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

Table of Contents Introduction Data Layout Strategy Matching Disk Bandwidth with Application Requirement Disk Optimization Strategies Conclusion

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.

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.

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.

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.

Matching Disk Bandwidth with Application Requirement The following diagram illustrates the placement of 3 movies on a single disk.

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

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.

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.

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.

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.

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.

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;

The Disk Pairing (DP) Strategy Movies are divided “horizontally”.

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.

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.