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bug#21949: git-2.6.3: 108M download, but 23M installed

From: Ludovic Courtès
Subject: bug#21949: git-2.6.3: 108M download, but 23M installed
Date: Wed, 18 Nov 2015 19:12:43 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Florian Paul Schmidt <address@hidden> skribis:

> I noticed this earlier and wonder if it's a bug or not:
> When downloading the git-2.6.3 substitute from hydra it is a hefty
> 108M download, but installed in the store it's just 23M.

The fact that the result is small is due to hard links (under
libexec/git-core), resulting from deduplication performed by the daemon:

--8<---------------cut here---------------start------------->8---
$ du -msc /gnu/store/8xk88mnx31b6i9jz0n7x7zm4xrnknj3b-git-2.6.3
23      /gnu/store/8xk88mnx31b6i9jz0n7x7zm4xrnknj3b-git-2.6.3
23      totalo
$ du -mscl /gnu/store/8xk88mnx31b6i9jz0n7x7zm4xrnknj3b-git-2.6.3
249     /gnu/store/8xk88mnx31b6i9jz0n7x7zm4xrnknj3b-git-2.6.3
249     totalo
--8<---------------cut here---------------end--------------->8---

The problem is that the Nix archive format (Nar) is dumb and doesn’t do
anything sensible with hard links.  So the actual archive is indeed that

--8<---------------cut here---------------start------------->8---
$ guix archive --export /gnu/store/8xk88mnx31b6i9jz0n7x7zm4xrnknj3b-git-2.6.3 | 
wc -c
--8<---------------cut here---------------end--------------->8---

Hydra compresses it with bzip2, hence the 108 MiB.

Git is obviously a pathological case.  However, the issue is a more
global one that I think we should address using content-addressable



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