1. Chapter 7 External Memory Computer Architecture Chapter Seven: 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 Magnetic Tape Chapter Seven: External Memory

3. Magnetic Disk A disk - a circular platter constructed of nonmagnetic material. - called substrate. Disk substrate coated with magnetizable material Substrate used – aluminium, glass substrate (more recently) Benefits of glass substrate:- –Improved magnetic film surface uniformity Increases reliability –Reduction in surface defects Reduced read/write errors –Lower fly heights –Better stiffness –Greater ability to withstand shock and damage. Chapter Seven: External Memory

4. Recording & retrieval via conductive coil called a head May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write –Current through coil produces magnetic field –Pulses sent to head –Magnetic pattern recorded on surface below Read (traditional) –Magnetic field moving relative to coil produces current –Coil is the same for read and write-one head for read/write Read (contemporary) –Separate read head, close to write head –Partially shielded magneto resistive (MR) sensor –Electrical resistance depends on direction of magnetic field –High frequency operation Higher storage density and speed Chapter Seven: External Memory Read and Write Mechanisms

5. Chapter Seven: External Memory Inductive Write MR Read

6. Data Organization and Formatting Concentric rings or tracks –Gaps between adjacent tracks-avoid misalignment of head/magnetic fields interference. –Reduce gap to increase capacity –Same number of bits per track (variable packing density) –Constant angular velocity (CAV) Tracks divided into sectors Minimum block size is one sector May have more than one sector per block Chapter Seven: External Memory

7. Disk Data Layout Chapter Seven: External Memory

8. Disk Velocity Bit near centre of rotating disk passes fixed point slower than bit on outside of disk Increase spacing between bits in different tracks Rotate disk at constant angular velocity (CAV) –Gives pie shaped sectors and concentric tracks –Individual tracks and sectors addressable –Move head to given track and wait for given sector –Waste of space on outer tracks Lower data density Can use zones to increase capacity –Each zone has fixed bits per track –More complex circuitry

9. Disk Layout Methods Diagram Overcome the waste space for the outer tracks. Same data size for short inner and long outer tracks. Chapter Seven: External Memory

10. Finding Sectors Must be able to identify –start of track. –start and end of each sector Disk Formatting –Means to control data recorded on disk –Additional information not available to user, only for the disk drive. –Marks tracks and sectors Chapter Seven: External Memory

11. Example of Winchester Disk Format Seagate ST506 Chapter Seven: External Memory

12. Physical Characteristics Head - Fixed (rare) – one head / track - movable with respect to the platter Disk - Removable (remove/replace with another) (e.g Floppy disk, pen drives) - Fixed (permanently mounted in a disk drive) (e.g : hard disk) Magnetizable coating - Single or double (usually) sided of platter Platter - Single or multiple (stacked vertically) Head mechanism –Contact (Floppy) –Fixed gap-air gap –Flying (Winchester) Chapter Seven: External Memory

13. Fixed/Movable Head Disk Fixed head –One read write head per track –Heads mounted on fixed rigid arm Movable head –One read write head per side –Mounted on a movable arm Chapter Seven: External Memory

14. Removable or Not Removable disk –Can be removed from drive and replaced with another disk –Provides unlimited storage capacity –Easy data transfer between systems Non-removable disk –Permanently mounted in the drive Chapter Seven: External Memory

15. Multiple Platter One head per side Heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder –reduces head movement –Increases speed (transfer rate) Chapter Seven: External Memory

16. Multiple Platters Chapter Seven: External Memory

17. Toshiba HDD2189 2.5" hard disk drive platters, head and actuator arm. near 1:1 macro.

18. Tracks and Cylinders Chapter Seven: External Memory

19. Floppy Disk 8”, 5.25”, 3.5” Small capacity –Up to 1.44Mbyte (2.88M never popular) Slow Universal Cheap Obsolete Chapter Seven: External Memory

20. Winchester Hard Disk Developed by IBM in Winchester (USA) Sealed unit One or more platters (disks) Heads fly on boundary layer of air as disk spins Very small head to disk gap Getting more robust Chapter Seven: External Memory

21. Winchester Hard Disk Universal Cheap Fastest external storage Getting larger all the time –250 Gigabyte now easily available Chapter Seven: External Memory

22. Speed Seek time –Moving head to correct track (Rotational) latency –Waiting for data to rotate under head Access time = Seek + Latency Transfer rate Chapter Seven: External Memory

23. Disk Performance Parameters When operating, disk rotating at constant speed. To read/write, the head must be positioned at the desired track and at the beginning at the desired sector on track. Time taken to position the head at the track – seek time. Time taken for the beginning of the sector to reach the head – rotational delay / latency Time taken to get into position to read or write - access time. The read/write operation performed as the sector moves under the head. Time required for data transfer – transfer time Chapter Seven: External Memory

24. Timing of Disk I/O Transfer Chapter Seven: External Memory

25. Inside a Hard Disk Chapter Seven: External Memory SCSI-Small computer sys interface

26. RAID (INTRODUCTION) The demand for better performance from secondary storage device introduced the development of RAID systems. To match the super fast processor. RAIDS system treat multiple disk drives as a single entity. Data recording, distributed on different drives which appear as a single drive to the application program. Data stripping is the common technique used in RAID systems. A single file is split into multiple strips that are written on multiple disk drives. Chapter Seven: External Memory

27. RAID (INTRODUCTION) Initially - Redundant Array of Inexpensive Disks Then - Redundant Array of Independent Disks 7 levels (0 to 6) Not a hierarchy relationship Share three common characteristics:- 1)Set of physical disks viewed as single logical drive by O/S 2) Data distributed across physical drives 3)Can use redundant capacity to store parity information Advantages : Performance improvement due to parallelism by multiple disk drives. Fault tolerance due to replication of data. Chapter Seven: External Memory

28. RAID 0 No redundancy Data striped across all disks Round Robin striping 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 Chapter Seven: External Memory

29. RAID 1 Mirrored Disks Data is striped across disks 2 copies of each stripe on separate disks Read from either Write to both Recovery is simple –Swap faulty disk & re-mirror –No down time Expensive Chapter Seven: External Memory

30. RAID 2 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 Lots of redundancy –Expensive –Not used Chapter Seven: External Memory

31. RAID 3 Similar to RAID 2 Only one redundant disk, no matter how large the array Simple parity bit for each set of corresponding bits Data on failed drive can be reconstructed from surviving data and parity info Very high transfer rates Chapter Seven: External Memory

32. RAID 4 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 Chapter Seven: External Memory

33. RAID 5 Like RAID 4 Parity striped across all disks Round robin allocation for parity stripe Avoids RAID 4 bottleneck at parity disk Commonly used in network servers Chapter Seven: External Memory

34. RAID 6 Two parity calculations Stored in separate blocks on different disks User requirement of N disks needs N+2 High data availability –Three disks need to fail for data loss –Significant write penalty because each write affects two parity blocks. Chapter Seven: External Memory

35. Data Mapping For RAID 0 Chapter Seven: External Memory

36. RAID 0, 1, 2 Chapter Seven: External Memory

37. RAID 3 & 4 Chapter Seven: External Memory

38. RAID 5 & 6 Chapter Seven: External Memory

39. RAID Levels (summary) RAID level PrincipleMeritsDemerits 0Data stripping and block interleave: different blocks of a file are written on different disk drives. High performance at low cost No redundancy 1Disk mirroring; each disk drive has a mirror disk drive; every byte is written in both drives. Good redundancy Performance improvement Expensive 2Data stripping with bit interleave and hamming codes for error detection and correction; each bit in a byte is written in a different disk drive; separate drive for hamming codes. High performance due to parallel storage/read No duplication of data. Expensive Chapter Seven: External Memory

40. RAID Levels (summary) RAID level PrincipleMeritsDemerits 3Data stripping with bit interleave and parity; each bit in a byte written in a different disk drive; separate drive for parity bit. High performance No duplication of data Good reliability Expensive 4Data stripping with block interleave and parity; each block of file written in a different disk drive; separate drive for parity. Multiple simultaneous operations on different drives. Good reliability Expensive Chapter Seven: External Memory

41. RAID Levels (summary) RAID level PrincipleMeritsDemerits 5Data stripping with block interleave and distributed check; data writing similar to level 4 but parity bits are distributed in all disk drives Same as previous expensive 6P + Q redundancy; uses Reed-Solomon codes; two different error codes (P and Q) are generated and stored (distributed) in two drive with block interleave data striping similar to level 5. High redundancy; up to two disk failures are tolerable Expensive and additional delays. Chapter Seven: External Memory

42. Optical Storage (Introduction) Information in magnetic disk is recorded in concentric tracks. With CAV system, the number of bits per track is constant. An increase in intensity is achieved with multiple zoned recording -the surface is divided into a number of zones with zones farther from the center containing more bits than zones closer to the center. This magnetic disk increases capacity, but still not optimum. Then CDs and CD ROMs (optical storage device) introduced but not organize information in concentric tracks. Contains a single spiral track beginning near the center and spiraling out to the outer edge of the disk. The disk rotates at constant linear velocity (CLV) during read operation Chapter Seven: External Memory

43. CAV vs CLV Concentric tracks Spiral tracks

44. Optical Storage CD-ROM CD (Compact Disk)  Originally for audio. CDROM (Compact Disk Read Only Memory) Digitally recorded information (computer data or music) 650Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminium or gold Constant packing density Constant linear velocity (CLV) Chapter Seven: External Memory

45. Optical Storage CD-ROM Data stored as microscopic pits Read by reflecting laser-shines through the clear polycarbonate while motor spins. The intensity of the laser changes as it encounters a pit (rough surface-light scatters and low intensity light reflected back to the source). Lands-the area between pits. A smooth surface- higher intensity. The intensity changes by lands and pits detected by photo sensor and converted into digital signal (0 or 1) Chapter Seven: External Memory

46. CD Operation Chapter Seven: External Memory

47. CD-ROM Drive Speeds Audio is single speed –Constant linear velocity (CLV) –1.2 ms -1 –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 Chapter Seven: External Memory

48. CD-ROM Format 2 bytes of all 0s + 10 bytes of all 1s Data on CDROM are organized as a seq. of block Sync – indicate the beginning of a block Header – block address and mode byte –Mode 0=blank data field –Mode 1=2048 byte data+error correction –Mode 2=2336 byte data Data – user data Auxiliary – additional user data in mode 2, mode 1  288 bytes error correcting codes (ECC) Chapter Seven: External Memory

49. Inside a CD player Chapter Seven: External Memory

50. Random Access on CD-ROM Difficult Move head to rough position Set correct speed Read address Adjust to required location Chapter Seven: External Memory

51. CD-ROM for & against Large capacity (?) Easy to mass produce Removable Robust Expensive for small runs Slow Read only Chapter Seven: External Memory

52. Other Optical Storage CD-R (CD-Recordable) –Also known as WORM-Write Once, Read Many –Now affordable –Compatible with CD-ROM drives –Dye layer change reflectivity activated by high intensity laser CD-RW (CD-Rewritable) –Erasable –Getting cheaper –Mostly CD-ROM drive compatible –Phase change Material has two different reflectivities in different phase states Chapter Seven: External Memory

53. DVD – (Digital Versatile Disk) Read Only and writeable like CDs Bits are packed more closely Spiral narrower than the CD one Two layers Two sides (17 GB) Chapter Seven: External Memory

54. DVD - technology Multi-layer Very high capacity (4.7G per layer) Full length movie on single disk –Using MPEG (moving pictures experts group) compression Finally standardized Chapter Seven: External Memory

55. DVD – Writable Loads of trouble with standards First generation DVD drives may not read first generation DVD-W disks First generation DVD drives may not read CD-RW disks Chapter Seven: External Memory

56. CD and DVD Chapter Seven: External Memory

57. Magnetic Tape Serial access Slow Very cheap Backup and archive Chapter Seven: External Memory