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

    ACP_DINDXCACHE controls the size of the directory index cache
    and the number of buffers used on a cachewide basis. Also,
    ACP_DINDXCACHE builds a temporary index into the directory
    file, thereby reducing search time and directory header lookup
    operations.

    ACP_DINDXCACHE is an AUTOGEN, DYNAMIC, and FEEDBACK parameter.