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December 1994
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1994.12.01 17:46 "Directory Access Time in TIFF Files", by Victor C Zandy
Greetings,
This message concerns the time it takes to retrieve individual
images from a multiple-directory (50-100 entries) TIFF image.
We are presently constructing a database of small (less than 1K,
uncompressed) TIFF images. We have approximately 180,000 images.
Applications which use this database generally work by retrieving a
large fraction of the images. We have found that the overhead
incurred by storing each image in a separate file affects other users
of the disk and cpu---there is too much I/O. We are investigating
alternatives to this organization.
A natural idea is to divide the database into fewer files, say
3500, and utilitize the directory facilitites of TIFF to store many
images in a single file. The advantages of this (over manually
concatenating the files and devising an index/retrieval scheme), is
that the TIFF{Read,Write,Set}Directory() routines in libtiff make it
very simple and convenient to implement.
However, we have discovered that this solution, while reducing the
I/O overhead, on average at least doubles the retrieval time.
TIFFSetDirectory() seeks from the first directory; it does not jump
immediately to the desired directory. It turns out to be faster to
just read the desired image directly off the disk.
We are thus discouraged from using the directory facilities as we
understand them.
We would greatly appreciate your input on the following:
* Is there a way to increase the speed of the directory access
routines without modifying libtiff?
* Are there other features of libtiff that we can use instead of
directories?
* Do you have any other suggestions?
Thank you,
Victor Zandy