VMS Help Sys Parameters, PFCDEFAULT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    On VAX systems during execution of programs, PFCDEFAULT controls
    the number of image pages read from disk per I/O operation when
    a page fault occurs. The PFCDEFAULT maximum default value is 127
    512-byte pages.

    On Alpha systems during execution of programs, PFCDEFAULT
    controls the number of image pagelets read from disk per I/O
    operation when a page fault occurs. The PFCDEFAULT maximum
    default value is 2032 512-byte pagelets (127 8192-byte Alpha
    pages).

    The read I/O operations can take place from an image file or
    from the page file. The actual size of the cluster can be less
    than PFCDEFAULT, depending on the size of image sections and the
    pattern of page references.

    The value should not be greater than one-fourth the default size
    of the average working set to prevent a single page fault from
    displacing a major portion of a working set. Too large a value
    for PFCDEFAULT can hurt system performance. PFCDEFAULT can be
    overridden on an image-by-image basis with the CLUSTER option of
    the OpenVMS linker.

    PFCDEFAULT has the AUTOGEN and DYNAMIC attributes.