A Novel Video Layout Strategy for Near-Video-on-Demand Servers

1
A Novel Video Layout Strategy for
Near-Video-on-Demand Servers

• Shenze Chen Manu Thapar
• Hewlett-Packard Labs
• 1501 Page Mill Rd.
• Palo Alto, CA 94304

2
Table of Contents

• Introduction
• Data Layout Strategy
• Matching Disk Bandwidth with Application
  Requirement
• Disk Optimization Strategies
• Conclusion

3
Introduction

• TVoD allocates each client a dedicated channels
  for video streaming.
• For NVoD system, the number of channels needed is
  significantly smaller.
• The cost of TVoD system is much higher than that
  of NVoD
• NVoD requires a much smaller number of video
  channels
• Usually, no VCR control is provided for NVoD
  system.
• Scalability is no longer the problem (with
  multicast /broadcast), instead disk throughput
  may now be the bottleneck.

4
Data Layout Strategy

• Movie is broken down in to segment or logical
  block.
• The segments are placed on the disk as in the
  following diagram.

5
Data Layout Strategy

• Movie is not placed across a disk array.
• By using this layout,
• The disk head can read continuously track by
  track without any seek within a period.
• Disk seeks are eliminated, except for the seeks
  from the innermost to the outermost track between
  sweeps.
• When a disk fails, only one movie is out of air,
  other movie storing on other disks are not
  affected.

6
Matching Disk Bandwidth with Application
Requirement

• Placing Multiple Movies on a Single Disk
• In most cases, the disk bandwidth is much more
  than that of the video playback rate
• To utilize these bandwidth, we can put multiple
  movies on a single disk.
• The data rates of the movies must be identical.
• However, the size of the movies need not be the
  same, approximately the same is already ok. ?
  movies need not to be of same length.

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Matching Disk Bandwidth with Application
Requirement

• The following diagram illustrates the placement
  of 3 movies on a single disk.

8
Disk Optimization strategies

• With zoning in disk, the disk is divided into
  multiple zones with different sizes and transfer
  rates.

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Disk Optimization strategies

• When doing deterministic performance analysis, we
  are limited to use the lower data rate at the
  inner zone for calculation.
• Thus the deterministic performance greatly
  reduced.
• Two strategies are developed to deal with this
  problem.

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The Segment-Group-Pairing (SGP) Strategy

• This strategy is based on track-pairing scheme.
• In track pairing scheme, an outer track is paired
  with an inner track so as to average out the
  discrepancy in transfer rate.
• However, since the each retrieval of data block
  involve a seek, the performance of the disk is
  greatly reduced.
• SGP is devised to tackle this problem.

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The Segment-Group-Pairing (SGP) Strategy

• In SGP,
• Instead of pairing an outer track with an inner
  track, we pair an outer zone with an inner zone.
• Data to be retrieved in a service round is broken
  down into two segments, one is placed in the
  outer zone, and the other is placed in the inner
  zone.
• Advantages
• Larger block size and longer service round can be
  used.
• Only one seek is involved in a single service
  round.
• Data rate of the disk is averaged out.
• The deterministic performance of the disk is
  improved.

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The Disk Pairing (DP) Strategy

• A similar approaches to average out the data
  rates of zones.
• Suppose we have movies of different data rates to
  be placed in n disk.
• In DP,
• Every movie is divided into n parts
• If movie A is of highest data rate, then each
  part of movie A is placed in the outer zones of
  the disks.
• If movie B is of second highest data rate, then
  after the data of movie A is allocated on disk, B
  will be allocated on the unused highest data rate
  zones remaining on the disks.

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The Disk Pairing (DP) Strategy

• Playback time of the movies are staggered by
  Tr/n. where Tr is the period of the transmission.
• This architecture also enable load balancing
  between the disk because at each time instant,
  every disk is scheduling the channels for a movie
  only.

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The Disk Pairing (DP) Strategy

• Example
• suppose we have 2 disk and 2 movies.
• Movie A and B are divided into 2 parts, according
  to the Data Layout Strategy.
• First part of movie A is stored on outer zone of
  disk 1 First part of movie B is stored on the
  inner zone of disk 1
• Second part of movie A is stored on outer zone of
  disk 2 Second part of movie B is stored on the
  inner zone of disk 2

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The Disk Pairing (DP) Strategy

• Movies are divided horizontally.

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The Disk Pairing (DP) Strategy

• By the time when the first part of movie A in
  disk 1 is serving the channels, the second part
  of movie A in disk 2 should be idle.
• By the time when the first part of movie B in
  disk 2 is serving the channels, the second part
  of movie B in disk 1 should be idle.
• Perfect Load Balancing can be achieved.
• Movies with higher data rate can utilize the
  higher data rate zones in the disk?performance
  improve.

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Conclusion

• In this paper, a disk layout strategy is
  proposed.
• According to the proposed disk layout strategy,
  two disk optimization techniques can be applied.
• Segment Group Pairing (SGP)
• Disk Pairing (DP)
• In SGP, the deterministic performance is improved
  by average out the discrepancy of data rate in
  different zones.
• In DP, movies with higher data rate are allocated
  to zone with higher throughput, thus improving
  the deterministic performance.