Chapter 7 device management


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Chapter 7 device management

  1. 1. Chapter Seven : Device Management <ul><li>System Devices </li></ul><ul><li>Sequential Access Storage Media </li></ul><ul><li>Direct Access Storage Devices </li></ul><ul><li>Components of I/O Subsystem </li></ul><ul><li>Communication Among Devices </li></ul><ul><li>Management of I/O Requests </li></ul><ul><li>Paper Storage Media </li></ul><ul><li>Magnetic Tape Storage </li></ul><ul><li>Magnetic Disk Storage </li></ul><ul><li>Optical Disc Storage </li></ul>
  2. 2. Device Management Functions <ul><li>Track status of each device (such as tape drives, disk drives, printers, plotters, and terminals). </li></ul><ul><li>Use preset policies to determine which process will get a device and for how long. </li></ul><ul><li>Allocate the devices. </li></ul><ul><li>Deallocate the devices at 2 levels: </li></ul><ul><ul><li>At process level when I/O command has been executed & device is temporarily released </li></ul></ul><ul><ul><li>At job level when job is finished & device is permanently released. </li></ul></ul>
  3. 3. System Devices <ul><li>Differences among system’s peripheral devices are a function of characteristics of devices, and how well they’re managed by the Device Manager. </li></ul><ul><li>Most important differences among devices </li></ul><ul><ul><li>Speeds </li></ul></ul><ul><ul><li>Degree of sharability. </li></ul></ul><ul><li>By minimizing variances among devices, a system’s overall efficiency can be dramatically improved. </li></ul>
  4. 4. Dedicated Devices <ul><li>Assigned to only one job at a time and serve that job for entire time it’s active. </li></ul><ul><ul><li>E.g., tape drives, printers, and plotters, demand this kind of allocation scheme, because it would be awkward to share. </li></ul></ul><ul><li>Disadvantage -- must be allocated to a single user for duration of a job’s execution. </li></ul><ul><ul><li>Can be quite inefficient, especially when device isn’t used 100 % of time. </li></ul></ul>
  5. 5. Shared Devices <ul><li>Assigned to several processes. </li></ul><ul><ul><li>E.g., disk pack (or other direct access storage device) can be shared by several processes at same time by interleaving their requests. </li></ul></ul><ul><li>Interleaving must be carefully controlled by Device Manager. </li></ul><ul><li>All conflicts must be resolved based on predetermined policies to decide which request will be handled first. </li></ul>
  6. 6. Virtual Devices <ul><li>Combination of dedicated devices that have been transformed into shared devices. </li></ul><ul><ul><li>E.g, printers are converted into sharable devices through a spooling program that reroutes all print requests to a disk. </li></ul></ul><ul><ul><li>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. </li></ul></ul><ul><ul><li>Because disks are sharable devices, this technique can convert one printer into several “virtual” printers, thus improving both its performance and use. </li></ul></ul>
  7. 7. Sequential Access Storage Media <ul><li>Magnetic tape used for secondary storage on early computer systems; now used for routine archiving & storing back-up data. </li></ul><ul><li>Records on magnetic tapes are stored serially, one after other. </li></ul><ul><li>Each record can be of any length. </li></ul><ul><ul><li>Length is usually determined by the application program. </li></ul></ul><ul><li>Each record can be identified by its position on the tape. </li></ul><ul><li>To access a single record, tape is mounted & “fast-forwarded” from its beginning until locate desired position. </li></ul>
  8. 8. Magnetic Tapes <ul><li>Data is recorded on 8 parallel tracks that run length of tape. </li></ul><ul><li>Ninth track holds parity bit used for routine error checking. </li></ul><ul><li>Number of characters that can be recorded per inch is determined by density of tape (e.g., 1600 or 6250 bpi). </li></ul>Parity Characters • • • • • • • • • • • •
  9. 9. Storing Records on Magnetic Tapes <ul><li>Can store records individually or grouped into blocks. </li></ul><ul><ul><li>If individually, each record is separated by a space to indicate its starting and ending places. </li></ul></ul><ul><ul><li>If blocks, then entire block is preceded by a space and followed by a space, but individual records are stored sequentially within block. </li></ul></ul><ul><li>Interrecord gap (IRG) is gap between records about 1 / 2 inch long regardless of the sizes of the records it separates. </li></ul><ul><li>Interblock gap ( IBG ) the gap between blocks of records; still 1 / 2 inch long. </li></ul>
  10. 10. Pros & Cons of Blocking <ul><li>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. </li></ul><ul><li>Less tape is wasted because size of physical record exceeds size of gap. </li></ul><ul><li>Overhead and software routines are needed for blocking, deblocking, and record keeping. </li></ul><ul><li>Buffer space may be wasted if you need only one logical record but must read an entire block to get it. </li></ul>
  11. 11. Transfer Rates & Speeds <ul><li>Block size set to take advantage of transfer rate. </li></ul><ul><li>Transfer rate -- density of the tape, multiplied by the tape transport speed (speed of the tape) </li></ul><ul><li>transfer rate = density * transport speed </li></ul><ul><li>If transport speed is 200 inches per second, at 1600 bpi, a total of 320,000 bytes can be transferred in one second, </li></ul><ul><ul><li>Theoretically optimal size of a block is 320,000 bytes. </li></ul></ul><ul><ul><li>Buffer must be equivalent. </li></ul></ul>
  12. 12. Magnetic Tape Access Times Vary Widely <ul><li>Benchmarks Access time </li></ul><ul><li>Maximum access 2.5 minutes </li></ul><ul><li>Average access 1.25 minutes </li></ul><ul><li>Sequential access 3 milliseconds </li></ul><ul><li>Variability makes magnetic tape a poor medium for routine secondary storage except for files with very high sequential activity. </li></ul>
  13. 13. Direct Access Storage Devices (R andom Access Storage Devices) <ul><li>Direct access storage devices (DASDs)-- any devices that can directly read or write to a specific place on a disk. </li></ul><ul><li>Two major categories: </li></ul><ul><li>DASD with fixed read/write heads </li></ul><ul><li>DASD with movable read/write heads. </li></ul><ul><li>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. </li></ul>
  14. 14. Fixed-Head Drums <ul><li>Magnetically recordable drums. </li></ul><ul><li>Resembles a giant coffee can covered with magnetic film and formatted so the tracks run around it. </li></ul><ul><li>Data is recorded serially on each track by the read/write head positioned over it. </li></ul><ul><li>Fixed-head drums were very fast but also very expensive, and they did not hold as much data as other DASDs. </li></ul>
  15. 15. Fixed Head Disks <ul><li>Fixed-head disks -- each disk looks like a phonograph album. </li></ul><ul><li>Covered with magnetic film that has been formatted, usually on both sides, into concentric circles. </li></ul><ul><li>Each circle is a track . Data is recorded serially on each track by the fixed read/write head positioned over it. </li></ul><ul><li>One head for each track. </li></ul>Rotation
  16. 16. Pros & Cons of Fixed Head Disks <ul><li>Very fast—faster than movable-head disks. </li></ul><ul><li>High cost. </li></ul><ul><li>Reduced storage space compared to a moveable-head disk </li></ul><ul><ul><li>because tracks must be positioned farther apart to accommodate width of the read/write heads. </li></ul></ul>
  17. 17. Movable-Head Drums and Disks <ul><li>Movable-head drums have only a few read/write heads that move from track to track to cover entire surface of drum. </li></ul><ul><ul><li>Least expensive device has only 1 read/write head for entire drum </li></ul></ul><ul><ul><li>More conventional design has several read/write heads that move together. </li></ul></ul><ul><li>One read/write head that floats over the surface of the disk. </li></ul><ul><li>Disks can be individual units (used with many PCs) or part of a disk pack (a stack of disks). </li></ul>
  18. 18. Cylinders <ul><li>It’s slower to fill a disk pack surface-by-surface than to fill it up track-by-track. </li></ul><ul><li>If fill Track 0 of all surfaces, got virtual cylinder of data. </li></ul><ul><ul><li>Are as many cylinders as there are tracks. </li></ul></ul><ul><ul><li>Cylinders are as tall as the disk pack. </li></ul></ul><ul><li>To access any given record, system needs: </li></ul><ul><ul><li>Cylinder number, so arm can move read/write heads to it. </li></ul></ul><ul><ul><li>Surface number, so proper read/write head is activated. </li></ul></ul><ul><ul><li>Record number, so read/write head know when to begin reading or writing. </li></ul></ul>
  19. 19. Optical Disc Storage (CD-ROM) <ul><li>Optical disc drives uses a laser beam to read and write to multi-layered discs. </li></ul><ul><li>Optical disc drives work in a manner similar to a magnetic disk drive. </li></ul><ul><ul><li>Head on an arm that moves forward and backward across the disc. </li></ul></ul><ul><li>Uses a high-intensity laser beam to burn pits (indentations) and lands (flat areas) in disc to represent ones and zeros, respectively. </li></ul>
  20. 20. Concentric Tracks vs. Spiraling Tracks <ul><li>Magnetic disk consists of concentric tracks of sectors and it spins at a constant speed (constant angular velocity). </li></ul><ul><ul><li>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. </li></ul></ul><ul><li>An optical disc consists of a single spiraling track of same-sized sectors running from center to rim of disc. </li></ul><ul><ul><li>Allows many more sectors & much more data to fit on optical disc compared to magnetic disk of same size . </li></ul></ul>
  21. 21. Measures of Performance for Optical Disc Drives <ul><li>Sustained data-transfer rate -- speed at which massive amounts of data can be read from disc. </li></ul><ul><ul><li>Measured in bytes per second (such as Mbps). </li></ul></ul><ul><ul><li>Crucial for applications requiring sequential access. </li></ul></ul><ul><li>Average access time -- average time required to move head to a specific place on disc. </li></ul><ul><ul><li>Expressed in milliseconds (ms). </li></ul></ul><ul><li>Cache size -- hardware cache acts as a buffer by transferring blocks of data from the disc </li></ul><ul><ul><li>Anticipates user may want to reread some recently retrieved info. </li></ul></ul><ul><ul><li>Act as read-ahead buffer, looking for next block of info on disc. </li></ul></ul>
  22. 22. CD-ROM Technology <ul><li>CD-ROM -- first commonly used optical storage DASD. </li></ul><ul><li>Stores very large databases, reference works, complex games, large software packages, system documentation, and user training material. </li></ul><ul><li>CD-ROM jukeboxes (autochangers or libraries) are capable of handling multiple discs and networked to distribute multimedia and reference works to distant user. </li></ul>
  23. 23. CD-Recordable Technology (CD-R) <ul><li>CD-R drives record data on optical discs using a write-once technique. </li></ul><ul><li>WORM (write once, read many). </li></ul><ul><li>Only a finite amount of data can be recorded on each disc and, once data is written, it can’t be erased or modified. </li></ul><ul><li>It has an extremely long shelf life. </li></ul>
  24. 24. CD-Rewritable Technology (CD-RW) <ul><li>CD-RW drives can read a standard CD-ROM, CD-R and CD-RW discs. </li></ul><ul><li>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. </li></ul><ul><li>Useful for storing large quantities of data and for sound, graphics, and multimedia applications. </li></ul>
  25. 25. Digital Video Disc ( DVD) Techonolgy <ul><li>DVD uses infrared laser to read disc (holds equivalent of 13 CD-ROM discs). </li></ul><ul><li>By using compression technologies, has more than enough space to hold a 2-hour of movie with enhanced audio. </li></ul><ul><ul><li>Single layered DVDs can hold 4.7 GB </li></ul></ul><ul><ul><li>Double-layered disc can hold 8.5 GB on each side of the disc. DVDs are used to store music, movies, and multimedia applications. </li></ul></ul><ul><li>DVD-RAM is a writable technology that uses a red laser to read, modify, and write data to DVD discs. </li></ul>
  26. 26. Three Factors Contribute To Time Required To Access a File <ul><li>Seek time -- time required to position the read/write head on the proper track. (Doesn’t apply to devices with fixed read/write heads.) </li></ul><ul><ul><li>Slowest of the three factors </li></ul></ul><ul><li>Search time (rotational delay) -- time it takes to rotate DASD until requested record is under read/write head. </li></ul><ul><li>Transfer time -- when data is actually transferred from secondary storage to main memory. </li></ul><ul><ul><li>Fastest. </li></ul></ul>
  27. 27. Access Time For Fixed-Head Devices <ul><li>Fixed-head devices can access a record by knowing its track number and record number. </li></ul><ul><li>Total amount of time required to access data depends on: </li></ul><ul><ul><li>Rotational speed is constant within each device (although it varies from device to device) </li></ul></ul><ul><ul><li>Position of record relative to position of the read/write head. </li></ul></ul><ul><ul><ul><li>search time (rotational delay) </li></ul></ul></ul><ul><ul><ul><li>+ transfer time (data transfer) </li></ul></ul></ul><ul><li>access time </li></ul>
  28. 28. Example of Access Time For Fixed-Head Devices <ul><li>How long will it take to access a record? </li></ul><ul><li>Typically, one complete revolution takes 16.8 ms, so average rotational delay is 8.4 ms. </li></ul><ul><li>Data transfer time varies from device to device, but a typical value is 0.00094 ms per byte </li></ul><ul><ul><li>size of record dictates this value. </li></ul></ul><ul><li>For example, it takes 0.094 ms (almost 0.1 ms) to transfer a record with 100 bytes. </li></ul>
  29. 29. Access Time For Movable-Head Devices <ul><li>Movable-head DASDs adds time required to move arm into position over the proper track ( seek time ). </li></ul><ul><ul><ul><li>seek time (arm movement) </li></ul></ul></ul><ul><ul><ul><li>search time (rotational delay) </li></ul></ul></ul><ul><ul><ul><li>+ transfer time (data transfer) </li></ul></ul></ul><ul><ul><ul><li>access time </li></ul></ul></ul><ul><li>Seek time is the longest and several strategies have been developed to minimize it. </li></ul>
  30. 30. 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
  31. 31. I/O Subsystem : I/O Channel <ul><li>I/O Channel -- keeps up with I/O requests from CPU and pass them down the line to appropriate control unit. </li></ul><ul><ul><li>Programmable units placed between CPU and control unit. </li></ul></ul><ul><ul><li>Synchronize fast speed of CPU with slow speed of the I/O device. </li></ul></ul><ul><ul><li>Make it possible to overlap I/O operations with processor operations so the CPU and I/O can process concurrently. </li></ul></ul><ul><li>Use channel programs that specifies action to be performed by devices & controls transmission of data between main memory & control units. </li></ul><ul><li>Entire path must be available when an I/O command is initiated. </li></ul>
  32. 32. I/O Subsystem : I/O Control Unit <ul><li>I/O control unit interprets signal sent by channel. </li></ul><ul><ul><li>One signal for each function. </li></ul></ul><ul><li>At start of I/O command, info passed from CPU to channel: </li></ul><ul><ul><li>I/O command (READ, WRITE, REWIND, etc.) </li></ul></ul><ul><ul><li>Channel number </li></ul></ul><ul><ul><li>Address of physical record to be transferred (from or to secondary storage) </li></ul></ul><ul><ul><li>Starting address of a memory buffer from which or into which record is to be transferred </li></ul></ul>
  33. 33. Device Manager Must <ul><li>Know which components are busy and which are free. </li></ul><ul><li>Be able to accommodate requests that come in during heavy I/O traffic. </li></ul><ul><li>Accommodate disparity of speeds between CPU and I/O devices. </li></ul>Handled by “buffering” records & queueing requests Solved by structuring interaction between units
  34. 34. Communication Among Devices <ul><li>Each unit in I/O subsystem can finish its operation independently from others. </li></ul><ul><li>CPU is free to process data while I/O is being performed, which allows for concurrent processing and I/O. </li></ul><ul><li>Success of operation depends on system’s ability to know when device has completed operation. </li></ul><ul><ul><li>Uses a hardware flag that must be tested by CPU. </li></ul></ul>
  35. 35. Hardware Flag Used To Communicate When A Device Has Completed An Operation <ul><li>Composed made up of three bits. </li></ul><ul><ul><li>Each bit represents a component of I/O subsystem. </li></ul></ul><ul><ul><li>One each for channel, control unit, and device. </li></ul></ul><ul><li>Resides in the Channel Status Word (CSW) </li></ul><ul><ul><li>In a predefined location in main memory and contains info indicating status of channel. </li></ul></ul><ul><li>Each bit is changed from zero to one to indicate that unit has changed from free to busy. </li></ul>
  36. 36. Testing the Flag : Polling or Interrupts <ul><li>Polling uses a special machine instruction to test flag. </li></ul><ul><ul><li>CPU periodically tests the channel status bit (in CSW). </li></ul></ul><ul><li>Major disadvantage with this scheme is determining how often the flag should be polled. </li></ul><ul><ul><li>If polling is done too frequently, CPU wastes time testing flag just to find out that channel is still busy. </li></ul></ul><ul><ul><li>If polling is done too seldom, channel could sit idle for long periods of time. </li></ul></ul>
  37. 37. Interrupts <ul><li>Use of interrupts is a more efficient way to test flag. </li></ul><ul><li>Hardware mechanism does test as part of every machine instruction executed by CPU. </li></ul><ul><li>If channel is busy flag is set so that execution of current sequence of instructions is automatically interrupted. </li></ul><ul><li>Control is transferred to interrupt handler , which resides in a predefined location in memory. </li></ul><ul><li>Some sophisticated systems are equipped with hardware that can distinguish between several types of interrupts. </li></ul>
  38. 38. Direct Memory Access (DMA) <ul><li>I/O technique that allows a control unit to access main memory directly. </li></ul><ul><li>Once reading or writing begins, remainder of data can be transferred to and from memory without CPU intervention. </li></ul><ul><li>To activate this process CPU sends enough info to control unit to initiate transfer of data </li></ul><ul><li>Then CPU goes to another task while control unit completes transfer independently. </li></ul><ul><li>This mode of data transfer is used for high-speed devices such as disks. </li></ul>
  39. 39. Buffers <ul><li>Buffers are temporary storage areas residing in convenient locations throughout system: main memory, channels, and control units. </li></ul><ul><li>Used extensively to better synchronize movement of data between relatively slow I/O devices & very fast CPU. </li></ul><ul><li>Double buffering --2 buffers are present in main memory, channels, and control units. </li></ul><ul><ul><li>While one record is being processed by CPU another can be read or written by channel </li></ul></ul>
  40. 40. Management of I/O Requests <ul><li>Device Manager divides task into 3 parts, with each handled by specific software component of I/O subsystem. </li></ul><ul><li>I/O traffic controller watches status of all devices, control units, and channels. </li></ul><ul><li>I/O scheduler implements policies that determine allocation of, and access to, devices, control units, and channels. </li></ul><ul><li>I/O device handler performs actual transfer of data and processes the device interrupts. </li></ul>
  41. 41. I/O Traffic Controller <ul><li>Monitors status of every device, control unit, and channel. </li></ul><ul><ul><li>Becomes more complex as number of units in I/O subsystem increases and as number of paths between these units increases. </li></ul></ul><ul><li>Three main tasks: (1) it must determine if there’s at least 1 path available; (2) if there’s 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. </li></ul><ul><li>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. </li></ul>
  42. 42. Traffic Controller Maintains Database For Each Unit In I/O Subsystem
  43. 43. I/O Scheduler <ul><li>I/O scheduler performs same job as Process Scheduler-- it allocates the devices, control units, and channels. </li></ul><ul><li>Under heavy loads, when # requests > # available paths, I/O scheduler must decide which request satisfied first. </li></ul><ul><li>I/O requests are not preempted: once channel program has started, it’s allowed to continue to completion even though I/O requests with higher priorities may have entered queue. </li></ul><ul><ul><li>Feasible because programs are relatively short (50 to 100 ms). </li></ul></ul>
  44. 44. I/O Scheduler - 2 <ul><li>Some systems allow I/O scheduler to give preferential treatment to I/O requests from “high-priority” programs. </li></ul><ul><ul><li>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. </li></ul></ul><ul><li>I/O scheduler must synchronize its work with traffic controller to make sure that a path is available to satisfy selected I/O requests. </li></ul>
  45. 45. I/O Device Handler <ul><li>I/O device handler processes the I/O interrupts, handles error conditions, and provides detailed scheduling algorithms, which are extremely device dependent. </li></ul><ul><li>Each type of I/O device has own device handler algorithm. </li></ul><ul><ul><li>first come first served (FCFS) </li></ul></ul><ul><ul><li>shortest seek time first (SSTF) </li></ul></ul><ul><ul><li>SCAN (including LOOK, N-Step SCAN, C-SCAN, and C-LOOK) </li></ul></ul><ul><li>Every scheduling algorithm should : </li></ul><ul><ul><li>Minimize arm movement </li></ul></ul><ul><ul><li>Minimize mean response time </li></ul></ul><ul><ul><li>Minimize variance in response time </li></ul></ul>
  46. 46. First Come First Served (FCFS) Device Scheduling Algorithm <ul><li>Simplest device-scheduling algorithm: </li></ul><ul><li>Easy to program and essentially fair to users. </li></ul><ul><li>On average, it doesn’t meet any of the three goals of a seek strategy. </li></ul><ul><li>Remember, seek time is most time-consuming of functions performed here, so any algorithm that can minimize it is preferable to FCFS. </li></ul>
  47. 47. Shortest Seek Time First (SSTF) Device Scheduling Algorithm <ul><li>Uses same underlying philosophy as shortest job next where shortest jobs are processed first & longer jobs wait. </li></ul><ul><li>Request with track closest to one being served (that is, one with shortest distance to travel) is next to be satisfied. </li></ul><ul><li>Minimizes overall seek time. </li></ul><ul><li>Favors easy-to-reach requests and postpones traveling to those that are out of way. </li></ul>
  48. 48. SCAN Device Scheduling Algorithm <ul><li>SCAN uses a directional bit to indicate whether the arm is moving toward the center of the disk or away from it. </li></ul><ul><li>Algorithm moves arm methodically from outer to inner track servicing every request in its path. </li></ul><ul><li>When it reaches innermost track it reverses direction and moves toward outer tracks, again servicing every request in its path. </li></ul>
  49. 49. LOOK (Elevator Algorithm) : A Variation of SCAN <ul><li>Arm doesn’t necessarily go all the way to either edge unless there are requests there. </li></ul><ul><li>“ Looks” ahead for a request before going to service it. </li></ul><ul><li>Eliminates possibility of indefinite postponement of requests in out-of-the-way places—at either edge of disk. </li></ul><ul><li>As requests arrive each is incorporated in its proper place in queue and serviced when the arm reaches that track. </li></ul>
  50. 50. Other Variations of SCAN <ul><li>N-Step SCAN -- holds all requests until arm starts on way back. New requests are grouped together for next sweep. </li></ul><ul><li>C-SCAN (Circular SCAN) -- arm picks up requests on its path during inward sweep. </li></ul><ul><ul><li>When innermost track has been reached returns to outermost track and starts servicing requests that arrived during last inward sweep. </li></ul></ul><ul><ul><li>Provides a more uniform wait time. </li></ul></ul><ul><li>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. </li></ul><ul><ul><li>Arm doesn’t necessarily return to the lowest-numbered track; it returns only to the lowest-numbered track that’s requested. </li></ul></ul>
  51. 51. Which Device Scheduling Algorithm? <ul><li>FCFS works well with light loads, but as soon as load grows, service time becomes unacceptably long. </li></ul><ul><li>SSTF is quite popular and intuitively appealing. It works well with moderate loads but has problem of localization under heavy loads. </li></ul><ul><li>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. </li></ul><ul><li>C-SCAN works well with moderate to heavy loads and has a very small variance in service times. </li></ul>
  52. 52. Search Strategies: Rotational Ordering <ul><li>Rotational ordering -- optimizes search times by ordering requests once read/write heads have been positioned. </li></ul><ul><ul><li>Nothing can be done to improve time spent moving read/write head because it’s dependent on hardware. </li></ul></ul><ul><li>Amount of time wasted due to rotational delay can be reduced. </li></ul><ul><ul><li>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. </li></ul></ul>
  53. 53. Redundant Array of Inexpensive Disks (RAID) <ul><li>RAID is a set of physical disk drives that is viewed as a single logical unit by OS. </li></ul><ul><li>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. </li></ul><ul><li>System shows improved I/O performance and improved data recovery in event of disk failure. </li></ul>
  54. 54. RAID -2 <ul><li>RAID introduces much-needed concept of redundancy to help systems recover from hardware failure. </li></ul><ul><li>Also requires more disk drives which increase hardware costs. </li></ul>
  55. 55. Six standard levels of RAID fall into 4 categories. Each offers a unique combination of advantages.
  56. 56. Terminology <ul><li>access time </li></ul><ul><li>blocking </li></ul><ul><li>buffers </li></ul><ul><li>Channel Status Word (CSW) </li></ul><ul><li>cylinder </li></ul><ul><li>dedicated device </li></ul><ul><li>direct access storage devices (DASDs) </li></ul><ul><li>direct memory access (DMA) </li></ul><ul><li>first come first served (FCFS) </li></ul><ul><li>I/O channel </li></ul><ul><li>I/O control unit </li></ul><ul><li>I/O device handler </li></ul><ul><li>I/O scheduler </li></ul><ul><li>I/O subsystem </li></ul><ul><li>I/O traffic controller </li></ul><ul><li>interblock gap (IBG) </li></ul><ul><li>interrecord gap (IRG) </li></ul><ul><li>interrupts </li></ul><ul><li>LOOK </li></ul><ul><li>magnetic tape </li></ul><ul><li>optical disc drive </li></ul><ul><li>polling </li></ul><ul><li>RAID </li></ul>
  57. 57. Terminology - 2 <ul><li>rotational ordering </li></ul><ul><li>SCAN </li></ul><ul><li>search strategy </li></ul><ul><li>search time </li></ul><ul><li>seek strategy </li></ul><ul><li>seek time </li></ul><ul><li>sequential access media </li></ul><ul><li>shared device </li></ul><ul><li>shortest seek time first (SSTF) </li></ul><ul><li>transfer rate </li></ul><ul><li>transfer time </li></ul><ul><li>transport speed </li></ul><ul><li>virtual device </li></ul>