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

    ACP_REBLDSYSD specifies whether the system disk should be rebuilt
    if it was improperly dismounted with extent caching, file number
    caching, or disk quota caching enabled. The ACP_REBLDSYSD default
    value (1)  ensures that the system disk is rebuilt. Setting the
    value to 0 means the disk is not rebuilt.

    Depending on the amount of caching enabled on the volume
    before it was dismounted, the rebuild operation may consume a
    considerable amount of time. Setting the value of ACP_REBLDSYSD
    to 0 specifies that the disk should be returned to active service
    immediately. If you set ACP_REBLDSYSD to 0, you can enter the DCL
    command SET VOLUME/REBUILD at any time to rebuild the disk.