grouping raster tiles for mapserv WMS

[Jeff de La Beaujardiere]

Suppose I have a large TIFF image cut into tiles forming a
multi-resolution pyramid (each tile is a separate TIFF, not
a tiled TIFF).  I would like to have mapserv access the
lowest-resolution tile that covers the area of interest at
the user-requested size.

I created a single .shp file using gdaltindex referencing
all of my tiles, in order from lowest to highest resolution,
but mapserv does not seem to be taking advantage of that
TILEINDEX (that is, mapserv quickly renders high-res local images
but is slow to create a global overview).  Have I misunderstood
how a .shp file is supposed to be created and used?

I would like to have a separate TILEINDEX at each resolution,
with appropriate MINSSCALE and MAXSCALE, but I don't see how
to group those in a single WMS LAYER since CLASS can contain
MINSCALE and MAXSCALE but cannot contain TILEINDEX (i.e., it
seems only 1 TILEINDEX is permitted for each LAYER).

Confused,
Jeff DLB
mapserv newbie

P.S. Also, the expected units of MIN/MAXSCALE do not seem to be
documented - is it coordinate units per pixel or what? My
input data are in lon-lat.