Hi Timothy, On +2019-11-24 02:17:20 -0500, Timothy Sample wrote: > Hi Bengt, > > Bengt Richter <b...@bokr.com> writes: > [...] > > ┌─────────────────────────────────────────────────────────────────────────────────────┐ > > │ So again, what exactly goes into computing those /gnu/store/.../file > > prefixes?? ;-) │ > > └─────────────────────────────────────────────────────────────────────────────────────┘ > > A store prefix is the hash of all the inputs used to build a store item. > It has nothing to do with the contents of the output [1]. Two store > items may have identical contents but different prefixes. > > Say we have a package A. If we build A we might get a file called > “/gnu/store/abc123-a” with contents “this is a”. Now imagine that we > make a change to A that doesn’t change its output. If we build this > changed package A′, we would get a file called “/gnu/store/xyz456-a” > (note the different prefix) but its contents would still be “this is a”. > > What’s more is that Guix will notice that these two files are the same > and deduplicate them through hard linking. From the manual: > > By default, files added to the store are automatically > “deduplicated”: if a newly added file is identical to another one > found in the store, the daemon makes the new file a hard link to the > other file. > > This explains why you have only one inode. > > As an aside, this is called the “extensional” model of functional > package management. There is also an “intensional” model where the > prefixes are hashes of the content (i.e., everything in the store is > content addressable). It has some neat properties, but also some > complications. This is all discussed in Eelco Dolstra’s Ph.D. thesis > which introduces Nix: <https://nixos.org/~eelco/pubs/phd-thesis.pdf>. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ a really good read, glad you added your "aside" > > Hope that helps!
Very much so! My mistaken assumption was that the current guix was an implementation of an "intensional" model whose implementation I just wasn't understanding ;-p Crystal lear now, thanks :) Preconceptions die hard ;-/ > > [1] There are “fixed-output” derivations where the prefix is the hash of > the contents. They are used for things like a source archive, where you > don’t care how you get it, as long as you have the right one. I found that explored and explained in the thesis too. I hope there's an incrementally growing index of guix reference docs like that, with urls and synopses in the citations. Your post here (before I cluttered it up :) could be a model entry. > > > -- Tim Thanks again. -- Regards, Bengt Richter
