Date: 12 Feb 1994 00:00:01 EST

  Mark Kilgard of SGI <mjk@hoot.asd.sgi.com> writes,
  SGI ships its X server with backing store disabled.  If you edit
  /usr/lib/X11/xdm/Xservers and remove the -bs option, then restart the
  X server, backing store will be available.

  The reason for not turning on backing store by default is two-fold.

  1) In IRIX 4.0.X the code for managing overlay planes didn't properly
     take overlays into account so backing store doesn't work if you
     have overlay windows on the screen. This is fixed in IRIX 5.x.

  2) In most cases, backing store hurts your performance.  It cases ALL
     X rendering operations to make an extra (small) traversal through
     backing store code even if backing store isn't in use.  Also when
     backing store is in use, it tends to grow the X server since a
     large amount of memory can easily be used to maintain backing
     store.

  One alternative to backing store is to render your image to a pixmap
  and then blit from the pixmap to the screen in response to expose
  events.  This will stress the X server a great deal less than using
  backing store.  Additionally, make sure you are compressing expose
  events when you redraw.