Galley: a new parallel file system for scientific applications

Most current multiprocessor file systems are designed to use multiple disks in parallel, using the high aggregate bandwidth to meet the growing I/O requirements of parallel scientific applications. Most multiprocessor file systems provide applications with a conventional Unix-like interface, allowing the application to access those multiple disks transparently. This interface conceals the parallelism within the file system, increasing the ease of programmability, but making it difficult or impossible for sophisticated application and library programmers to use knowledge about their I/O to exploit that parallelism. In addition to providing an insufficient interface, most current multiprocessor file systems are optimized for a different workload than they are being asked to support. In this work we examine current multiprocessor file systems, as well as how those file systems are used by scientific applications. Contrary to the expectations of the designers of current parallel file systems; the workloads on those systems are dominated by requests to read and write small pieces of data. Furthermore, rather than being accessed sequentially and contiguously, as in uniprocessor and supercomputer workloads, files in multiprocessor file systems are accessed in regular, structured, but non-contiguous patterns. Based on our observations of multiprocessor workloads, we have designed Galley; a new parallel file system that is intended to efficiently support realistic scientific multiprocessor workloads. In this work, we introduce Galley and discuss its design and implementation. We describe Galley's new three-dimensional file structure and discuss how that structure can be used by parallel applications to achieve higher performance. We introduce several new data-access interfaces, which alloy applications to explicitly describe the regular access patterns we found to be common in parallel file system workloads. We show how these new interfaces allow parallel applications to achieve tremendous increases in I/O performance. Finally, we discuss how Galley's new file structure and data-access interfaces can be useful in practice.