Chapter 7 - Device Management

1. Chapter 7 - Device Management

2. Introduction - Device Management Device Manager: • Manages every peripheral device of the system • Maintains a balance of the user’s demand and the system’s finite supply of devices See Illustration p.145

3. Introduction - Device Management • Four Basic Functions • tracking the status of each device, tape & disk drives, printers, terminals, etc. • uses policies to determine which process will get a device & for how long • allocating devices • deallocating devices • process level • job level

4. System Devices • 3 Peripheral Device categories • dedicated • assigned only one job at a time, tape drives, printers, plotters • shared • assigned to several processes, disk pack, other storage devices by interleaving requests • virtual • combination dedicated & shared, printers, disks

5. Sequential Access Storage Media • 2 Groups of Storage Media • Sequential access media • store records sequentially, one after another • Direct access storage devices • can store either sequentially or direct access, directly reading or writing to a specific place

6. Sequential Access Storage Media Magnetic Tape • developed for early systems for routine secondary storage, archiving, back-up data • records are stored serially, each record can be any length stored in any location, record length usually determined by application • time consuming to locate records, “fast-forwarded” to desired position • parity bit used for routine error checking • interrecord gap (IRG) inserted in between each record See Fig. 7.1 p.147

7. Sequential Access Storage Media Magnetic Tape • blocking, grouping records into blocks before recording them, performed while file is created • transfer rate, density of tape (number or records in a block) • transport speed, speed of tape: transfer rate = density * transport speed

8. Sequential Access Storage Media Magnetic Tape Blocking Advantages: • fewer I/O operations • less tape is wasted Blocking Disadvantages • overhead & software routines are needed, deblocking, and record keeping • buffer space may be wasted Access times can vary widely

9. Direct Access Storage Devices • (DASDs) any devices that can directly read or write to a specific place on disk • Also called random access storage devices • Grouped into 2 categories • fixed read/write heads • movable read/write heads

10. Direct Access Storage Devices • Fixed-Head Drums & Disks • one of the first DASDs developed in early 1950’s • fixed-drum resembles a giant coffee can covered with magnetic film & formatted so tracks run around it • fixed-drum data is recorded serially on each track by the read/write head positioned over it See Fig. 7.4 & 7.5 p.150

11. Direct Access Storage Devices • Fixed-Head Drums & Disks • fixed-disk looks like a phonograph album covered with magnetic film that has been formatted, into concentric circles (tracks) • fixed-disk data is recorded serially on each track by the read/write head positioned over it

12. Direct Access Storage Devices • Movable-Head Drums & Disks • Movable-Head drums have only a few read/write heads that move from track to track to cover the entire surface of the drum • least expensive device with only one read/write head for the entire drum • conventional design device with several read/write heads that move together See Fig. 7.6 p.151

13. Direct Access Storage Devices • Movable-Head Drums & Disks • Movable-Head disk have only one read/write head that floats over the surface of the disk • individual units, such as those with many PCs • disk packs, stack of disks, each disk has 2 surfaces for recording (top & bottom) • tracks on disk varies by manufacturer, but typically range from 200 to 800 • tracks are numbered, Track 0 is the outermost concentric circle See Fig. 7.7 p.151

14. Direct Access Storage Devices • Optical Disc Storage (ODS) • Includes CD-ROM, provides reliable high-density storage for large amounts of data • 1995 ODS were read only, now CD-R (recordable) available • function similar to magnetic disk drive • read/write head on an arm • measure performance in data-transfer rate and average access time

15. Direct Access Storage Devices • Optical Disc Storage (ODS) • data transfer rate measured in kilobytes per second (Kps) refers to speed that data can be read off disc • access time indicates the average time required to move the read head to a specific place on the disc, expressed in milliseconds (ms) • hardware cache acts like a buffer by transferring blocks of data from the disk See Table 7.2 p.153

16. Direct Access Storage Devices Access Time Required: 3 factors that contribute to the time required to access a file • seek time • time required to position the read/write head on the proper track; slowest of 3 factors • search time • also known as rotational delay; time it takes to rotate the drum or disk until the requested record is moved under the read/write head • transfer time • when the data is actually transferred from secondary storage to main memory; fastest of 3 factors

17. Direct Access Storage Devices Access Time for Fixed-Head Devices • access a record by knowing its track and record number • total amount of time required to access data depends on 2 factors: • rotational speed • position of the record relative to the position of the read/write head Access Time = Search Time + Transfer Time See Fig 7.8 p.154

18. Direct Access Storage Devices Access Time for Movable-Head Devices • adds a third time element to the computation of access time: Access Time = Seek Time + Search Time + Transfer • overall, moveable-head devices are much more common than fixed-head DASDs • less costly and larger capacity; even though retrieval time is longer

19. I/O System Components • I/O subsystems work together • I/O Channels (dispatcher) • keeps up with the I/O requests from the CPU & pass them on to the appropriate control unit • programmable units placed between the CPU and the control unit • sends one signal for each function • synchronizes the fast speed of the CPU with the slow speed of the I/O device See Fig. 7.9 p.157

20. I/O System Components • I/O subsystems work together • I/O Control Unit (driver) • interprets the channel signal • sometimes part of the device • each control unit can direct several devices • flexibility, connect more than one channel for a control unit or connect more than one control unit to a single device • multiple paths increase reliability of the I/O subsystem See Fig. 7.10 & 11 p.158 -159

21. CommunicationAmong Devices • Demanding conditions of a busy computer system • Device Manager (3 Problems) • needs to know which components are busy & which are free • must be able to handle requests that come in during heavy I/O traffic • must accommodate disparity of speeds between CPU & I/O devices

22. CommunicationAmong Devices • Concurrent processing & I/O • each unit in the I/O subsystem can finish its operation independently • after a device has begun writing a record, & before completing the task, the connection between the device & its controller can be cut off to begin another I/O task with another device • CPU is free to process data while I/O is being performed

23. CommunicationAmong Devices • How does the system know when a device has completed an operation? • Hardware flag must be tested by the CPU • made up of 3 bits & resides in the Channel Status Word (CSW) • CSW, predefined location in main memory, contains channel status information • each bit represents one of the components indicating channel status • channel bit, control unit, & device • 0 free, 1 busy

24. CommunicationAmong Devices • Each component has access to the flag • can be tested before proceeding with the next I/O operation • Common ways to perform this test: • polling • interrupts

25. CommunicationAmong Devices • Polling • use special machine instructions to test the flag • major disadvantage CPU wastes time testing flag • Interrupts • more efficient way to test flags • interrupt handler determines best course for action

26. CommunicationAmong Devices • Direct Memory Access (DMA) • allows a control unit to access main memory more directly • data can be transferred to & from memory without CPU interaction • Buffers • more efficient way to test flags • interrupt handler determines best course for action See Fig. 7.12 p.161

27. Management of I/O Requests • Device Manager divides the task into 3 parts handled by a software component of the I/O subsystem • I/O traffic controller • I/O scheduler • I/O device handler

28. Management of I/O Requests • I/ traffic controller • monitors every device status, control unit, & channel • maintains control blocks • three main tasks: • determines if path is available • determines best path • determines when a path will become available See Table 7.5 p.162

29. Management of I/O Requests • I/O scheduler • performs same job as the Process Scheduler • allocates the devices, control units, & channels • decides which request will take priority • synchronizes its work with the traffic controller

30. Management of I/O Requests • I/O Device handler • processes the I/O interrupts • handles error conditions • provides detailed scheduled algorithms • each type of I/O device has its own device handler algorithm

31. Device Handler Seek Strategies • Seek Strategy determines the order processes get the device, keeping seek time to a minimum • Every scheduling algorithm should do the following: • minimize movement • minimize mean response time • minimize the variance in response time

32. Device Handler Seek Strategies • First come first served (FCFS) • simplest device-scheduling algorithm • easy to users & fair to users • seek time delay disadvantage • seek time is the most time consuming See Fig. 7.13 p.164

33. Device Handler Seek Strategies • Shortest seek time first (SSTF) • shortest jobs are processed first • longer jobs are made to wait • requests the track closest to the one being served • favors easy-to-reach requests & postpones long jobs farther away See Fig. 7.14 p.164

34. Device Handler Seek Strategies • SCAN uses a directional bit to indicate arm movement • LOOK (elevator algorithm/SCAN variation), arm does not go all the way to either edge unless requested • N-step SCAN holds all requests until the arm starts on its way back See Fig. 7.15 p.165

35. Device Handler Seek Strategies • C-SCAN (Circular SCAN) the arm picks up requests on its path during inward sweep • C-LOOK arm does not move all the way back to last track unless required The best scheduling algorithm may be a combination of more than one scheme

36. Search Strategies:Rotational Ordering • Optimize search times by ordering requests once the read/write heads have been positioned

37. Search Strategies:Rotational Ordering • “Rotational Ordering” • amount of time wasted due to rotational delay can be reduced • requests are ordered within each track • device controller must provide “rotational sensing” so the device driver can “see” which sector is under the read/write head See Fig. 7.16 & table 7.6 p.167

38. Summary • Device Manager’s job to manage all the devices despite varying speeds & sharing abilities • Direct Access and Sequential Access • One or many read/write heads • Fixed position or heads move across surface

39. Summary • Complex task of balancing demand for devices divided among hardware components: • channels • control units • devices • Success of the I/O subsystem depends on the communications that link these parts

40. Summary • Seek strategies to optimize seek time • FCFS: First Come, First Served • SSTF: Shortest Seek Time First • SCAN/LOOK • N-step SCAN • C-SCAN/C-LOOK