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

    ACP_MAPCACHE sets the number of pages for caching index file
    bitmap 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 bitmap cache.

    ACP_MAPCACHE is an AUTOGEN, DYNAMIC, and FEEDBACK parameter.