l...@gnu.org (Ludovic Courtès) writes: > Lukas Gradl <lgr...@openmailbox.org> skribis: > >> l...@gnu.org (Ludovic Courtès) writes: >> >>> Leo Famulari <l...@famulari.name> skribis: >>> >>>> I _think_ that `gc --references` works by querying the database for a >>>> list of references in the store item you give it as an argument. I think >>>> that the information in the database is created after building, by >>>> scanning the files of the package's output in /gnu/store, looking for >>>> strings that appear to be paths in /gnu/store. I say "I think" because I >>>> am not sure. I don't understand that part of the code very well yet. >>> >>> That’s 100% correct! :-) Namely, ‘guix gc --references’ makes an RPC to >>> the daemon, which then looks things up in the database (see the ‘Refs’ >>> table in nix/libstore/schema.sql.) >>> >>> Scanning for references indeed happens at the end of a successful build, >>> in ‘scanForReferences’ in libstore/references.cc. Since scanning is >>> expensive (I/O-intensive), the result is stored in the database. >> >> Sorry, this might be a dumb question, I don't quite understand the >> concept of these references. Why are references important? Shouldn't >> every store item know which other store-items are related to it from the >> "inputs"-field in its definition? Why is it necessary to keep track of >> the references? > > The “references” of a store item are its run-time dependencies, a subset > of the ‘inputs’ etc. fields, which are themselves the compile-time > dependencies. > > If those run-time dependencies were not inferred automatically by the > daemon, we’d have to maintain them individually, and this would be > error-prone and imprecise. > > Run-time dependency information is what allows Guix to know which > substitutes need to be downloaded when installing from substitutes, and > it’s what allows the garbage collector to determine which store items > are “live”, and which ones are not. >
OK, Thank you for the explanation, this makes more sense to me now! Best, Lukas
