I wondered, "What does the Solaris tmpfs do?"

File Systems                                           tmpfs(7FS)

...

     Users of tmpfs should be aware of some constraints  involved
     in  mounting  a   tmpfs  file  system. The resources used by
     tmpfs are the same as those used when commands are  executed
     (for example, swap space allocation). This means that  large
     sized  tmpfs files can affect the amount of space left  over
     for  programs to execute. Likewise, programs requiring large
     amounts of memory use up the space available to tmpfs. Users
     running  into  this  constraint (for example, running out of
     space on tmpfs) can allocate more swap space  by  using  the
     swap(1M) command.

...

     df(1M) output is of limited accuracy since a tmpfs file sys-
     tem  size  is not static and the space available to tmpfs is
     dependent on the swap space demands of the entire system.

        - Nathan