Multimedia Storage Issues

1. Multimedia Storage Issues

2. Media vs. Documents • large file size • write once, read many • deadlines! NUS.SOC.CS5248 OOI WEI TSANG

3. OS Review: Disk

4. Disk head, spindle, track, sector, cylinder seek time, rotational latency NUS.SOC.CS5248 OOI WEI TSANG

5. Disk Scheduler read/write command disk sched I/O Request NUS.SOC.CS5248 OOI WEI TSANG

6. Disk Scheduling Algorithm

7. Best Effort Scheduler • FCFS: First come first serve • SSTF: Shortest seek time first • SCAN: Serve all requests in one direction, then in the other direction • C-SCAN: Serve all requests in one direction only, then “jump” back to the other end and repeat. NUS.SOC.CS5248 OOI WEI TSANG

8. Other Schemes • EDF: Earliest deadline first • EDF-SCAN: Use SCAN to schedule request with same deadline. • FD-SCAN: SCAN towards direction with earliest feasible deadline • SSEDO: Rank request by deadline and use formula (seektime*rank) • SSEDV: Use formula (lifetime* + seektime*(1- ) NUS.SOC.CS5248 OOI WEI TSANG

9. Data Placement

10. Contiguous NUS.SOC.CS5248 OOI WEI TSANG

11. Fragmented NUS.SOC.CS5248 OOI WEI TSANG

12. Stripping (RAID-0) NUS.SOC.CS5248 OOI WEI TSANG

13. Mirroring (RAID-1) NUS.SOC.CS5248 OOI WEI TSANG

14. Parity (RAID-5) NUS.SOC.CS5248 OOI WEI TSANG

15. Parity NUS.SOC.CS5248 OOI WEI TSANG

16. Summary of Concerns • High Throughput • Fault Tolerant • Load balancing NUS.SOC.CS5248 OOI WEI TSANG

17. Design Parameters • unit of striping (block size) • degree of striping (num of disks) NUS.SOC.CS5248 OOI WEI TSANG

18. Stripe Unit NUS.SOC.CS5248 OOI WEI TSANG

19. Stripe Unit NUS.SOC.CS5248 OOI WEI TSANG

20. Effects of Block Size service time Block Size NUS.SOC.CS5248 OOI WEI TSANG

21. Problem • Find block size such that the service time for the most heavily loaded disk is minimize NUS.SOC.CS5248 OOI WEI TSANG

22. Analysis Model: A Plan • service time of the busiest disk as a function of block size • expected num of blocks accessed on the busiest disk • expected num of blocks accessed on any disk NUS.SOC.CS5248 OOI WEI TSANG

23. Assumptions • Non-redundant array • Analyze read operation only NUS.SOC.CS5248 OOI WEI TSANG

24. Variables Declaration • Nd: Number of Disks • Nc: Number of Clients • B : Block size • Nb(i,j): Number of blocks client i access from disk j • Nb(i,*): Total number of blocks client i access NUS.SOC.CS5248 OOI WEI TSANG

25. Analysis Suppose i request m blocks NUS.SOC.CS5248 OOI WEI TSANG

26. Reading 1 block Prob(Nb(i,j)=1) NUS.SOC.CS5248 OOI WEI TSANG

27. Reading k block Prob(Nb(i,j)=k) NUS.SOC.CS5248 OOI WEI TSANG

28. Total Num of Blocks from Disk j • Nb(*,j) = NUS.SOC.CS5248 OOI WEI TSANG

29. Busiest Disk • Nmax = NUS.SOC.CS5248 OOI WEI TSANG

30. Access Time T = Nmax(ts + tr + Btt) Depends on: • block size B • disk characteristic ts,tr,tt • server design Nd • workload characteristic Nc, N(i,j)_ NUS.SOC.CS5248 OOI WEI TSANG

31. Model Verification Service Time Block Size NUS.SOC.CS5248 OOI WEI TSANG

32. Imbalance and Overhead • I = 1 – Tavg/Tmax • O = 1 – NmaxBTt/Tmax NUS.SOC.CS5248 OOI WEI TSANG

33. Effects of Block Size Imbalance Overhead ~optimum Block Size NUS.SOC.CS5248 OOI WEI TSANG

34. Effects of Clients Nc incr, I decr, opt B incr Imbalance Overhead Block Size NUS.SOC.CS5248 OOI WEI TSANG

35. Effects of Clients Block Size Nc NUS.SOC.CS5248 OOI WEI TSANG

36. Effects of Num of Disks Nd incr, N(i,j) decr, I incr, opt B decr Block Size Nd NUS.SOC.CS5248 OOI WEI TSANG

37. Choosing Block Size • Given Nc, assume rest is fixed • find B that minimize T • if T < duration of a round incr Nc and try again NUS.SOC.CS5248 OOI WEI TSANG

38. There is more.. • redundant array • finding optimum degree of striping NUS.SOC.CS5248 OOI WEI TSANG

39. Cello: Disk Scheduler

40. Problem • How to co-exist with other applications? NUS.SOC.CS5248 OOI WEI TSANG

41. Classes of Applications • Real-time • hard/soft • periodic/aperiodic • Best-effort • interactive • troughput-intensive NUS.SOC.CS5248 OOI WEI TSANG

42. Attempt 1 • Priority-based scheduler • Always schedule real-time tasks ahead of best-effort tasks NUS.SOC.CS5248 OOI WEI TSANG

43. Attempt 2 • Partition time slots into real-time and best-effort NUS.SOC.CS5248 OOI WEI TSANG

44. Attempt 3 • Assign weight to application class, based on their priority • Service based on weight NUS.SOC.CS5248 OOI WEI TSANG

45. Attempt 3: Cello • Two-level scheduling NUS.SOC.CS5248 OOI WEI TSANG

46. Class Independent Scheduler NUS.SOC.CS5248 OOI WEI TSANG

47. Allocating Disk Bandwidth • Allocate in proportion to time • Allocate in proportion to bytes NUS.SOC.CS5248 OOI WEI TSANG

48. Notations • P : Interval of a round • I : Total idle time so far in this round • Ui : Time allocated so far to class i • wi : Weight for class i • W : Total weight for all class NUS.SOC.CS5248 OOI WEI TSANG

49. Proportionate Time-Allocation Ui < P-I NUS.SOC.CS5248 OOI WEI TSANG

50. Proportional Time Allocation Ui < (P-I)wi/W NUS.SOC.CS5248 OOI WEI TSANG