What are physical swap sapce & virtual swap space ? - Chinaunix

On operating systems that do not provide virtual swap space, you must
configure large amounts of physical swap space on systems to
accommodate the reservations. You must always reserve swap space to
accommodate for the possibility that a task gets paged out for a task with
a higher priority.
Because of the virtual swap space provided by the swapfs file system in
the Solaris 9 OE, there is less need for physical swap space on systems
with a large available memory. The decreased need for physical swap
space occurs because the swapfs file system provides virtual swap space
addresses rather than real physical swap space addresses in response to
swap space reservation requests. Therefore, you need physical swap space
on disk, only in the event that the physical man pages containing private
data need to be paged out.