VMS Help SET, VOLUME, Qualifiers, /WRITETHROUGH  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /WRITETHROUGH
       /NOWRITETHROUGH

    This qualifier only affects applications such as PATHWORKS that
    can request deferred writes to file headers. You use it to
    control whether these applications can use the deferred write
    feature.

    Use /NOWRITETHROUGH to allow these applications to use the
    deferred write feature. When one of these applications explicitly
    requests a deferred write when updating a file header, control
    returns to the application when the I/O request has been queued;
    the application does not have to wait until the data is on disk.

    Note that although the SHOW DEVICES/FULL command shows the volume
    status as write-back caching enabled, the extended file cache
    (XFC) will still be in write-through mode.

    Use /WRITETHROUGH to disable the deferred write feature, so that
    no applications can use it. The SHOW DEVICES/FULL command shows
    the volume status as write-through caching enabled.

    The deferred write feature is not available on Files-11 ODS-1
    volumes.