| DRUM(4) | Device Drivers Manual | DRUM(4) | 
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.
The drum special file appeared in
  3.0BSD.
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.