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Storing and Serving Multimedia

Storing and Serving Multimedia. What is a Media Server?. A scalable storage manager Allocates multimedia data optimally among disk resources Performs memory and disk-based I/O optimization Supports real-time and non-real-time clients Supports presentation of continuous-media data

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Storing and Serving Multimedia

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  1. Storing and Serving Multimedia

  2. What is a Media Server? • A scalable storage manager • Allocates multimedia data optimally among disk resources • Performs memory and disk-based I/O optimization • Supports real-time and non-real-time clients • Supports presentation of continuous-media data • Supports mixed workloads – schedules the retrieval of blocks • Performs admission control

  3. Task 1 How should multimedia data be placed on a disk?

  4. Constrained Allocation • Intuition: presentations are sequential, so place sequential segments of a MM object as closely as possible • MM object X[i] = {X[i,1], X[i,2] …X[i,n]} • | Track(X[i,j+1]) - Track(X[i,j]) |  d • Problem?

  5. Scheme Bidirectional • Disk has R equal regions • Max length of seek= 2/R X disk radius • What combination of R and segment size S will maximize throughput N without violating continuity and memory availability? • Time between two I/O requests = T, T  S/DR • For N requests, total time T = TregionTR + T regionSeek • Time to transfer N segments TregionTR = N  (S/TR) • Worst seek = d = CYL/R tracks  (CYL/R) with overhead • TregionSeek = N  (CYL/R) • N  (S/TR + (CYL/R) )  S/DR • What is wrong with this model? data display rate data transfer rate (d)=a1+b1d, often piecewise continuous

  6. Scheme Bidirectional • T is never constant • The i-th I/O request has to wait till the (i-1)th request has been serviced + seek time + transfer time • Time between two consecutive requests: • T + i (CYL/R) • Solution • Increase the segment by DR  i (CYL/R) for the i-th request • For N requests • What is the worst initial latency under this model?

  7. Other Allocation Schemes • Unidirectional allocation • Initial Latency • Sequential allocation • Idea: pick up whatever segments in on-disk sequential order • Expected seek distance: CYL/(R X N) • Worst case: 2(N-1) accesses between segments X[i,j] and X[i,j+1] • Solution • Put X[i,1] into a buffer (size S) and start playback only after the region k containing X[i,1] is fully read • If X[i,2] occurs at the start of region k+1, put in a cushion buffer, else if occurs at the end of region k+1play back from disk. • Memory constraint: 2  N  S  Mem Concurrent presentations

  8. Other Allocation Schemes • Arrival Time Problem: X[i] comes just when the disk passes over X[i,1] • Wait period: R region scans + time to read region k • Group Sweeping Scheme (GSS) • Divide N requests into G groups • Service N/G requests as a chunk • When is this beneficial? • How does initial latency of the scheme compare with others? • Effects of Replication • Memory cost? • Disk cost? • Latency?

  9. Service Models • Random Access • Maximize the number of clients that can be served concurrently at any time with a low response time • Minimize latency • Enhanced Pay-per-view (EPPV) • Increase the number of clients that can be serviced concurrently beyond the available disk and memory bandwidth, while guaranteeing a constraint on the response time

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