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From "Burton-West, Tom" <tburt...@umich.edu>
Subject Understanding lucene indexes and disk I/O
Date Mon, 12 Apr 2010 21:57:30 GMT
Hi all,

Please let me know if this should be posted instead to the Lucene java-dev list.

We have very large tis files (about 36 GB).  I have not been too concerned as I assumed that
due to the indexing of the tis file by the tii file, only a small portion of the file needed
to be read.  However, upon re-reading the Lucene Index File Formats document: http://lucene.apache.org/java/3_0_1/fileformats.html#Term%20Dictionary
I am now wondering whether most of the tis file may need to be read if a term is near the
end of the file.

I'm trying to understand whether lucene has enough information stored in the *tii and *tis
files to determine what byte offset in the prx file to seek to in order to get the freq or
positions list for a particular term and whether a sequential read of the entire *tis file
up to the term being sought is needed in order to decode ProxDeltas and determint the byte
offset.

 What has me confused is that the Lucene Index File Formats document says:

"ProxDelta determines the position of this term's TermPositions within the .prx file. In particular,
it is the difference between the position of this term's data in that file and the position
of the previous term's data (or zero, for the first term in the file. For fields with omitTf
true, this will be 0 since prox information is not stored."

I assumed that Lucene implements a binary search of the tii file, then reads the appropriate
128 (IndexDivisor) entries from the tis file and does a binary search on that.   (So that
should be one disk seek when the searcher starts up to read in the entire tii file and then
a second disk seek to load in the appropriate data for 128 terms from the tis file.  However,
once a term's entry in the tis file is found, if only the difference between this term and
the previous term's position in the prx file is stored, it seems that in order to get the
actual byte offset of a term in the prx file, all of the previous term's ProxDelta's  starting
at the first term in the file, would have to be read and added together.  If that is true
then we are talking a sequential read of the entire tis file up to the current term.

Is this correct?   Can someone point me to the area of the code base where this is implemented
?   Am I missing something here?


Tom Burton-West


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