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Recall: Technology Access Time Cost/GB SRAM 0.5 – 5 ns $4,000 - $10,000

Today: External Memory Thurs: Computer Busses Final Exam Discussion Don’t Forget to clean up your Breadboard. External Memory. Recall: Technology Access Time Cost/GB SRAM 0.5 – 5 ns $4,000 - $10,000 DRAM 50 – 70 ns $100 - $200

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Recall: Technology Access Time Cost/GB SRAM 0.5 – 5 ns $4,000 - $10,000

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  1. Today: External MemoryThurs: Computer BussesFinal ExamDiscussionDon’t Forget to clean up your Breadboard External Memory Recall: Technology Access Time Cost/GB SRAM 0.5 – 5 ns $4,000 - $10,000 DRAM 50 – 70 ns $100 - $200 Disk 5 – 20 ms $0.50 - $2

  2. Types of External Memory • Magnetic Disk • Hard Disk Drives • Removable (Slower than Hard Drives) • RAID (Redundant Array of Independent Disks) • Solid State Disk • Flash Memory • Optical Disk • CD-ROM • CD-Recordable (CD-R) • CD-R/W • DVD • DVD-R • DVD-RW - Blue Ray • 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. HDD Capacity

  5. 2.5” and 5.25” Hard Disk Drives

  6. HDD – all the pieces

  7. HDD

  8. HDD – heads, connections

  9. HDD heads

  10. Disk Layout Methods Diagram

  11. Disk Data Layout - Platter

  12. Multiple Platters

  13. Inductive Write / Read Heads

  14. Physical Characteristics of Disk Systems

  15. Hard Drives • A typical HDD might: - have platters between .85 in and 5.25 in, - store between 120 GB and 1 TB of data , - rotate at 5,400 to 10,000 rpm and - have a media transfer rate of 100 MB/s or higher • As of July 2008, the highest capacity HDDs are 1.5  TB • The fastest “enterprise” HDDs: - spin at 10,000 or 15,000 rpm, and - can achieve sequential media transfer speeds above 160 GB/s and - a sustained transfer rate up to 125 MB/s. • Drives running at 10,000 or 15,000 rpm use smaller platters because of air drag and therefore generally have lower capacity than the highest capacity desktop drives. • Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. A typical mobile HDD spins at 5,400 rpm.

  16. Typical Hard Disk Drive Parameters

  17. Winchester Disk Format (Seagate ST506) • Formatting: Must be able to identify position of data: start of track and sector • - Additional information not available to user • - Marks tracks and sectors 30 fixed-length sectors per track

  18. Timing of Disk I/O Transfer 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/rev Note: How does organization on disk (e.g. random vs sequential) effect total time?

  19. RAID: Disk Arrays • Arrays of small and inexpensive disks • Increase potential Throughput by having many disk drives • Data is spread over multiple disk • Multiple accesses are made to several disks at a time • Reliability is lower than a single disk • Availability can be improved by adding redundant disks (RAID) • Lost information can be reconstructed from redundant information • MTTR: mean time to repair is in the order of hours • MTTF: mean time to failure of disks is tens of years Redundant Array of Independent Disks (or Inexpensive Disks)

  20. Redundant Array of Independent Disks

  21. RAID 0, 1, 2

  22. RAID: Level 0 (No Redundancy; Striping) • Multiple smaller disks as opposed to one big disk • Spreading the blocks over multiple disks – striping – means that multiple blocks can be accessed in parallel increasing the performance • A 4 disk system gives four times the throughput of a 1 disk system • Same cost as one big disk – assuming 4 small disks cost the same as one big disk • No redundancy, so what if one disk fails? • Failure of one or more disks is more likely as the number of disks in the system increases blk1 blk2 blk3 blk4

  23. RAID: Level 1 (Redundancy via Mirroring) • Uses twice as many disks as RAID 0 (e.g., 8 smaller disks with second set of 4 duplicating the first set) so there are always two copies of the data • # redundant disks = # of data disks so twice the cost of one big disk • writes have to be made to both sets of disks, so writes would be only 1/2 the performance of RAID 0 • What if one disk fails? • If a disk fails, the system just goes to the “mirror” for the data blk1.1 blk1.2 blk1.3 blk1.4 blk1.1 blk1.2 blk1.3 blk1.4 redundant (check) data

  24. RAID: Level 0+1 (Striping with Mirroring) • Combines the best of RAID 0 and RAID 1, data is striped across four disks and mirrored to four disks • Four times the throughput (due to striping) • # redundant disks = # of data disks so twice the cost of one big disk • writes have to be made to both sets of disks, so writes would be only 1/2 the performance of RAID 0 • What if one disk fails? • If a disk fails, the system just goes to the “mirror” for the data blk1 blk2 blk3 blk4 blk1 blk2 blk3 blk4 redundant (check) data

  25. RAID 3 & 4

  26. RAID: Level 3 (Bit-Interleaved Parity) blk1,b0 blk1,b1 blk1,b2 blk1,b3 • Cost of higher availability is reduced to 1/N where N is the number of disks in a protection group • # redundant disks = 1 × # of protection groups • writes require writing the new data to the data disk as well as computing the parity, meaning reading the other disks, so that the parity disk can be updated • Can tolerate limited disk failure, since the data can be reconstructed • reads require reading all the operational data disks as well as the parity disk to calculate the missing data that was stored on the failed disk 1 0 1 0 1 (odd) bit parity disk disk fails

  27. RAID: Level 4 (Block-Interleaved Parity) • Cost of higher availability still only 1/N but the parity is stored as blocks associated with sets of data blocks • Four times the throughput (striping) • # redundant disks = 1 × # of protection groups • Supports “small reads” and “small writes” (reads and writes that go to just one (or a few) data disk in a protection group) • by watching which bits change when writing new information, need only to change the corresponding bits on the parity disk • the parity disk must be updated on every write, so it is a bottleneck for back-to-back writes • Can tolerate limited disk failure, since the data can be reconstructed blk1 blk2 blk3 blk4 block parity disk

  28. New D1 data D1 D2 D3 D4 P   D1 D2 D3 D4 P Small Writes RAID 3 New D1 data D1 D2 D3 D4 P 3 reads and 2 writes involving all the disks D1 D2 D3 D4 P RAID 4 2 reads and 2 writes involving just two disks

  29. RAID 5 & 6 RAID 6 Not Used

  30. RAID: Level 5 (Distributed Block-Interleaved Parity) • Cost of higher availability still only 1/N but the parity block can be located on any of the disks so there is no single bottleneck for writes • Still four times the throughput (striping) • # redundant disks = 1 × # of protection groups • Supports “small reads” and “small writes” (reads and writes that go to just one (or a few) data disk in a protection group) • Allows multiple simultaneous writes as long as the accompanying parity blocks are not located on the same disk • Can tolerate limited disk failure, since the data can be reconstructed one of these assigned as the block parity disk

  31. Distributing Parity Blocks RAID 4 RAID 5 • By distributing parity blocks to all disks, some small writes can be performed in parallel 1 2 3 4 P0 1 2 3 4 P0 5 6 7 P1 8 5 6 7 8 P1 9 10 11 12 P2 9 10 P2 11 12 13 P3 14 15 16 13 14 15 16 P3

  32. Raid Summary • Four components of disk access time: • Seek Time: advertised to be 3 to 14 ms but lower in real systems • Rotational Latency: 5.6 ms at 5400 RPM and 2.0 ms at 15000 RPM • Transfer Time: 30 to 80 MB/s • Controller Time: typically less than .2 ms • RAIDS can be used to improve availability • RAID 0 and RAID 5 – widely used in servers, one estimate is that 80% of disks in servers are RAIDs • RAID 1 (mirroring) – EMC, Tandem, IBM • RAID 3 – Storage Concepts • RAID 4 – Network Appliance • RAIDS have enough redundancy to allow continuous operation, but not used to support hot swapping

  33. RAID Comparison (1)

  34. Raid Comparison (2)

  35. Solid State Disks • Solid State Disks • USB Thumb Drives

  36. Solid State Drive – up to 128GB – 100 MB/s Similar speed to magnetic disks – more robust – limited life

  37. USB Flash Drive – 32 GB - 15 MB/s

  38. USB Flash Drive

  39. usbFlash Drive

  40. Flash Memory Densities

  41. Optical Disks

  42. CD & DVD Disks Capacity 650 to 700 MB

  43. DVD Drive

  44. DVD Lens

  45. Optical Storage CD-ROM • Originally for audio • 650MB (or 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 • Option of error correcting

  46. 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 (for data) of the speed in relation to the basic “music” speed. • e.g. 24x • Quoted figure is maximum drive can achieve

  47. CD Construction

  48. Size Perspective

  49. CD reader

  50. CD-ROM Sector Format (333,000 Sectors/disk) • Mode 0 = blank data field • Mode 1 = 2048 byte data + error correction • Mode 2 = 2336 byte data

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