very large tile index

[Frank Warmerdam]

Ed McNierney wrote:
> Your overviews can make things considerably worse.  

Folks,

Ed has hit on a dirty little secret of overviews here.  Because of
the way they are stored in TIFF files and the way libtiff and GDAL
scan for their availability, opening a TIFF file with overviews is
presumably substantially more expensive than opening a file without
them.

The TIFF directories will tend to be distributed through the file meaning
quite a bit of seeking to get them all, and a standard GDALOpen will scan
for them all.

I don't want to discourage use of internal overviews.  They are great from
many angles.  But when in use, it is still important to avoid having to
touch too many files.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org