/sys$common/syshlp/helplib.hlb Sys Parameters, ACP_HDRCACHE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    ACP_HDRCACHE sets the number of pages for caching file header
    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 file header caches.

    ACP_HDRCACHE is an AUTOGEN, DYNAMIC, and FEEDBACK parameter.