Kernel Interfaces Manual
NAME drum — paging device
DESCRIPTION This file refers to the paging device in use by the system. This may actually be a subdevice of one of the disk drivers, but in a system with paging interleaved across multiple disk drives it provides an indirect driver for the multiple drives.
HISTORY The drum special file appeared in 3.0BSD.
BUGS Reads from the drum are not allowed across the interleaving boundaries. Since these only occur every .5Mbytes or so, and since the system never allocates blocks across the boundary, this is usually not a problem.