The IO_OPTIONS clause allows you to specify I/O options used by the operating system for reading the data files. The only options available for specification are DIRECTIO (the default) and NODIRECTIO.
DIRECTIO is used by default, so an attempt is made to open the data file and read it using direct I/O. If successful, then the operating system and NFS server (if the file is on an NFS server) do not cache the data read from the file. This can improve the read performance for the data file, especially if the file is large. If direct I/O is not supported for the data file being read, then the file is opened and read but the DIRECTIO option is ignored.
If the IO_OPTIONS clause is not specified at all, or if it is specified with the NODIRECTIO option, then direct I/O is not used to read the data files.