Non-Contiguous I/O Support for Object-Based Storage

The access patterns performed by disk-intensive applications vary widely, from simple contiguous reads or writes through an entire file to completely unpredictable random access. Often, applications will be able to access multiple disconnected sections of a file in a single operation. Application programming interfaces such as POSIX and MPI encourage the use of non-contiguous access with calls that process I/O vectors. Under the level of the programming interface, most storage protocols do not implement I/O vector operations (also known as scatter/gather). These protocols, including NFSv3 and block-based SCSI devices, must instead issue multiple independent operations to complete the single I/O vector operation specified by the application, at a cost of a much slower overall transfer time. Scatter/gather I/O is critical to the performance of many parallel applications, hence protocols designed for this area do tend to support I/O vectors. Parallel Virtual File System (PVFS) in particular does so; however, a recent specification for object-based storage devices (OSD) does not. Using a software implementation of an OSD as storage devices in a Parallel Virtual File System (PVFS) framework, we show the advantages of providing direct support for non-contiguous data transfers. We also implement the feature in OSDs in a way that is both efficient for performance and appropriate for inclusion in future specification documents.