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

    RMS_EXTEND_SIZE specifies the number of blocks by which files
    are extended as they are written. This number should be chosen to
    balance the amount of extra disk space wasted at the ends of each
    file against the performance improvement provided by making large
    extents infrequently.

    When small disk quotas are used, specify a small number such as
    the disk cluster size to prevent the user's disk quota from being
    consumed. If the value of 0 is used, RMS allocates large extents
    and truncates the file back to its actual usage when it closes.

    You can set this system parameter with the DCL command
    SET RMS_DEFAULT/SYSTEM and display the parameter with SHOW RMS_
    DEFAULT.

    RMS_EXTEND_SIZE is a DYNAMIC parameter.