From: Bengt Richter <bokr@bokr.com>
To: Timothy Sample <samplet@ngyro.com>
Cc: Guix Devel <guix-devel@gnu.org>
Subject: Re: Package inputs in manifests
Date: Sun, 24 Nov 2019 19:42:47 -0800 [thread overview]
Message-ID: <20191125034247.GA34155@PhantoNv4ArchGx.localdomain> (raw)
In-Reply-To: <87h82tg2v3.fsf@ngyro.com>
Hi Timothy,
On +2019-11-24 02:17:20 -0500, Timothy Sample wrote:
> Hi Bengt,
>
> Bengt Richter <bokr@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
next prev parent reply other threads:[~2019-11-25 3:43 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-23 16:37 Profiles/manifests-related command line interface enhancements Pierre Neidhardt
2019-10-24 9:00 ` Mark H Weaver
2019-10-24 9:32 ` Pierre Neidhardt
2019-10-24 16:28 ` Pierre Neidhardt
2019-10-24 16:42 ` Danny Milosavljevic
2019-10-24 18:16 ` Pierre Neidhardt
2019-10-24 19:23 ` Mark H Weaver
2019-10-24 20:04 ` Pierre Neidhardt
2019-10-24 21:35 ` Mark H Weaver
2019-10-25 9:29 ` Pierre Neidhardt
2019-10-31 11:38 ` Pierre Neidhardt
2019-11-03 14:18 ` Ludovic Courtès
2019-11-04 10:39 ` Pierre Neidhardt
2019-11-04 11:06 ` zimoun
2019-11-05 6:26 ` Konrad Hinsen
2019-11-05 8:35 ` Hartmut Goebel
2019-11-05 9:03 ` Konrad Hinsen
2019-11-05 9:09 ` Hartmut Goebel
2019-11-05 9:22 ` Pierre Neidhardt
2019-11-05 15:36 ` zimoun
2019-11-05 16:05 ` Konrad Hinsen
2019-11-06 12:09 ` zimoun
2019-11-07 13:07 ` Konrad Hinsen
2019-11-06 17:07 ` Ludovic Courtès
2019-11-06 22:21 ` Bengt Richter
2019-11-07 13:52 ` Konrad Hinsen
2019-11-06 16:35 ` Ludovic Courtès
2019-11-07 7:46 ` Konrad Hinsen
2019-11-07 9:04 ` Pierre Neidhardt
2019-11-07 11:14 ` Konrad Hinsen
2019-11-07 11:36 ` Pierre Neidhardt
2019-11-09 17:59 ` Ludovic Courtès
2019-11-10 9:36 ` Konrad Hinsen
2019-11-11 15:56 ` A better XML, config is code (was Re: Profiles/manifests-related command line...) Giovanni Biscuolo
2019-11-13 15:28 ` Konrad Hinsen
2019-11-12 8:55 ` Profiles/manifests-related command line interface enhancements Andy Wingo
2019-11-12 20:07 ` Konrad Hinsen
2019-11-13 20:58 ` Bengt Richter
2019-11-16 22:02 ` Ludovic Courtès
2019-11-17 10:44 ` Konrad Hinsen
2019-11-18 14:25 ` zimoun
2019-11-19 10:24 ` Konrad Hinsen
2019-11-23 17:10 ` Ludovic Courtès
2019-11-25 11:06 ` Konrad Hinsen
2019-11-26 9:51 ` On DSLs Ludovic Courtès
2019-12-02 19:05 ` zimoun
2019-12-02 19:11 ` Julien Lepiller
2019-12-03 10:19 ` Konrad Hinsen
2019-12-03 14:12 ` Ricardo Wurmus
2019-12-03 15:46 ` zimoun
2019-12-04 6:33 ` Bengt Richter
2019-12-10 16:26 ` Ludovic Courtès
2019-12-08 8:48 ` Konrad Hinsen
2019-12-03 10:26 ` Konrad Hinsen
2019-12-03 12:00 ` zimoun
2019-11-11 14:13 ` Profiles/manifests-related command line interface enhancements Hartmut Goebel
2019-11-16 22:27 ` Ludovic Courtès
2019-11-17 11:30 ` Konrad Hinsen
2019-11-18 14:40 ` zimoun
2019-12-22 19:40 ` Andreas Enge
2019-12-22 20:39 ` Pjotr Prins
2019-11-18 14:15 ` zimoun
2019-11-26 9:36 ` Ludovic Courtès
2019-11-06 16:42 ` Ludovic Courtès
2019-11-07 12:57 ` zimoun
2019-11-17 10:35 ` Package inputs in manifests Ludovic Courtès
2019-11-17 23:11 ` Bengt Richter
2019-11-18 17:14 ` zimoun
2019-11-23 14:05 ` Ludovic Courtès
2019-11-24 5:49 ` Bengt Richter
2019-11-24 7:17 ` Timothy Sample
2019-11-25 3:42 ` Bengt Richter [this message]
2019-11-18 16:18 ` zimoun
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