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The Metadata Server Overhead

The overall and raw write throughput is measured in CEFT-PVFS with a configuration of 8 data servers mirroring 8 under two access patterns: all clients concurrently write different files and all clients concurrently write different portions of the same file. Figure 6 and 7 plot the overall and raw write performance of Protocol 2 as a function of the number of client nodes, in which each measurement is repeated 20 times. As the experiment indicates, the aggregate write performance increases with the number of client nodes and reaches its maximum values when the cache at the data server side achieves best utilization. When the client number continues to increase, the aggregate write performance will decrease since on the data server side the context-switching overhead among different I/O requests increases while the benefit of cache decreases. The aggregate throughput will eventually saturate the disk throughput.

An important observation from these figures is that the performance gap between overall and raw write throughput does not increase significantly with the total number of clients. This implies that the metadata server is most likely not the performance bottleneck even when that client number is 100, close to the total available client number of 128 in our cluster. Experimental results of the other three protocols also show the same pattern of performance gap between the overall and raw throughputs. This further validates the claim made in [10][2] that the metadata server only introduces insignificant performance degradation and is not the performance bottleneck in a moderate-size cluster.

Figure 6: Aggregate write performance when each client writes to a different file using Synchronous Server Duplication with 8-mirroring-8 data servers (20 measurements, discarding 5 highest and 5 smallest).
\begin{figure}\centering \centerline{\epsfig{figure=blackfigures/DiffMetaBLACK.eps, width=3in}} \end{figure}

Figure 7: Aggregate write performance when all clients write to the same file using Synchronous Server Duplication with 8-mirroring-8 data servers (20 measurements, discarding 5 highest and 5 smallest).
\begin{figure}\centering \centerline{\epsfig{figure=blackfigures/SameMetaBLACK.eps, width=3in}} \end{figure}


next up previous
Next: Write Performances of the Up: Experimental Results on Write Previous: Benchmark
Yifeng Zhu 2003-10-16