
9
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.

10
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, deblocking, and record keeping.
• Buffer space may be wasted if you need only one
  logical record but must read an entire block to
  get it.

11
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.

12
Magnetic Tape Access Times Vary Widely

• Benchmarks Access time
• Maximum access 2.5 minutes
• Average access 1.25 minutes
• Sequential access 3 milliseconds
• Variability makes magnetic tape a poor medium for
  routine secondary storage except for files with
  very high sequential activity.

13
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 isnt as
  wide as with magnetic tape, location of specific
  record still has a direct effect on amount of
  time required to access it.

14
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.

15
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
16
Pros Cons of Fixed Head Disks

• Very fastfaster than movable-head disks.
• High cost.
• Reduced storage space compared to a moveable-head
  disk
• because tracks must be positioned farther apart
  to accommodate width of the read/write heads.

17
Movable-Head Drums and Disks

• Movable-head drums 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).

18
Cylinders

• Its 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.

19
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.

20
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
Measures of Performance for Optical Disc Drives

• Sustained data-transfer rate -- speed at which
  massive amounts of data can be read from disc.
• Measured in bytes per second (such as Mbps).
• Crucial for applications requiring sequential
  access.
• Average access time -- average time required to
  move head to a specific place on disc.
• Expressed in milliseconds (ms).
• Cache size -- hardware cache acts as a buffer by
  transferring blocks of data from the disc
• Anticipates user may want to reread some recently
  retrieved info.
• Act as read-ahead buffer, looking for next block
  of info on disc.

22
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.

23
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 cant be
  erased or modified.
• It has an extremely long shelf life.

24
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.

25
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.

26
Three Factors Contribute To Time Required To
Access a File

• Seek time -- time required to position the
  read/write head on the proper track. (Doesnt
  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.

27
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

28
Example of Access Time For Fixed-Head Devices

• How long will it take to access a record?
• Typically, one complete revolution takes 16.8 ms,
  so average rotational delay is 8.4 ms.
• Data transfer time varies from device to device,
  but a typical value is 0.00094 ms per byte
• size of record dictates this value.
• For example, it takes 0.094 ms (almost 0.1 ms) to
  transfer a record with 100 bytes.

29
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.

30
Components of the I/O Subsystem
Control Unit 1
Channel 1
Control Unit 2
CPU
Control Unit 3
Channel 2
Control Unit 4
31
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.

32
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

33
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.

Solved by structuring interaction between units
Handled by buffering records queueing requests
34
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 systems ability
  to know when device has completed operation.
• Uses a hardware flag that must be tested by CPU.

35
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.

36
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.

37
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.

38
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.

39
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

40
Management of I/O Requests

• Device Manager divides task into 3 parts, with
  each handled by specific software component of
  I/O subsystem.
• I/O traffic controller watches status of all
  devices, control units, and channels.
• I/O scheduler implements policies that determine
  allocation of, and access to, devices, control
  units, and channels.
• I/O device handler performs actual transfer of
  data and processes the device interrupts.

41
I/O Traffic Controller

• Monitors status of every device, control unit,
  and channel.
• Becomes more complex as number of units in I/O
  subsystem increases and as number of paths
  between these units increases.
• Three main tasks (1) it must determine if
  theres at least 1 path available (2) if theres
  more than 1 path available, it must determine
  which to select and (3) if paths are all busy,
  it must determine when one will become available.
• Maintains a database containing status and
  connections for each unit in I/O subsystem,
  grouped into Channel Control Blocks, Control Unit
  Control Blocks, and Device Control Blocks.

42
Traffic Controller Maintains Database For Each
Unit In I/O Subsystem
43
I/O Scheduler

• I/O scheduler performs same job as Process
  Scheduler-- it allocates the devices, control
  units, and channels.
• Under heavy loads, when requests gt available
  paths, I/O scheduler must decide which request
  satisfied first.
• I/O requests are not preempted once channel
  program has started, its allowed to continue to
  completion even though I/O requests with higher
  priorities may have entered queue.
• Feasible because programs are relatively short
  (50 to 100 ms).

44
I/O Scheduler - 2

• Some systems allow I/O scheduler to give
  preferential treatment to I/O requests from
  high-priority programs.
• If a process has high priority then its I/O
  requests also has high priority and is satisfied
  before other I/O requests with lower priorities.
• I/O scheduler must synchronize its work with
  traffic controller to make sure that a path is
  available to satisfy selected I/O requests.

45
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

46
First Come First Served (FCFS) Device Scheduling
Algorithm

• Simplest device-scheduling algorithm
• Easy to program and essentially fair to users.
• On average, it doesnt meet any of the three
  goals of a seek strategy.
• Remember, seek time is most time-consuming of
  functions performed here, so any algorithm that
  can minimize it is preferable to FCFS.

47
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.

48
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.

49
LOOK (Elevator Algorithm) A Variation of SCAN

• Arm doesnt 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 placesat either
  edge of disk.
• As requests arrive each is incorporated in its
  proper place in queue and serviced when the arm
  reaches that track.

50
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
  doesnt move all the way to last track unless
  its required to do so.
• Arm doesnt necessarily return to the
  lowest-numbered track it returns only to the
  lowest-numbered track thats requested.

51
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.

52
Search Strategies Rotational Ordering

• Rotational ordering -- optimizes search times by
  ordering requests once read/write heads have been
  positioned.
• Nothing can be done to improve time spent moving
  read/write head because its dependent on
  hardware.
• Amount of time wasted due to rotational delay can
  be reduced.
• If requests are ordered within each track so that
  first sector requested on second track is next
  number higher than one just served, rotational
  delay is minimized.

53
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.

54
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.

55
Six standard levels of RAID fall into 4
categories. Each offers a unique combination of
advantages.
56
Terminology

• access time
• blocking
• buffers
• Channel Status Word (CSW)
• cylinder
• dedicated device
• direct access storage devices (DASDs)
• direct memory access (DMA)
• first come first served (FCFS)
• I/O channel
• I/O control unit

• I/O device handler
• I/O scheduler
• I/O subsystem
• I/O traffic controller
• interblock gap (IBG)
• interrecord gap (IRG)
• interrupts
• LOOK
• magnetic tape
• optical disc drive
• polling
• RAID

57
Terminology - 2

• rotational ordering
• SCAN
• search strategy
• search time
• seek strategy
• seek time
• sequential access media
• shared device
• shortest seek time first (SSTF)
• transfer rate
• transfer time

• transport speed
• virtual device