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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

  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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