.fdt is one of the two file extensions that are used for stored fields in Lucene.
loading a document from disk requires two disk seeks:
one in the fields index file (.fdx),
one in the fields data file (.fdt).
The fields index file being usually small (~ 8 * maxDoc bytes), the I/O cache should be able to serve most disk seeks in this file. However the fields data file is often much larger (a little more than the original data) so the seek in this file is more likely to translate to an actual disk seek.
it is usually faster to compress the fields data file so that most of it can fit into the I/O cache.
Read full article from Efficient compressed stored fields with Lucene - Adrien Grand
loading a document from disk requires two disk seeks:
one in the fields index file (.fdx),
one in the fields data file (.fdt).
The fields index file being usually small (~ 8 * maxDoc bytes), the I/O cache should be able to serve most disk seeks in this file. However the fields data file is often much larger (a little more than the original data) so the seek in this file is more likely to translate to an actual disk seek.
it is usually faster to compress the fields data file so that most of it can fit into the I/O cache.
Read full article from Efficient compressed stored fields with Lucene - Adrien Grand
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