External Memory. Patrick Happ Raul Queiroz Feitosa. Parts of these slides are from the support material provided by W. Stallings

Transcription

1 External Memory Patrick Happ Raul Queiroz Feitosa Parts of these slides are from the support material provided by W. Stallings

2 Objective This chapter examines a range of external memory devices and systems. W. Stallings External Memory 2

3 Outline Magnetic Disc RAID Solid State Drives Optical Memory Magnetic Tape External Memory 3

4 Magnetic Disc Disc substrate of non magnetic material coated with magnetizable material (iron oxide rust) Substrate used to be aluminium; now glass Improved surface uniformity Increases reliability Reduction in surface defects Reduced read/write errors Lower flight heights (See later) Better stiffness Better shock/damage resistance External Memory 4

5 Read and Write Mechanisms Underlying Physics Current flowing through a conducting coil creates a magnetic field the orients the magnetic domains over the metal Changes in the magnetic field intensity induces a current in a coil External Memory 5

6 Write Operation The electronic in the drive receives binary data and converts it into a current that flows through the coil. The current flow direction changes at each 1 and keeps unchanged at each 0 The interaction with the media magnetizes the material, whose direction depends on the current direction in the coil. See Writing mechanism External Memory 6

7 Read Operation (traditional) The same coil for read and write Magnetic field variation due to the movement relative to coil produces current. The direction of the induced current indicates what is recorded. See Reading mechanism External Memory 7

8 Read Operation (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 External Memory 8

9 Inductive Write MR Read External Memory 9

10 Disc Data Layout Concentric rings or tracks Gaps between tracks Reduce gap to increase capacity Same number of bits per track (variable packing density) Constant angular velocity Tracks divided into sectors External Memory 10

11 Disc Velocity Bit near centre of rotating disc passes fixed point slower than bit on outside of disc. Increase spacing between bits in external tracks. Disc rotates 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 Within a zone (typically 16) bits per track is constant Zones farther/closer to the centre contain more/less sectors. More complex circuitry External Memory 11

12 Disc Layout Methods Diagram External Memory 12

13 Characteristics Fixed (rare) or movable head Removable or fixed Single or double (usually) sided Single or multiple platter Head mechanism Contact (Floppy) Fixed gap Flying (Winchester) External Memory 13

14 Fixed/Movable Head Disc Fixed head One read write head per track Heads mounted on fixed ridged arm Movable head One read write head per side Mounted on a movable arm External Memory 14

15 Removable or Not Removable disc Can be removed from drive and replaced with another disc Provides unlimited storage capacity Easy data transfer between systems Non-removable disc Permanently mounted in the drive External Memory 15

16 Head Mechanism Contact (Floppy) Fixed gap Flying (Winchester) See Slider External Memory 16

17 Conventional Hard Disc External Memory 17

18 Hard Disc 3D Visualization Click here to watch the video External Memory 18

19 Inside the Hard Disc Click here to watch the video External Memory 19

20 Cylinders through Multiple Platters External Memory 20

21 A Portion of a Disc Track. Two sectors External Memory 21

22 Winchester Disc Format Seagate ST506 External Memory 22

23 Disc Controller Typically embedded in the disc drive, which acts as an interface between the CPU and the disc hardware. The controller has an internal cache (typically a number of MBs) that it uses to buffer data for read/write requests. External Memory 23

24 Speed Seek time Time to move head to correct track (Rotational) latency Time it takes for the disc to rotate so that the desired sector is under the read/write head Transfer Time Once the read/write head is positioned over the data, this is the time it takes for transferring data Access time Seek + Latency (according to Stalling) Seek + Latency + Transfer (according to Parhami) External Memory 24

25 Speed Exercise 1: Given average seek time = 4 ms, rotation speed =15,000 rpm, 512 bytes/sector, 500 sectors/track, 5 tracks per cylinder. What is the time to read a file consisting of 2500 sectors for a total of 1.28 Mbyte? External Memory 25

26 Speed Exercise 1 - Solution 1 We assume that the file is stored as compactly as possible. That is, the file occupies 5 tracks of one cylinder (sequential organization). 15,000 rpm time for a complete rotation = 60/ ms Transfer data to transfer / data from track * rotation / ( ) *4ms = 4ms (Full track) (avg. seek) (avg. rotational latency) (transfer) To read the first track = 10 ms We assume that the tracks are aligned across the cylinders and the time to switch between tracks of the same cylinder is close to zero. (transfer) Time to read the other four tracks 4 4 = 16 ms Time to read the file = 26 ms External Memory 26

27 Speed Exercise 1 - Solution 2 We assume random access rather than sequential access. That is, the accesses are distributed randomly over the disc. For each sector we have. 15,000 rpm time for a complete rotation = 4ms Transfer data to transfer / data from track * rotation 512/ ( ) *4ms = 4/500 ms (seek) (rotational latency) (transfer) To read the first sector /500 = ms. Time to read the file = = 15,020 ms = seconds! Fragmentation! See Defragmentation External Memory 27

28 Defragmentation Click here to watch the video External Memory 28

29 Speed Exercise 2: A hard disc has 500 cylinders, 5 tracks/cylinder, 100 sectors per track and operates at 3000 rpm. The time to move from the most external to the most internal cylinder is equal 10 ms. Assume that the time to switch between tracks of the same cylinder is negligible, and disregard acceleration time when the heads move. Compute the average time to: a) Read a single sector, b) Read the whole first cylinder starting with the first track and going track by track till the last track c) Read the whole disc starting with the first cylinder and first track, going track by track, cylinder by cylinder till the final cylinder and track External Memory 29

30 Speed Exercise 3: A hard disc has 600 cylinders, 6 tracks/cylinder, 60 sectors per track and operates at rpm. The time to move from the most external to the most internal cylinder is equal 24 ms. Assume that the time to switch between tracks of the same cylinder is negligible, and disregard acceleration time when the heads move. How long does it take to read a file stored in 10 sectors, assuming that a) The file is stored in 10 adjacent sectors of the same cylinder and track. b) The file is stored in 5 adjacent sectors of the same track of the first cylinder and then in 5 adjacent sectors of the same track of the last cylinder. c) The file is stored in 5 adjacent sectors of one track of the first cylinder and then in 5 adjacent sectors of another track of the same cylinder. External Memory 30

31 Outline Magnetic Disc RAID Solid State Drives Optical Memory Magnetic Tape External Memory 31

32 RAID - what s in a name? Redundant Array of Independent/ Inexpensive Discs Set of physical discs viewed as single logical drive by O/S The two keywords are: Redundant Redundant data on multiple discs provides fault tolerance Array. An array of multiple discs accessed in parallel will give greater throughput than a single disc. External Memory 32

33 Non-Redundant - RAID 0 Data striped across all discs Round Robin striping Increase speed Multiple data requests probably not on same disc discs seek in parallel A set of data is likely to be striped across multiple discs No redundancy strip 0 strip 1 strip 2 strip 3 strip 4 strip 5 strip 6 strip 7 strip 8 strip 9 strip 10 strip 11 External Memory 33

34 Mirrored - RAID 1 Redundancy is achieved by duplicating all data A read request can be serviced by either of the two discs performance dictated by the fastest one A write request requires that both discs be updated performance dictated by the slowest one Simple recovery if driver fails data is available in the second one mirrored External Memory 34

35 RAID 1+0 Data is striped across discs 2 copies of each strip on separate discs (mirroring) Positive aspects: Read from either (the one with least seek) Write to both no write penalty (see later) Recovery is simple - Swap faulty disc & re-mirror (no down time) Negative aspect: expensive strip 0 strip 1 strip 2 strip 3 strip 0 strip 1 strip 2 strip 3 strip 4 strip 5 strip 6 strip 7 strip 4 strip 5 strip 6 strip 7 strip 8 strip 9 strip 10 strip 11 strip 8 strip 9 strip 10 strip 11 External Memory 35

36 Memory Style - RAID 2 Typically, discs are synchronized head in same position Very small stripes - often single byte/word Error correction calculated across corresponding bits. On a single write, all data on parity discs must be accessed. Lots of redundancy Expensive Only effective if many disc errors occur not used nowadays b 0 b 1 b 2 b 3 parity parity parity External Memory 36

37 Bit-Interleaved Parity - RAID 3 Similar to RAID 2 Only one redundant disc, 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 bit 1 bit 2 bit 3 bit 4 parity External Memory 37

38 Block-Interleaved Parity - RAID 4 Each disc operates independently separate I/O requests can be satisfied in parallel. Good for high I/O request rate Large stripes Bit by bit parity calculated across stripes on each disc Parity stored on parity disc Writes involve 2 reads and writes - user and parity stripes. Parity disc becomes a bottleneck and overloaded. strip 0 strip 1 strip 2 strip 3 P0-3 strip 4 strip 5 strip 6 strip 7 P4-7 strip 8 strip 9 strip 10 strip 11 P8-11 External Memory 38

39 Block-Interleaved Distributed-Parity - RAID 5 Like RAID 4 Parity striped across all discs Round robin allocation for parity stripes Avoids RAID 4 bottleneck at parity disc Commonly used in network servers strip 0 strip 1 strip 2 strip 3 P0-3 strip 4 strip 5 strip 6 P4-7 strip 7 strip 8 strip 9 P8-11 strip 10 strip 11 strip 12 P12-15 strip 13 strip 14 strip 15 P16-19 strip 16 strip 17 strip 18 strip 19 External Memory 39

40 P+Q redundancy - RAID 6 Two parity calculations Stored in separate blocks on different discs User requirement of N discs needs N+2 High data availability Three discs need to fail for data loss Significant write penalty (30% compared with RAID 5) strip 0 strip 1 strip 2 strip 3 P0-3 Q0-3 strip 4 strip 5 strip 6 P4-7 Q4-7 strip 7 strip 8 strip 12 strip 9 P12-15 P8-11 Q12-15 Q8-11 strip 13 strip 10 strip 14 strip 11 strip 15 P16-19 Q16-19 strip 16 strip 17 strip 18 strip 20 External Memory 40

41 RAID - A short Break Raid Kills Bugs Dead External Memory 41

42 RAID Comparison External Memory 42

43 Outline Magnetic Disc RAID Solid State Drives Optical Memory Magnetic Tape External Memory 43

44 Solid State Drives One of the most significant developments in computer architecture in recent years! Generally complement or even replace hard disk drives (HDDs). Solid state refers to electronic circuitry built with semiconductors. Uses a type of semiconductor memory referred to as flash memory. External Memory 44

45 Flash Memory Has been around for years and is used in many consumer electronic products: smart phones, GPS devices, MP3 players, digital cameras, and USB devices. The cost and performance has evolved to the point where it is feasible to use flash memory drives (even to replace HDDs). NOR flash memory: the basic unit of access is a bit, and the logical organization resembles a NOR logic device. NAND flash memory: the basic unit is 16 or 32 bits, and the logical organization resembles NAND devices. External Memory 45

46 Flash Memory NOR flash memory: Provides high-speed random access. Can read and write data to specific locations, and can reference and retrieve a single byte. E.g. Cell phone operating system code; BIOS program. NAND flash memory: Provides higher bit density and greater write speed. Does not provide a random-access external address bus. E.g.: flash drives, memory cards (in digital cameras, MP3 players, etc.), and SSDs External Memory 46

47 Flash Memory Transistors exploit the properties of semiconductors: a small voltage applied to the gate can be used to control the flow of a large current between source and drain. A floating gate (insulated by a thin oxide layer) does not interfere with the transistor when the state is 1. Applying a large voltage across the oxide layer causes electrons to tunnel through it and become trapped on the floating gate, where they remain even if the power is disconnected. State is 0. The state of the cell can be read by using external circuitry to test whether the transistor is working or not. External Memory 47

48 SSD X HDD High-performance input/output operations per second (IOPS): Significantly increases performance I/O subsystems. Durability: Less susceptible to physical shock and vibration. Longer lifespan: SSDs are not susceptible to mechanical wear. Lower power consumption: SSDs use as little as 2.1 watts of power per drive, considerably less than comparable-size HDDs. Quieter and cooler running capabilities: Less floor space required, lower energy costs, and a greener enterprise. Lower access times and latency rates: Over 10 times faster than the spinning disks in an HDD. HDDs still has a cost per bit advantage and a capacity advantage. External Memory 48

49 SSD X HDD External Memory 49

50 SSD Practical Issues SDD performance has a tendency to slow down as the device is used due to a high level of fragmentation. A flash memory becomes unusable after a certain number of writes. As flash cells are stressed, they lose their ability to record and retain values. Most flash devices are capable of estimating their own remaining lifetimes so systems can anticipate failures. External Memory 50

51 Outline Magnetic Disc RAID Solid State Drives Optical Memory Magnetic Tape External Memory 51

52 Optical Storage CD Compact Disc 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 (CLV) External Memory 52

53 CD Drive Speeds Audio is single speed Constant linear velocity=1.2 ms -1 Track (spiral) = 5.27km Gives 73.2 minutes Other speeds are quoted as multiples e.g. 24x Quoted figure is maximum drive can achieve External Memory 53

54 CD Operation Optical media ¼ wave plate Collimator lens Cylindrical lens Spot detector Polarizing prism Laser diod See CD Operation External Memory 54

55 CD - Single Layer See CD-ROM Reading External Memory 55

56 CD Format CD block format Mode 0=blank data field Mode 1=2048 byte data+error correction Mode 2=2336 byte data External Memory 56

57 CD - Products CD (audio) CD-ROM (data non erasable) CD R (data recordable just once) CD-RW (dara recordable multiple times) External Memory 57

58 DVD - what s in a name? Digital Video Disc Used to indicate a player for movies Only plays video discs Digital Versatile Disc Used to indicate a computer drive Will read computer discs and play video discs External Memory 58

59 DVD - technology Very high capacity (4.7G per layer) Bits packed more closely Spacing between loops (1.5 μm 0.74 μm) Distance between pits (0.834 μm 0.4 μm) Up to two layer per side May be two sided 133 min video Using MPEG compression (otherwise 4 min) External Memory 59

60 DVD - Dual Layer See DVD External Memory 60

61 How a DVD works Click here to watch the video External Memory 61

62 DVD - Capacity Name Midia Capacity (GB) DVD-5 Single Side / Single Layer 4.7 DVD-9 Single Side / Dual Layer 8.54 DVD-10 Double Side / Single Layer 9.4 DVD-18 Double Side / Dual Layer DVD-R Single ou Double Side / Single Layer 3.95 / 7.9 DVD-RAM Single ou Double Side / Single Layer 2.6 / 5.2 External Memory 62

63 DVD Region Codes External Memory 63

64 Blu-Ray Essentially the same DVD architecture Blu ray HD DVD DVD Capacity 25/26 (single) * 15 (single) 4,7 (single) (Gb) 50/54 (double) 30 (double) 8,5 (double) wavelength 405 nm 400 nm 650 nm Transfer rate 54 Mbps 36 Mbps 11 Mbps Scratch resistant yes no no formats MPEG-2, MPEG-4 AVC, VC-1 MPEG-2, VC-1 (Baseado no WMV), H.264/MPEG-4 AVC MPEG-2 * 6 hour of Full HDV - video External Memory 64

65 Optical Memory Characteristics External Memory 65

66 Blu-Ray Código de Regiões External Memory 66

67 Outline Magnetic Disc RAID Solid State Drives Optical Memory Magnetic Tape External Memory 67

68 Magnetic Tape Same reading/recording as disc systems Serial access Slow Very cheap (1/3) High durability (up to 30 years) Backup and archive Linear Tape-Open (LTO) Tape Drives Developed late 1990s Open source alternative to proprietary tape systems External Memory 68

69 LTO Tape Format Bands: Guard no data Data data Servo location info Lossless compression Encryption Serpentine recording External Memory 69

70 Linear Tape-Open (LTO) YEAR source: wikipedia.org External Memory 70

71 LTO-6 Driver Click here to watch the video External Memory 71

72 Text Book References These topics are covered in Stallings - chapter 6 Tanenbaum - section 2.3 Parhami - chapter 19 External Memory 72

73 External Memory END External Memory 73