/sys$common/syshlp/helplib.hlb MERGE, Output File Qualifiers, /CONTIGUOUS, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    By default, Sort/Merge does not allocate contiguous disk blocks
    for the output file. You can request, however, that the output
    file be stored in contiguous disk blocks by specifying the
    /CONTIGUOUS qualifier, thereby decreasing access time. If you use
    the /CONTIGUOUS qualifier, you must also specify the /ALLOCATION
    qualifier because if the preallocated space is too small, OpenVMS
    RMS may be unable to extend the file contiguously.