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

    ACP_DIRCACHE sets the number of pages for caching directory
    blocks. Too small a value causes excessive XQP I/O operations,
    while too large a value causes excessive physical memory to be
    consumed by the directory data block cache.

    ACP_DIRCACHE is an AUTOGEN, DYNAMIC, and FEEDBACK parameter.