File Manipulation

MPI_File_open and MPI_File_close are collective operations that open and close a file, respectively -- that is, all processes in a communicator group must together open or close a file. To achieve a single-user, UNIX-like open, set the communicator to MPI_COMM_SELF.

MPI_File_delete deletes a specified file.

The routines MPI_File_set_size, MPI_File_get_size, MPI_File_get_group, and MPI_File_get_amode get and set information about a file. When using the collective routine MPI_File_set_size on a UNIX file, if the size that is set is smaller than the current file size, the file is truncated at the position defined by size. If size is set to be larger than the current file size, the file size becomes size.

When the file size is increased this way with MPI_File_set_size, new regions are created in the file with displacements between the old file size and the larger, newly set file size. Sun MPI I/O does not necessarily allocate file space for such new regions. You may reserve file space either by using MPI_File_preallocate or by performing a read or write to unallocated bytes. MPI_File_preallocate ensures that storage space is allocated for a set quantity of bytes for the specified file; however, its use is very "expensive" in terms of performance and disk space.

The routine MPI_File_get_group returns a communicator group, but it does not free the group.

File Info

The opaque info object allows you to provide hints for optimization of your code, making it run faster or more efficiently, for example. These hints are set for each file, using the MPI_File_open, MPI_File_set_view, MPI_File_set_info, and MPI_File_delete routines. MPI_File_set_info sets new values for the specified file's hints. MPI_File_get_info returns all the hints that the system currently associates with the specified file.

When using UNIX files, Sun MPI I/O provides four hints for controlling how much buffer space it uses to satisfy I/O requests: noncoll_read_bufsize, noncoll_write_bufsize, coll_read_bufsize, and coll_write_bufsize. These hints may be tuned for your particular hardware configuration and application to improve performance for both noncollective and collective data accesses. For example, if your application uses a single MPI I/O call to request multiple noncontiguous chunks that form a regular strided pattern inthe file, you may want to adjust the noncoll_write_bufsize to match the size of the stride. Note that these hints limit the size of MPI I/O's underlying buffers but do not limit the size of how much data a user can read or write in asingle request.