iBridge: Improving Unaligned Parallel File Access with Solid-State Drives

When files are striped in a parallel I/O system, requests to the files are decomposed into a number of sub-requests that are distributed over multiple servers. If a request is not aligned with the striping pattern such decomposition can make the first and last sub-requests much smaller than the striping unit. Because hard-disk-based servers can be much less efficient in serving small requests than large ones, the system exhibits heterogeneity in serving sub-requests of different sizes, and the net throughput of the entire system can be severely degraded by the inefficiency of serving the smaller requests, or fragments. Because a request is not considered complete until its slowest sub-request is, the penalty is yet greater for synchronous requests. To make the situation even worse, the larger the request, or the more data servers the requested data is striped over, the larger the detrimental performance effect of serving fragments can be. This effect can become the Achilles' heel of a parallel I/O system performance seeking scalability with large sequential accesses. In this paper we propose iBridge, a scheme that uses solid-state drives to serve request fragments and thereby bridge the performance gap between serving fragments and serving large sub-requests. We have implemented iBridge in the PVFS file system. Our experimental results with representative MPI-IO benchmarks show that iBridge can significantly improve the I/O throughput of storage systems, especially for large requests with fragments.