Chapter 6

1. Chapter 6 External Memory

2. Types of External Memory • Magnetic Disk • RAID (Redundant Array of Independent Disks) • Removable • Optical • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • DVD-R • DVD-RW • Magnetic Tape

3. Magnetic Disk • Disk substrate coated with magnetizable material (iron oxide…rust) • Substrate originally was aluminium - Is now glass • Improved surface uniformity • Increases reliability • Reduction in surface defects • Reduced read/write errors • Lower flight heights (head rides on air gap) • Better stiffness • Better shock/damage resistance

4. Disk Data Layout - Platter

5. Tracks and Cylinders

6. Multiple Platters

7. Disk Layout Methods Diagram

8. Physical Characteristics of Disk Systems

9. Inductive Write MR Read

10. Typical Hard Disk Drive Parameters

11. Formating • Why? • Must be able to identify position of data: start of track and sector • Marks tracks and sectors

12. Winchester Disk Format 30 fixed-length sectors per track

13. Speed Parameters • Seek time • Time to position head at track • Latency (Rotational) • Time for head to rotate to beginning of sector • Access time - Seek time + Latency time • Transfer rate - The rate at which data can be transferred after access T = b / N * 1/r Transfer time = bytes transferred / bytes/track * sec/revolution (track) Note: How does organization on disk (e.g. random vs sequential) effect total time?

14. Timing of Disk I/O Transfer

15. RAID – Goals: Speed, Reliability, Standardization • Redundant Array of Independent Disks (or Redundant Array of Inexpensive Disks ?) • Set of physical disks viewed as single logical drive by O/S • Data distributed across physical drives • Can use redundant capacity to store parity information • 7 “levels” of RAID organization (not a hierarchy) • 0 not really a RAID organization (no redundancy) • 1, 3 used for high transfer rate • 5, 6 used for high transaction rate • 2, 4 not commercially available • Requirements for high transfer rate: • High transfer rate along entire path between host memory to disk drives • Application must make I/O requests that drive disks efficiently

16. Redundant Array of Independent Disks Not RAID Not used Not used

17. RAID 0, 1, 2

18. RAID 0 Description: • No redundancy (Not “really” RAID) • Data striped across all disks • Round Robin striping Characteristics: • Increase speed • Multiple data requests probably not on same disk • Disks seek in parallel • A set of data is likely to be striped across multiple disks

19. RAID 1 Description: • Mirrored Disks • Data is striped across disks • 2 copies of each stripe on separate disks • Read from either • Write to both Characteristics: • Recovery is simple • Swap faulty disk & re-mirror • No down time • Expensive

20. RAID 2(not used) Description: • Disks are synchronized • Very small stripes • Often single byte/word • Error correction calculated across corresponding bits on disks • Multiple parity disks store Hamming code error correction in corresponding positions Characteristics: • Lots of redundancy • Very Expensive • Not used commercially

21. RAID 3 & 4

22. RAID 3 Description: • Similar to RAID 2 • Only one “redundant” disk, no matter how large the array • Simple parity bit for each set of corresponding bits Characteristics: • Data on failed drive can be reconstructed from surviving data and parity info • Very high transfer rates • Not very expensive or complex

23. RAID 4(not used) Description: • Each disk operates independently • Good for high I/O request rate • Large stripes • Bit by bit parity calculated across stripes on each disk • Parity stored on parity disk Characteristics: • Good for high request rates rather than high transfer rates • Every write impacts the parity disk so it becomes a bottleneck. • Not used commercially

24. RAID 5 & 6

25. RAID 5 Description: • Very similar to RAID 4 • Parity striped across all disks • Round robin allocation for parity stripe Characteristics: • Avoids RAID 4 bottleneck at parity disk • Commonly used in network servers

26. RAID 6 Description: • Two parity calculations • Stored in separate blocks on different disks • User requirement of N disks needs N+2 disks Characteristics: • High data availability • Three disks need to fail for data loss • Significant write penalty (two parity calculations)

27. RAID Comparison (1) Not RAID Not used

28. Raid Comparison (2) Not used

29. Types of External Memory • Magnetic Disk • RAID (Redundant Array of Independent Disks) • Removable • Optical • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • DVD-R • DVD-RW • Magnetic Tape

30. Optical Products

31. Optical Storage CD-ROM • Originally for audio • 650Mbytes giving over 70 minutes audio • Polycarbonate coated with highly reflective coat, usually aluminium • Data stored as pits • Read by reflecting laser • Constant packing density • Constant linear velocity

32. CD Construction

33. CD Layout

34. CD reader

35. CD-ROM Drive Speeds • Audio is single speed • Constant linear velocity • 1.2 m/sec • Track (spiral) is 5.27km long • Gives 4391 seconds = 73.2 minutes • Other speeds are quoted as multiples • e.g. 24x • Quoted figure is maximum drive can achieve • Note: CD-ROM has option of error correction (not on CD)

36. CD-ROM Format • Mode 0 = blank data field • Mode 1 = 2048 byte data + error correction • Mode 2 = 2336 byte data

37. Random Access on CD-ROM & CD-R • Difficult • Process: • Move head to rough position • Set correct speed • Read address • Adjust to required location

38. CD-ROM CD-R for & against • Large capacity (?) • Easy to mass produce • Removable • Robust • Expensive for small runs • Slow • Read only

39. CD-RW • Erasable • Getting cheaper • Mostly CD-ROM drive compatible • Phase change • Material has two different reflectivities in different phase states

40. DVD - technology • Multi-layer • Very high capacity (4.7G per layer) • Full length movie on single disk • Using MPEG compression

41. CD vs DVD

42. DVD’s Two objectives had to be resolved to make the DVDs viable. • The linear velocity of a DVD must be held constant and be able to reproduce a vertical frame rate of 29.97 frames/second • Every DVD player had to have absolute tracking accuracy to insure the extremely narrow laser beam would scan exactly in the middle of the track where the data was recorded.  The solution: • The disk is pressed with the track grooves accurately pre-cut and encoded with a constant bit rate frequency.Thus a blank DVD disk isn't really blank at all.

43. Types of External Memory • Magnetic Disk • RAID (Redundant Array of Independent Disks) • Removable • Optical • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • DVD-R • DVD-RW • Magnetic Tape

44. Magnetic Tape • Serial access • Slow • Very cheap • Used for backup and archive

45. Chapter 7 Input/Output

46. Input/Output Challenges • Must support a wide variety of peripherals • Delivering different amounts of data • At different speeds • In different formats • All slower than CPU and RAM • Need standardized I/O Interfaces (modules) or channels

47. Generic Model of I/O Device Interface (Module)

48. I/O Interface Function • Support single or multiple devices • Hide or reveal device properties • Provide • Control & Timing • CPU Communication • Device Communication • Data Buffering • Perhaps Error Detection

49. I/O Device Interface Diagram

50. Input/Output Alternatives • Programmed I/O Interface • Interrupt driven I/O Interface • Direct Memory Access (DMA) I/O Interface