Reported 2 years ago by Jari Aalto in the Debian BTS, still applies.
The xvfb manual page refers to /usr/tmp, but it would be better to
use normal /tmp for examples
Xvfb -pixdepths 3 27 -fbdir /usr/tmp
The server will listen for connections as server number 0, will
have the default screen configuration (one screen,
1280x1024x8), will also support pixmap depths of 3 and 27, and
will use memory mapped files in /usr/tmp for the framebuffer.
xwud -in /usr/tmp/Xvfb_screen0
Displays screen 0 of the server started by the preceding exam-
Heh - I just noticed this a couple of days ago and changed to /var/tmp
in master already: