CHAP 6

• Dedicated device


  Assigned to one job at a time, for entire time that job is active (or until released), examples: tape drives, printers, and plotters
• Disadvantage of dedicated devices


  Must be allocated for duration of job's execution, inefficient if device is not used 100 percent of time
• Shared device


  Assigned to several processes, example: direct access storage device (DASD), processes share DASD simultaneously, requests interleaved, device manager supervision controls interleaving, predetermined policies determine conflict resolution
• Virtual device


  Dedicated and shared device combination, dedicated device transformed into shared device, example: printer converted by spooling program, spooling speeds up slow dedicated I/O devices, universal serial bus (USB) controller assigns bandwidth to each device: high, medium, or low priority
• I/O traffic controller


  • Watches status of devices, control units, channels, determines if path available, if more than one path available determines which one to select, if paths all busy determines when one is available, maintains database containing each unit's status and connections
• I/O scheduler


  • Allocates devices, control units, and channels, if requests greater than available paths decides which request to satisfy first based on different criteria, in many systems I/O requests not preempted, for some systems allow preemption with I/O request subdivided and preferential treatment for high-priority requests
• I/O device handler


  • Performs actual data transfer, processes device interrupts, handles error conditions, provides detailed scheduling algorithms, device dependent, each I/O device type has its own device handler algorithm
• Local operating system's role in accessing remote I/O devices is essentially the same role performed accessing local devices, cloud provides access to many more devices
• Magnetic tape


  Records stored serially, record length determined by application program, record identified by position on tape, record access requires tape to rotate passing under read/write head only when access requested for read or write

  
• Tape density


  Characters recorded per inch, depends upon storage method (individual or blocked records)

  
• Interrecord gap (IRG)


  1/2 inch gap inserted between each record, same size regardless of sizes of records it separates
• Blocking


  Group records into blocks
• Transfer rate


  (tape density) x (transport speed)
• Interblock gap (IBG)


  1/2 inch gap inserted between each block, more efficient than individual records and IRG
• Optimal block size


  Entire block fits in buffer
• Blocking advantages


  • Fewer I/O operations needed, less wasted tape space
• Blocking disadvantages


  • Overhead and software routines needed for blocking, deblocking, and record keeping, buffer space wasted when only one logical record needed
• Access time for magnetic tape

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

  
• Direct access storage devices

  Directly read or write to specific disk area, random access storage devices, three categories: magnetic disks, optical discs, solid state (flash) memory
• Magnetic disk storage


  Computer hard drives, single platter or stack of magnetic platters, two recording surfaces (top and bottom), each surface formatted with concentric tracks numbered from track 0 on outside to highest track number in center, read/write heads move in unison: virtual cylinder

  
• Accessing a record on magnetic disk

  System needs cylinder number, surface number, and sector number

  
• File access time factors

  Seek time (slowest): time to position read/write head on track, search time: rotational delay, time to rotate DASD until desired record under read/write head, transfer time (fastest): time to transfer data from secondary storage to main memory

  
• Fixed-head magnetic drives


  Record access requires track number and record number, total access time = search time + transfer time, DASD rotates continuously, three 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

  
• Movable-head magnetic drives


  Access time = seek time + search time + transfer time, search time and transfer time calculation same as fixed-head DASD, blocking good way to minimize access time

  
• Device handler seek strategies

  • Predetermined device handler determines device processing order to 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), goals: minimize arm movement, mean response time, and variance in response time
• First-come, first-served (FCFS) seek strategy

  On average does not meet the three seek strategy goals, disadvantage: extreme arm movement
• Shortest seek time first (SSTF) seek strategy


  Requests with track closest to one being served, minimizes overall seek time, postpones traveling to out of way tracks
• SCAN seek strategy


  Arm moves methodically from outer to inner track servicing every request in its path, when innermost track reached reverses direction and moves toward outer tracks servicing every request in its path
• LOOK (elevator algorithm) seek strategy

  Arm does not go to either edge unless requests exist, eliminates indefinite postponement
• 1. Step SCAN, C-SCAN, and C-LOOK seek strategies


  Variations of SCAN strategy to provide more uniform wait time and eliminate unnecessary last track access
• Seek strategy comparison


  • FCFS best with light loads, SSTF best with moderate loads, SCAN best with light to moderate loads, C-SCAN best with moderate to heavy loads
• Rotational ordering


  Optimizes search times by ordering requests once read/write heads positioned to reduce time wasted due to rotational delay
• Optical disc storage


  Single spiralling track, same-sized sectors from center to disc rim, spins at constant linear velocity (CLV), more sectors and more disc data than magnetic disk, two important performance measures: sustained data-transfer rate (speed to read massive data amounts from disc) and access time

  
• Seek time


  5 ms/track
• Search time


  1 ms/sector
• Data transfer


  1 ms
• 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
• Optical disc storage design features

  • Single spiralling track
  • Same-sized sectors: from center to disc rim
  • Spins at constant linear velocity (CLV)
  • More sectors and more disc data than magnetic disk
• Sustained data-transfer rate


  Speed 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 location, expressed in milliseconds (ms)