tile index performance

[Ed McNierney]

Travis -

This is a surprisingly complicated question, as there are a number of
subtle factors involved.  Most of them involve the access speed and
throughput of the disk subsystem on which the tileindex is stored, and
those numbers depend a lot, of course, on your particular installation.

Remember that a tileindex is a shapefile.  So think of the question as
"does a shapefile containing 100,000 shapes display faster than 10
shapefiles each displaying 10,000 shapes?"

First, you should always create a spatial index on your shapefile using
shptree.  If you perceive a performance problem, that's the first thing
to check.

Second, avoid what my friend Bob Frankston calls "premature
optimization".  Get it working in a simple implementation first, then
look at performance tuning.  Don't add extra complexity right up front -
it's easier to understand and fix a machine with fewer parts.

Remember that a tileindex is designed to help you quickly identify the
small number of source files you need to open to render a particular
layer.  If your tiles or view scale/extent are such that you need to
open lots of source files to draw the map, the index won't help much.
That's unlikely to be the case if you're using very large index files,
but be aware of it.

There will also be a bit more MapServer overhead to deal with 10 layers
instead of one.  Opening 10 tileindexes to find the one tile you need is
slower than opening one tileindex to find that one tile.

In general I would strongly suspect that the management hassles of
increasing the number of layers by a factor of 10 isn't worth it.  I
have quite a few tileindex files, the largest of which contains about
25,000 tiles, and I have never felt that my tileindexes are a
performance problem.

The short version of that long answer is, "Don't worry about it."

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Travis Kirstine GIS Tech
Sent: Friday, December 16, 2005 9:54 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] tile index performance

I was wondering if there any difference in performance based on the size
of the tile index i.e. does a tile index containing 100,000 tile display
faster than 10 tileindexes each displaying 10,000 tile?

Travis Kirstine
First Base Solutions Limited
140 Renfrew Dr, Suite 100
Markham ON
L3R 6B3
Phone 905.477.3600
Fax 905.477.3882
tkirstine at jdbarnes.com