scda: A Minimal, Serial-Equivalent Format for Parallel I/O

We specify a file-oriented data format suitable for parallel, partition-independent disk I/O. Here, a partition refers to a disjoint and ordered distribution of the data elements between one or more processes. The format is designed such that the file contents are invariant under linear (i. e., unpermuted), parallel repartition of the data prior to writing. The file contents are indistinguishable from writing in serial. In the same vein, the file can be read on any number of processes that agree on any partition of the number of elements stored. In addition to the format specification we propose an optional convention to implement transparent per-element data compression. The compressed data and metadata is layered inside ordinary format elements. Overall, we pay special attention to both human and machine readability. If pure ASCII data is written, or compressed data is reencoded to ASCII, the entire file including its header and sectioning metadata remains entirely in ASCII. If binary data is written, the metadata stays easy on the human eye. We refer to this format as scda. Conceptually, it lies one layer below and is oblivious to the definition of variables, the binary representation of numbers, considerations of endianness, and self-describing headers, which may all be specified on top of scda. The main purpose of the format is to abstract any parallelism and provide sufficient structure as a foundation for a generic and flexible archival and checkpoint/restart. A documented reference implementation is available as part of the general-purpose libsc free software library.Comment: 17 pages, 7 figures and 2 table