Mike, I believe that the answer to your question is in this PR review comment: https://github.com/apache/lucene/pull/601#discussion_r783711025.
Merging is currently implemented by looping over fields once, and merging them. Writing the vec file first would require merging flat vectors for all fields first, and then doing a second pass over all fields to create their HNSW graph. This sounds doable, but we never got to it. On Fri, Jun 27, 2025 at 2:19 PM Michael Sokolov <msoko...@gmail.com> wrote: > Without this temp file we would need to load the entire set of vectors > for the new merged segment into RAM in order to support building an > HNSW graph from it. This way we can read the vectors off the disk in > the same way we would do during normal searches. I'm not sure, but I > think the temp file simply gets renamed into the new segment and > doesn't have to be physically copied a second time. It would be good > to confirm that. > > On Thu, Jun 26, 2025 at 4:52 PM Viliam Ďurina <viliam.dur...@gmail.com> > wrote: > > > > Hi all, > > > > I noticed that during merging in an index that contains vector fields, > the > > new segment contains a temporary file with ".vec_temp_N.tmp" extension, > > which contains all the vectors being merged. This file is used to search > > for neighbors for the new HNSW graph. It is later deleted, and the > segment > > will contain a ".vec" file with the same vectors. So vectors are copied > two > > times and more space is temporarily needed on disk. > > > > In my index, the ".vec" file is 98% of the index size and the index is > many > > GB. Is it really necessary to have the temp file? Couldn't Lucene query > the > > "vec" file directly? I checked the code around it, one temp file is > created > > per field and the temp file is probably deleted before starting the next > > field, but still, there is another copy of the vector, so the temp file > > seems unnecessary. > > > > Is there some specific need for the temp file? I might try to do a PR > > removing the need for it. > > > > Viliam > > --------------------------------------------------------------------- > To unsubscribe, e-mail: java-user-unsubscr...@lucene.apache.org > For additional commands, e-mail: java-user-h...@lucene.apache.org > > -- Adrien
