Transferred File Size

We conducted the bulk of our experiments with the size of the transferred files equal to 512MB. This decision is consistent with our interest in transferred files that don't fit simultaneously in the buffer cache of the server, while keeping the experimentation time within reasonable limits. In the present section, we investigate how the file size affects the performance metrics of the system. In Figure 11(a) we measure the server network throughput in a file size range between 256MB and 1GB. We observe that, with out-of-order transfers, the network throughput remains above 50MByte/s, consistently with the 40% load that we apply. Sequential transfers make the server network throughput to drop as low as less than 20MByte/s, getting close to the disk throughput. As a result, download duration (not shown) in sequential transfers increases dramatically to several tens of minutes, unlike the out-of-order case where downloads complete within a few minutes at all the file sizes that we examined.

  

Figure: We measure the server network throughput and disk throughput for transfers along T3 links of a file with size between 256MB and 1GB. The system load is set equal to 40%. Out- of-order transfers keep the network throughput always high and the disk throughput constant regardless of the size of the requested file.

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Figure: We depict the server network throughput and disk throughput when the number of files, concurrently requested from the server, varies between 1 and 16. The system load is set equal to 40% and the file size is always 512MB. All the files are downloaded equiprobably by the clients.

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Figure: In the base case we assume round-trip delay less than 1ms and packet loss close to 0%. In the delay case we increase the round-trip delay to 75ms and in the loss case we increase the loss rate to 10%, correspondingly, with respect to the base case. Both the download time and the miss ratio of sequential and out-of-order transfers can be affected when combining round-trip delay of 75ms with packet loss rate of 10% (delay+loss). T1 and T3 links are used equiprobably for connecting a single server to multiple clients.