The listserv maintainer can be alerted when disks start getting full.  This
is controlled by the LOCAL SYSVARS file, normally stored on LISTSERV's A
disk.  There are MDISK statements that determine the % to start the
warnings.  Most of mine are set to 95%, so I start getting messages when a
disk is 95% full.  There is also a CHECKMDISK statement that is required to
actually trigger the checking each time LISTSERV starts up.