SIONlib is a library for writing and reading data from many thousands of parallel tasks/processes into or from one or a small number of physical files. Only the file open and close functions are collective, while each task or process can access files independently. This lends itself to drop-in I/O replacement for applications that use standard I/O in a process-local way – where each parallel process writes to a different file. SIONlib can be easily used as a replacement for standard I/O APIs (e.g. POSIX, ANSI-C), as file access is performed using SIONlib equivalents to standard C-I/O with similar semantic. SIONlib bundles this data into one or a few physical files to avoid sequential bottlenecks in the filesystem, e. g., metadata processing and contention. The process-per-file picture is maintained for the application, so each process has access to its logical file only. This enables in-place parallelisation for simple I/O schemes and is particularly suited for checkpoint and restart files. Figure 1 illustrates the operation of SIONlib for the case of a single physical file.