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ECE 6160: Advanced Computer Networks Introduction to Storage Devices

ECE 6160: Advanced Computer Networks Introduction to Storage Devices. Instructor: Dr. Xubin (Ben) He Email: Hexb@tntech.edu Tel: 931-372-3462. Prev… . Gigabit Ethernet MAC PHY VLAN. Anatomy: Key components for a typical computer. Motivation: Who Cares About I/O?.

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ECE 6160: Advanced Computer Networks Introduction to Storage Devices

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  1. ECE 6160: Advanced Computer NetworksIntroduction to Storage Devices Instructor: Dr. Xubin (Ben) He Email: Hexb@tntech.edu Tel: 931-372-3462

  2. Prev… • Gigabit Ethernet • MAC • PHY • VLAN ECE6160:Advanced Computer Networks

  3. Anatomy: Key components for a typical computer ECE6160:Advanced Computer Networks

  4. Motivation: Who Cares About I/O? • CPU Performance: 60% per year • I/O system performance limited by mechanical delays (disk I/O) < 10% per year (IO per sec) • Amdahl's Law: system speed-up limited by the slowest part! 10% IO & 10x CPU => 5x Performance (lose 50%) 10% IO & 100x CPU => 10x Performance (lose 90%) • I/O bottleneck: Diminishing fraction of time in CPU Diminishing value of faster CPUs ECE6160:Advanced Computer Networks

  5. I/O Systems interrupts Processor Cache Memory - I/O Bus Main Memory I/O Controller I/O Controller I/O Controller Graphics Disk Disk Network ECE6160:Advanced Computer Networks

  6. Storage Technology Drivers • Driven by the prevailing computing paradigm • 1950s: migration from batch to on-line processing • 1990s: migration to ubiquitous computing • computers in phones, books, cars, video cameras, … • nationwide fiber optical network with wireless tails • Effects on storage industry: • Embedded storage • smaller, cheaper, more reliable, lower power • Data utilities • high capacity, hierarchically managed storage ECE6160:Advanced Computer Networks

  7. Outline • Disk Basics/Disk History • Current Disk options • Disk fallacies and performance • FLASH • Tapes • RAID ECE6160:Advanced Computer Networks

  8. Inner Track Outer Track Sector Head Arm Platter Actuator Disk Device Terminology • Several platters, with information recorded magnetically on both surfaces (usually) • Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes) • Actuator moves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write • “Cylinder”: all tracks under heads

  9. Tracks, Sectors, and Cylinders A cylinder is comprised of the set of tracks described by all the heads (on separate platters) at a single seek position ECE6160:Advanced Computer Networks

  10. { Platters (12) Photo of Disk Head, Arm, Actuator Spindle Arm Head Actuator ECE6160:Advanced Computer Networks source: www.seagate.com

  11. Disk Device Performance • Disk Latency = Seek Time + Rotation Time + Transfer Time + Controller Overhead • Seek Time? depends on tracks move arm, seek speed of disk • Rotation Time? depends on speed disk rotates, how far sector is from head • Transfer Time? depends on data rate (bandwidth) of disk (bit density), size of request Inner Track Outer Track Sector Head Controller Arm Spindle Platter Actuator ECE6160:Advanced Computer Networks

  12. Disk Device Performance • Average rotation time: • Average distance sector from head? • 1/2 time of a rotation • 10000 Revolutions Per Minute  166.67 Rev/sec • 1 revolution = 1/ 166.67 sec  6.00 milliseconds • 1/2 rotation (revolution)  3.00 ms • Average seek time? • Sum all possible seek distances from all possible tracks / # possible • Assumes average seek distance is random • Disk industry standard benchmark • About 10ms (ATA) and 5ms (SCSI) ECE6160:Advanced Computer Networks

  13. Data Rate: Inner vs. Outer Tracks • To keep things simple, orginally kept same number of sectors per track • Since outer track longer, lower bits per inch • Competition  decided to keep BPI the same for all tracks (“constant bit density”)  More capacity per disk  More of sectors per track towards edge  Since disk spins at constant speed, outer tracks have faster data rate • Bandwidth outer track 1.7X inner track! • Inner track highest density, outer track lowest, so not really constant • 2.1X length of track outer / inner, 1.7X bits outer / inner ECE6160:Advanced Computer Networks

  14. Disk Performance Model /Trends • Capacity + 100%/year (2X / 1.0 yrs) • Transfer rate (BW) + 40%/year (2X / 2.0 yrs) • Rotation + Seek time – 8%/ year (1/2 in 10 yrs) • MB/$ > 100%/year (2X / 1.0 yrs) 60GB/$300 => $5/GB =>0.5c/MB ECE6160:Advanced Computer Networks

  15. Latency = Queuing Time + Controller time + Seek Time + Rotation Time + Size / Bandwidth { per access + per byte State of the Art: Barracuda 180 • 181.6 GB, 3.5 inch disk • 12 platters, 24 surfaces • 24,247 cylinders • 7,200 RPM; • 7.4/8.2 ms avg. seek (r/w) • 64 to 35 MB/s (internal) • 0.1 ms controller time • 10.3 watts (idle) Track Sector Cylinder Track Buffer Arm Platter Head Q:Calculate time to read 64 KB (128 sectors) for Barracuda 180, sector is on outer track. source: www.seagate.com ECE6160:Advanced Computer Networks

  16. Disk Performance Example • Calculate time to read 64 KB (128 sectors) for Barracuda 180 X using advertised performance; sector is on outer track Disk latency = average seek time + average rotational delay + transfer time + controller overhead = 7.4 ms + 0.5 * 1/(7200 RPM) + 64 KB / (64 MB/s) + 0.1 ms = 7.4 ms + 0.5 /(7200 RPM/(60000ms/M)) + 64 KB / (64 KB/ms) + 0.1 ms = 7.4 + 4.2 + 1.0 + 0.1 ms = 12.7 ms ECE6160:Advanced Computer Networks

  17. Areal Density: linear density x track density • Bits recorded along a track • Metric is Bits Per Inch (BPI) along a track • Number of tracks per surface • Metric is Tracks Per Inch (TPI) • Disk Designs Brag about bit density per unit area • Metric is Bits Per Square Inch • Called Areal Density • Areal Density =BPI x TPI ECE6160:Advanced Computer Networks

  18. Areal Density • Areal Density =BPI x TPI • Change slope 30%/yr to 60%/yr about 1991 ECE6160:Advanced Computer Networks

  19. Historical Perspective • 1956 IBM Ramac — early 1970s Winchester • Developed for mainframe computers, proprietary interfaces • Steady shrink in form factor: 27 in. to 14 in • Form factor and capacity drives market, more than performance • 1970s: Mainframes  14 inch diameter disks • 1980s: Minicomputers,Servers  8”,5 1/4” diameter • PCs, workstations Late 1980s/Early 1990s: • Mass market disk drives become a reality • industry standards: SCSI, IPI, IDE • Pizzabox PCs  3.5 inch diameter disks • Laptops, notebooks  2.5 inch disks • Palmtops didn’t use disks, so 1.8 inch diameter disks didn’t make it • 2000s: • 1 inch for cameras, cell phones? ECE6160:Advanced Computer Networks

  20. Disk History Data density Mbit/sq. in. Capacity of Unit Shown Megabytes 1973: 1. 7 Mbit/sq. in 140 MBytes 1979: 7. 7 Mbit/sq. in 2,300 MBytes source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even more data into even smaller spaces”

  21. Disk History 1989: 63 Mbit/sq. in 60,000 MBytes 1997: 1450 Mbit/sq. in 2300 MBytes 1997: 3090 Mbit/sq. in 8100 MBytes source: New York Times, 2/23/98, page C3, “Makers of disk drives crowd even mroe data into even smaller spaces”

  22. 1 inch disk drive! • 2003 IBM MicroDrive: • 1.7” x 1.4” x 0.2” • 1 GB, 3600 RPM, 13.3 MB/s, 15 ms seek • Digital camera, PalmPC? • 2003 USB mini flash drive • 75mmx28mmx12mm, 17g • 256MB • 0.5-1MB/s • 2006 MicroDrive? • 9 GB, 50 MB/s! • Assuming it finds a niche in a successful product • Assuming past trends continue

  23. Seagate Cheetah 3146807LW Hitachi TravelStar 7K60 IBM Microdrive (07N5574) Dimension 3.5”, 1.5 lbs 2.5”, 0.2 lbs 1.7”, 0.6 lbs Interface Ultra 320 SCSI ATA-100 (Ultra) CF+ Capacity 146.8GB 60GB 1GB Data Transfer Rate 320MB/s 100MB/s 13.3MB/s Average seek time 4.7ms 10ms 15ms Spindle Speed 10000RPM 7200RPM 3600RPM Buffer 8MB 8MB 128KB Price (warehouse.com) $899.95 $299.95 $319.95 Disk Characteristics in 2003

  24. Seagate Cheetah ST3146707LW Hitachi TravelStar 7K60 Seagate Barracuda ES ST3750640NS Dimension 3.5” 2.5”, 0.2 lbs 3.5” Interface Ultra 320 SCSI ATA-100 (Ultra) Serial ATA-300 Capacity 146.8GB 60GB 750GB Data Transfer Rate 320MB/s 100MB/s Average seek time 4.7ms 10ms Spindle Speed 10000RPM 7200RPM 7200RPM Buffer 8MB 8MB 16MB Price (warehouse.com) $259 $93.99 (80GB) $319.95 Disk Characteristics in 2007

  25. Today’s Hard Drive (09/29/2008)(warehouse.com) • Seagate Barracuda 7200.11 - hard drive - 750 GB - SATA-300 Hard drive - 750 GB - internal - 3.5" - SATA-300 - 7200 rpm - buffer: 32 MB, $116 • Seagate FreeAgent Pro - hard drive - 500 GB - FireWire/Hi-Speed USB/eSATA Hard drive – 500 GB – external – 3.5” – USB 2.0, eSATA-300, FireWire interfaces – 7200 rpm – buffer: 32 MB, $100 ECE6160:Advanced Computer Networks

  26. Fallacy: Use Data Sheet “Average Seek” Time • Manufacturers needed standard for fair comparison (“benchmark”) • Calculate all seeks from all tracks, divide by number of seeks => “average” • Real average would be based on how data laid out on disk, where seek in real applications, then measure performance • Usually, tend to seek to tracks nearby, not to random track • Rule of Thumb: observed average seek time is typically about 1/4 to 1/3 of quoted seek time (i.e., 3X-4X faster) • Barracuda 180 X avg. seek: 7.4 ms  2.5 ms ECE6160:Advanced Computer Networks

  27. Fallacy: Use Data Sheet Transfer Rate • Manufacturers quote the speed off the data rate off the surface of the disk. • Sectors contain an error detection and correction field (can be 20% of sector size) plus sector number as well as data • There are gaps between sectors on track • Rule of Thumb: disks deliver about 3/4 of internal media rate (1.3X slower) for data • For example, Barracuda 180X quotes 64 to 35 MB/sec internal media rate  47 to 26 MB/sec external data rate (74%) ECE6160:Advanced Computer Networks

  28. Disk Performance Example: revision • Calculate time to read 64 KB for UltraStar 72 again, this time using 1/3 quoted seek time, 3/4 of internal outer track bandwidth; (12.7 ms before) Disk latency = average seek time + average rotational delay + transfer time + controller overhead = (0.33 * 7.4 ms) + 0.5 * 1/(7200 RPM) + 64 KB / (0.75 * 65 MB/s) + 0.1 ms = 2.5 ms + 0.5 /(7200 RPM/(60000ms/M)) + 64 KB / (47 KB/ms) + 0.1 ms = 2.5 + 4.2 + 1.4 + 0.1 ms = 8.2 ms (64% of 12.7) ECE6160:Advanced Computer Networks

  29. Future Disk Size and Performance • Continued advance in capacity (60%/yr) and bandwidth (40%/yr). • Slow improvement in seek, rotation (8%/yr). • Time to read whole disk. Year Sequentially Randomly (1 sector/seek) 1990 4 minutes 6 hours 2000 12 minutes 1 week(!) • 3.5” form factor make sense in 5 yrs? • What is capacity, bandwidth, seek time, RPM? • Assume today 80 GB, 100 MB/sec, 6 ms, 10000 RPM • 838GB, 537MB/s, 4ms, 15000RPM ECE6160:Advanced Computer Networks

  30. What about FLASH • Compact Flash Cards • Intel Strata Flash • 16 Mb in 1 square cm. (.6 mm thick) • 100,000 write/erase cycles. • Standby current = 100uA, write = 45mA • Compact Flash 256MB~=$120 512MB~=$542 • Transfer @ 3.5MB/s • IBM Microdrive 1G~370 • Standby current = 20mA, write = 250mA • Efficiency advertised in wats/MB • VS. Disks • Nearly instant standby wake-up time • Random access to data stored • Tolerant to shock and vibration (1000G of operating shock) ECE6160:Advanced Computer Networks

  31. Disk Operations • Seek: move head to track; • Rotation: wait for sector under head; • Transfer:Move data to/from disks; • Overhead: • Controller delay • Queuing delay Access time = seek time + rotational delay + transfer time+overhead ECE6160:Advanced Computer Networks

  32. Improving disk performance. • Use large sectors to improve bandwidth • Use track caches and read ahead; • Read entire track into on-controller cache • Exploit locality (improves both latency and BW) • Design file systems to maximize locality • Allocate files sequentially on disks (exploit track cache) • Locate similar files in same cylinder (reduce seeks) • Locate simlar files in near-by cylinders (reduce seek distance) • Pack bits closer together to improve transfer rate and density. • Use a collection of small disks to form a large, high performance one--->disk array Stripping data across multiple disks to allow parallel I/O, thus improving performance. ECE6160:Advanced Computer Networks

  33. Use Arrays of Small Disks? • Katz and Patterson asked in 1987: • Can smaller disks be used to close gap in performance between disks and CPUs? Conventional: 4 disk designs 3.5” 5.25” 10” 14” High End Low End Disk Array: 1 disk design 3.5” ECE6160:Advanced Computer Networks

  34. Replace Small Number of Large Disks with Large Number of Small Disks! (1988 Disks) IBM 3390K 20 GBytes 97 cu. ft. 3 KW 15 MB/s 600 I/Os/s 250 KHrs $250K x70 23 GBytes 11 cu. ft. 1 KW 110 MB/s 3900 IOs/s ??? Hrs $150K IBM 3.5" 0061 320 MBytes 0.1 cu. ft. 11 W 1.5 MB/s 55 I/Os/s 50 KHrs $2K Capacity Volume Power Data Rate I/O Rate MTTF Cost 9X 3X 8X 6X Disk Arrays have potential for large data and I/O rates, high MB per cu. ft., high MB per KW, but what about reliability?

  35. Array Reliability • MTTF: Mean Time To Failure: average time that a non - repairable component will operate before experiencing failure. • Reliability of N disks = Reliability of 1 Disk ÷N • 50,000 Hours ÷ 70 disks = 700 hours • Disk system MTTF: Drops from 6 years to 1 month! • Arrays without redundancy too unreliable to be useful! • Solution: redundancy. ECE6160:Advanced Computer Networks

  36. Redundant Arrays of (Inexpensive) Disks • Replicate data over several disks so that no data will be lost if one disk fails. • Redundancy yields high data availability • Availability: service still provided to user, even if some components failed • Disks will still fail • Contents reconstructed from data redundantly stored in the array  Capacity penalty to store redundant info  Bandwidth penalty to update redundant info ECE6160:Advanced Computer Networks

  37. ECE6160:Advanced Computer Networks

  38. Levels of RAID • Original RAID paper described five categories (RAID levels 1-5). (Patterson et al, “A case for redundant arrays of inexpensive disks (RAID)”, ACM SIGMOD, 1988) • Disk striping with no redundant now is called RAID0 or JBOD(Just a bunch of disks). • Other kinds have been proposed in literature, Level 6 (P+Q Redundancy), Level 10, RAID53, etc. • Except RAID0, all the RAID levels trade disk capacity for reliability, and the extra reliability makes parallism a practical way to improve performance. ECE6160:Advanced Computer Networks

  39. file data block 0 block 1 block 2 block 3 Disk 1 Disk 0 Disk 2 Disk 3 RAID 0: Nonredundant (JBOD) • High I/O performance. • Data is not save redundantly. • Single copy of data is striped across multiple disks. • Low cost. • Lack of redundancy. • Least reliable: single disk failure leads to data loss. ECE6160:Advanced Computer Networks

  40. RAID 0: Striped Disk Array without Fault Tolerance RAID Level 0 requires a minimum of 2 drives to implement ECE6160:Advanced Computer Networks

  41. Redundant Arrays of Inexpensive DisksRAID 1: Disk Mirroring/Shadowing recovery group • • Each disk is fully duplicated onto its “mirror” • Very high availability can be achieved • • Bandwidth sacrifice on write: • Logical write = two physical writes • • Reads may be optimized, minimize the queue and disk search time • • Most expensive solution: 100% capacity overhead Targeted for high I/O rate , high availability environments

  42. RAID 1: Mirroring and Duplexing For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair. RAID Level 1 requires a minimum of 2 drives to implement ECE6160:Advanced Computer Networks

  43. block 0 block 1 block 2 block 3 P(0-3) block 4 block 5 block 6 block 7 P(4-7) block 9 block 10 block 11 block 8 P(8-11) block 13 block 15 block 14 P(12-15) block 12 RAID 4: Block Interleaved Parity • Blocks: striping units • Allow for parallel access by multiple I/O requests, high I/O rates • Doing multiple small reads is now faster than before. (allows small read requests to be restricted to a single disk). • Large writes(full stripe), update the parity: • P’ = d0’ + d1’ + d2’ + d3’; • Small writes(eg. write on d0), update the parity: • P = d0 + d1 + d2 + d3 • P’ = d0’ + d1 + d2 + d3 = P + d0’ + d0; • However, writes are still very slow since the parity • disk is the bottleneck.

  44. RAID 4: Independent Data disks with shared Parity disk Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 4 requires a minimum of 3 drives to implement ECE6160:Advanced Computer Networks

  45. D0 D1 D2 D3 P D7 P D4 D5 D6 Inspiration for RAID 5 • RAID 4 works well for small reads • Small writes (write to one disk): • Option 1: read other data disks, create new sum and write to Parity Disk • Option 2: since P has old sum, compare old data to new data, add the difference to P • Small writes are limited by Parity Disk: Write to D0, D5 both also write to P disk. Parity disk must be updated for every write operation!

  46. Redundant Arrays of Inexpensive Disks RAID 5: High I/O Rate Interleaved Parity Increasing Logical Disk Addresses D0 D1 D2 D3 P Independent writes possible because of interleaved parity D4 D5 D6 P D7 D8 D9 P D10 D11 D12 P D13 D14 D15 Example: write to D0, D5 uses disks 0, 1, 3, 4 P D16 D17 D18 D19 D20 D21 D22 D23 P . . . . . . . . . . . . . . . Disk Columns

  47. block 0 block 2 block 3 P(0-3) block 1 block 5 block 4 block 6 P(4-7) block 7 block 9 block 10 block 11 P(8-11) block 8 block 12 P(12-15) block 13 block 14 block 15 P(16-19) block 16 block 17 block 18 block 19 RAID 5: Block Interleaved Distributed-Parity Left Symmetric Distribution • Parity disk = (block number/4) mod 5 • Eliminate the parity disk bottleneck of RAID 4 • Best small read, large read and large write performance • Can correct any single self-identifying failure • Small logical writes take two physical reads and two • physical writes. • Recovering needs reading all nonfailed disks

  48. RAID 5: Independent Data disks with distributed parity blocks Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement ECE6160:Advanced Computer Networks

  49. Comparison of RAID Levels (N disks, each with capacity of C)

  50. Other RAIDs • HP AutoRAID • AFRAID • RAPID • SwiftRAID • TickerTAIP • SMDA • …… ECE6160:Advanced Computer Networks

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