I guess I should have explained more.  I know the error code means the
disk is full; the disk is in fact NOT full when I look at it.  The
archiver runs every 3 hours to reduce it to 80% of the 20 (3380) cylinders
it has allocated.  I find it hard to believe that so many files would
arrive in 3 hours to fill 4 cylinders.  I never see it above 82%.
 
What I think MIGHT be happening is that the VMARCH program was not closing
the files it archives, and when subsequently erased (if VMARCH RC == 0)
they may be remaining mapped for whoever is "still" reading it.  This could
result in days of open files accumulating that would not show up after
LISTSERV bounces, or maybe not even in the QUERY DISK command, which would
only show for files in the directory, not OPEN ERASED ones.
 
I added a FINIS command after the archiving to just be sure.   If it still
happens, then back to the drawing board.