CS 414 – Multimedia Systems Design Lecture 27 – Media Server (Part 2)

1. CS 414 – Multimedia Systems DesignLecture 27 – Media Server (Part 2) Klara Nahrstedt Spring 2009 CS 414 - Spring 2009

2. Administrative MP3 is out – deadline April 6, 5-7pm in 216 SC Sign-up sheet for demonstrations will be provided on Monday, April 6, during the class. CS 414 - Spring 2009

3. Client/Server Video-on-Demand System CS 414 - Spring 2009

4. Media Server Architecture Delivered data Incoming request Network Attachment Content Directory Memory Management File System Storage management Disk controller Storage device CS 414 - Spring 2008

5. Outline • Disk/Storage Management • Multimedia Data Placement Strategies on Disk • Multiple Disks and Multimedia • RAID • Data Striping • Group Creation • Disk Management • Data Interleaving • Disk Scheduling • EDF ; SCAN-EDF CS 414 - Spring 2009

6. Scattered Non-continuous Placement CS 414 - Spring 2009

7. Constrained Placement • Approach: separation between successive file blocks is bounded • Bound on separation – not enforced for each pair of successive blocks, but only on average over finite sequence of blocks • Attractive for small block sizes • Implementation – expensive • For constrained latency to yield full benefit, scheduling algorithm must retrieve immediately all blocks for a given stream before switching to another stream CS 414 - Spring 2009

8. Log-Structure Placement • This approach writes modified blocks sequentially in a large contiguous space, instead of requiring seek for each block in stream when writing (recording) • Reduction of disk seeks • Large performance improvements during recording, editing video and audio • Problem: bad performance during playback • Implementation: complex CS 414 - Spring 2009

9. Placement of Multiple MM Files on Single Disk • Popularity concept among multimedia content - very important • Take popularity into account when placing movies on disk • Model of popularity distribution – Zipf’s Law • Movies are kth ranked • if their probability of customer usage is C/k, • C = normalization factor • Condition holds: C/1 + C/2 + … C/N = 1, • N is number of customers CS 414 - Spring 2009

10. Example • Assume N = 5 movies • Problem: what is the probability that the next customer picks 3rd ranked movie? • Solution: • Solve C from the equation • C/1 + C/2 + C/3 + C/4 + C/5 = 1 • C = 0.437 • Probability to pick 3rd ranked movie is C/3 = 0.437/3 = 0.1456 CS 414 - Spring 2009

11. Placement Algorithm for Multiple Files on Single Disk 1st rank (most popular movie) 2nd ranked movie 3rd 4th 5th 6th 7th 8th 9th Middle of disk (in case of traditional disk layout) Note: In case of ZBR disk layout , place most popular disks at the outer tracks Organ-Pipe Algorithms (Grossman and Silverman 1973) CS 414 - Spring 2009

12. Need for Multiple Disks Solutions for Media Server • Limitation of Single Disk: Disk Throughput • Approach: 1 Maintain multiple copies of the same file on different disks • Very expensive • Approach 2: Scatter multimedia file across multiple disks CS 414 - Spring 2008

13. Approach: Data Striping • RAID (Redundant Arrays of Inexpensive Disks) • Addresses both performance and security • (0-6) RAID levels – different approach at combining performance enhancements with security/fault-tolerance enhancements • Disks spindle synchronously • Operate in lock-step parallel mode • Striping improves BW, but does not improve seek or rotational delay CS 414 - Spring 2008

14. Data Striping – Group Creation Multiple RAID: Creation of Subgroups of disks into independent logical disk arrays; limits # of disks per file Declustering: Groups are not made up of complete disks ; # of disks for any stripe is fixed and of same size, but disks on which stripe is located differs Dynamic Declustering: Non - static strip allocation to disks CS 414 - Spring 2008

15. Storage/Disk Management • Disk access – slow and costly • Reduce disk access • Use block caches (anticipate future reads or writes) • Reduce disk arm motion • Blocks accesses in sequence (continuously) , place together on one cylinder • Interleaved vs non-interleaved storage CS 414 - Spring 2008

16. Data Interleaving Data Interleaving On single disk (consecutive blocks are placed on The same cylinder But in interleaved way Data Interleaving On Multiple Disks (Disks are not Synchronized) CS 414 - Spring 2008

17. Disk Scheduling Policies • Goal of Scheduling in Traditional Disk Management • Reduce cost of seek time • Achieve high throughput • Provide fair disk access • Goal of Scheduling in Multimedia Disk Management • Meet deadline of all time-critical tasks • Keep necessary buffer requirements low • Serve many streams concurrently • Find balance between time constraints and efficiency CS 414 - Spring 2008

18. EDF (Earliest Deadline First) Disk Scheduling • Each disk block request is tagged with deadline • Policy: • Schedule disk block request with earliest deadline • Excessive seek time – high overhead • Pure EDF must be adapted or combined with file system strategies CS 414 - Spring 2008

19. EDF Example Note: Consider that block number Implicitly encapsulates the disk track number CS 414 - Spring 2008

20. Conclusion Disk Scheduling – important component in the timely delivery of streams Admission should be done if one cares not to over subscribe CS 414 - Spring 2008