Lecture 9. I/O Management and Disk Scheduling Algorithms

Transcription

1 Lecture 9 I/O Management and Disk Scheduling Algorithms 1

2 Lecture Contents 1. I/O Devices 2. Operating System Design Issues 3. Disk Scheduling Algorithms 4. RAID (Redundant Array of Independent Disks) 2

3 1. I/O Devices Three categories of I/O devices are Human readable: Suitable for communicating with the computer user. Examples: printers, video display/screen/monitor, keyboard, mouse Machine readable: Suitable for communicating with electronic equipment. Examples: disk drives, USB, sensors, controllers. 3

4 1. I/O Devices Three categories of I/O devices are Communications: Suitable for communicating with remote devices. Examples: modems, routers. 4

5 Differences in I/O Devices Devices differ in a number of areas Data Rate: i.e., data transfer rates (bps). Application: e.g., disk used for files requires support of file management software. Complexity of control: printer requires simple control interface. Disk is much more complex. Unit of transfer: Data may be transferred as a stream of bytes, or characters (e.g., terminal I/O), or blocks (e.g., disk I/O). 5

6 Differences in I/O Devices Devices differ in a number of areas Data Representation: Different data encoding schemes are used by different devices, Error conditions: The nature of errors differs widely from one device to another. Aspects include the way in which they are reported, their consequences, the available range of responses 6

7 Lecture Contents 1. I/O Devices 2. Operating System Design Issues 3. Disk Scheduling Algorithms 4. RAID (Redundant Array of Independent Disks) 7

8 2. OS Design Issues Two objectives in designing I/O facility are efficiency and generality. 8

9 2. OS Design Issues: Efficiency Most I/O devices extremely slow compared to main memory and processor - Solution: use of multiprogramming allows for some processes to be waiting on I/O while another process executes I/O cannot keep up with processor speed - Swapping is used to bring in ready processes to keep processor busy 9

10 2. OS Design Issues: Generality For simplicity and freedom from error, it is desirable to handle all I/O devices in a uniform manner Because of diversity of devices, it is difficult to achieve true generality. - Solution: Use a hierarchical, modular approach to the design of the I/O functions. This approach hides most of the details of I/O device in lower-level routines. 10

11 Hierarchical Design A hierarchical philosophy leads to organizing an OS into layers. Each layer relies on the next lower layer to perform more primitive functions. Each layer provides services to the next higher layer. Changes in one layer should not require changes in other layers. 11

12 Example of Hierarchical Approach: File System Directory management layer - is concerned with user operations (e.g., add, delete, reorganize) affecting files File system layer - deals with logical structure of files and operations (e.g., open, read, write, close) Physical organization layer - converts logical references and records to physical addresses 12

13 Lecture Contents 1. I/O Devices 2. Operating System Design Issues 3. Disk Scheduling Algorithms 4. RAID (Redundant Array of Independent Disks) 13

14 3. Disk Scheduling Algorithms Disks are currently at least four orders of magnitude slower than main memory. - This gap is expected to continue into the foreseeable future. Thus, the performance of disk storage subsystem is of vital concern, and many researchers proposed schemes for improving the disk storage subsystem performance. 14

15 Disk Performance Parameters The actual details of disk I/O operation depend on - computer system, - operating system, - the nature of I/O channel, and - disk controller hardware. 15

16 Components of Disk Drive. 16

17 Disk Performance Parameters When disk drive is operating, disk is rotating at constant speed. To read or write, the head must be positioned at the desired track/cylinder and at the beginning of the desired sector on that track. Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system. 17

18 Disk Performance Parameters On a movable-head system, the time it takes to position the head at the track/cylinder is known as seek time. Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head. The time it takes for the beginning of the sector to reach the head is known as rotational delay, or rotational latency. 18

19 Disk Performance Parameters The sum of the seek time, if any, and the rotational delay equals the access time, which is the time it takes to get into position to read or write. Once the head is in position, the read or write operation is then performed as the sector moves under the head. This is the data transfer portion of the operation. The time required for the data transfer is the transfer time. 19

20 Disk Scheduling Example 1 Suppose that a disk drive has 200 cylinders (or tracks), numbered 0 to 199. The current position of the head is 53 and the head is moving in the direction of increasing cylinder number. The queue of pending requests, in FIFO order, is 98, 183, 37, 122, 14, 124, 65, 67. What is the total distance (in cylinders/tracks) that the head moves to satisfy all the pending requests? 20

21 3.1 FCFS Scheduling Algorithm The FCFS (First-Come, First-Served) scheduling algorithm is also called First-In, First-Out (FIFO) scheduling algorithm. FCFS processes the pending requests in the queue sequentially. The FCFS algorithm is fair, but it generally does not provide the fastest service. 21

22 3.1 FCFS Scheduling Algorithm. 22

23 3.1 FCFS Scheduling Algorithm Head positions: 53, 98, 183, 37, 122, 14, 124, 65, 67 The total distance is (98-53) + (183-98) + (183-37) + (122-37) + (122-14) + (124-14) + (124-65) + (67-65) = = 640 The total distance is 640 cylinders 23

24 3.2 SSTF Scheduling Algorithm The SSTF (Shortest-Seek-Time-First) scheduling algorithm SSTF selects the request with the least seek time from the current head position. Thus, SSTF chooses the pending request closest to the current head position. There may be new arriving requests that will be chosen before an existing request. Thus, SSTF has possibility of starvation. 24

25 3.2 SSTF Scheduling Algorithm. 25

26 3.2 SSTF Scheduling Algorithm Head positions: 53, 65, 67, 37, 14, 98, 122, 124, 183 The total distance is (65-53) + (67-65) + (67-37) + (37-14) + (98-14) + (122-98) + ( ) + ( ) = = 236 The total distance is 236 cylinders 26

27 3.3 SCAN Scheduling Algorithm In the SCAN algorithm, the disk arm starts at one end of the disk and moves toward the other end, servicing requests as it reaches each cylinder, until it gets to the other end of the disk. At the other end, the direction of head movement is reversed, and servicing continues. The head continuously scans back and forth across the disk. 27

28 3.3 SCAN Scheduling Algorithm The SCAN algorithm is sometimes called the elevator algorithm, since the disk arm behaves just like an elevator in a building, first servicing all the requests going up and then reversing to service requests along the other way. 28

29 3.3 SCAN Scheduling Algorithm. 29

30 3.3 SCAN Scheduling Algorithm Head positions: 53, 65, 67, 98, 122, 124, 183, 199, 37, 14 The total distance is (65-53) + (67-65) + (98-67) + (122-98) + ( ) + ( ) + ( ) + (199-37) + (37-14) = = 331 cylinders 30

31 3.4 C-SCAN Scheduling Algorithm Circular SCAN (C-SCAN) scheduling is a variant of SCAN. C-SCAN moves the head from one end of the disk to the other, servicing requests along the way. When the head reaches the one end, the head immediately returns to the other end without servicing any requests on the return trip. The C-SCAN scheduling algorithm essentially treats the cylinders as a circular list that wraps around from the final cylinder to the first one. 31

32 3.4 C-SCAN Scheduling Algorithm. 32

33 3.4 C-SCAN Scheduling Algorithm Head positions: 53, 65, 67, 98, 122, 124, 183, 199, 0, 14, 37 The total distance is (65-53) + (67-65) + (98-67) + (122-98) + ( ) + ( ) + ( ) + (199-0) + (14-0) + (37-14) = = 382 cylinders 33

34 3.5 LOOK Scheduling Algorithm Both SCAN and C-SCAN move the disk arm across the full width of the disk. LOOK is a variation of SCAN. The arm goes only as far as the final request in each direction. Then, it reverses direction immediately, without going all the way to the end of the disk. 34

35 3.5 LOOK Scheduling Algorithm. 35

36 3.5 LOOK Scheduling Algorithm Head positions: 53, 65, 67, 98, 122, 124, 183, 37, 14 The total distance is (65-53) + (67-65) + (98-67) + (122-98) + ( ) + ( ) + (183-37) + (37-14) = = 299 The total distance is 299 cylinders 36

37 3.6 C-LOOK Scheduling Algorithm The C-LOOK scheduling algorithm is the circular LOOK algorithm. C-LOOK is a variation of C-SCAN. 37

38 3.6 C-LOOK Scheduling Algorithm. 38

39 3.6 C-LOOK Scheduling Algorithm Head positions: 53, 65, 67, 98, 122, 124, 183, 14, 37 The total distance is (65-53) + (67-65) + (98-67) + (122-98) + ( ) + ( ) + (183-14) + (37-14) = = 322 The total distance is 322 cylinders 39

40 Lecture Contents 1. I/O Devices 2. Operating System Design Issues 3. Disk Scheduling Algorithms 4. RAID (Redundant Array of Independent Disks) 40

41 4. RAID The reliability of data storage can be improved by storing data on multiple disks (i.e., improvement of reliability via redundancy). RAID (Redundant Array of Independent Disks) is a disk-organization technique used to achieve data reliability. Data are distributed across the array of physical disks. If a disk out of a set of N disks fails, data are not lost. 41

42 Improvement of Reliability via Mirroring The simplest (but most expensive) approach to introducing redundancy is to duplicate every disk. This technique is called mirroring. With mirroring, a logical disk consists of two physical disks, and every write is carried out on both disks. The result is called a mirrored volume. If one of the disks in the volume fails, the data can be read from the other. Data will be lost only if the second disk fails before the first failed disk is replaced. 42

43 Improvement of Data-Transfer Rate via Striping Storing data across multiple disks is called data striping. Bit-level striping: splits the bits of each byte across multiple disks. - Example 1: if we have an array of eight disks, we write bit i of each byte to disk i. - Example 2: if we use an array of four disks, bits i and 4 + i of each byte go to disk i. 43

44 Improvement of Data-Transfer Rate via Striping Block-level striping: blocks of a file are striped across multiple disks - if we have an array of n disks, block i of a file goes to disk (i mod n)

45 Improvement of Data-Transfer Rate via Striping The array of n disks can be treated as a single disk with sectors that are n times the normal size and that have n times the access rate. In such an organization, every disk participates in every access (read or write). Thus, the number of accesses that can be processed per second is about the same as on a single disk, but each access can read n times as many data in the same time as on a single disk. 45

46 RAID Levels Mirroring provides high reliability, but it is expensive. Striping provides high data-transfer rates, but it does not improve reliability. Numerous schemes to provide redundancy at lower cost by using disk striping combined with parity bits (which we describe next) have been proposed. These schemes have different costperformance trade-offs and are classified according to levels called RAID levels. 46

47 RAID Levels RAID level 0. RAID level 0 refers to disk arrays with block-level striping but without any redundancy (e.g., mirroring or parity bits). 47

48 RAID Levels RAID level 1. RAID level 1 refers to disk mirroring. 48

49 RAID Levels RAID level 2. RAID level 2 is also known as memory-style error-correcting code (ECC) organization. Each byte in a memory system may have a parity bit associated with it that records whether the number of bits in the byte set to 1 is even (parity = 0) or odd (parity = 1). 49

50 RAID Levels - If one of the bits in the byte is damaged (either a 1 becomes a 0, or a 0 becomes a 1), the parity of the byte changes and thus does not match the stored parity. - If the stored parity bit is damaged, it does not match the computed parity. Thus, all single-bit errors are detected by the memory system. The idea of ECC can be used directly in disk arrays via striping of bytes across disks. The disks labeled P store the error-correction bits. 50

51 RAID Levels RAID level 3. RAID level 3, or bit-interleaved parity organization, improves on level 2 by taking into account the fact that, disk controllers can detect whether a sector has been read correctly, so a single parity bit can be used for error correction as well as for detection. 51

52 RAID Levels RAID level 4. RAID level 4, or blockinterleaved parity organization, uses block-level striping, as in RAID 0, and in addition keeps a parity block on a separate disk for corresponding blocks from N other disks. If one of the disks fails, the parity block can be used with the corresponding blocks from the other disks to restore the blocks of the failed disk. 52

53 RAID Levels RAID level 5. RAID level 5, or blockinterleaved distributed parity, differs from level 4 by spreading data and parity among all N + 1 disks, rather than storing data in N disks and parity in one disk. 53

54 RAID Levels - For each block, one of the disks stores the parity and the others store data. For example, with an array of five disks, the parity for the nth block is stored in disk (n mod 5) + 1; the nth blocks of the other four disks store actual data for that block. 54

55 RAID Levels RAID level 6. RAID level 6, also called the P + Q redundancy scheme, is much like RAID level 5 but stores extra redundant information to guard against multiple disk failures. 55

56 RAID Levels - Instead of parity, error-correcting codes such as the Reed-Solomon codes are used. In the scheme shown in Figure 11.11(g), 2 bits of redundant data are stored for every 4 bits of data - compared with 1 parity bit in level 5 - and the system can tolerate two disk failures. 56

57 RAID Levels RAID levels RAID level refers to a combination of RAID levels 0 and 1. RAID 0 provides the performance, while RAID 1 provides the reliability. 57

58 RAID Levels - In RAID 0 + 1, a set of disks are striped, and then the stripe is mirrored to another, equivalent stripe. - Strengths: This level provides better performance than RAID 5. It is common in environments where both performance and reliability are important. - Weakness: like RAID 1, it doubles the number of disks needed for storage, so it is also relatively expensive. 58

59 RAID Levels RAID levels RAID level 1 + 0, in which disks are mirrored in pairs and then the resulting mirrored pairs are striped. 59

60 RAID Levels - This scheme has some theoretical advantages over RAID For example, if a single disk fails in RAID 0 + 1, an entire stripe is inaccessible, leaving only the other stripe available. With a failure in RAID 1 + 0, a single disk is unavailable, but the disk that mirrors it is still available. 60

61 Exercises 1. Suppose that a disk drive has 200 cylinders (or tracks), numbered 0 to 199. The current position of the head is 53 and the head is moving in the direction of decreasing cylinder number. The queue of pending requests, in FIFO order, is 98, 183, 37, 122, 14, 124, 65, 67. What is the total distance (in cylinders/tracks) that the head moves to satisfy all the pending requests? 61