[Nix-dev] Please test Nix store auto-optimise

[Vladimír Čunát]

On 02/15/2015 09:54 AM, James Cook wrote:
> Hard linking is an impurity which can
> cause bugs, as Wout pointed out at the start of the thread (e.g.
> https://github.com/NixOS/nixpkgs/issues/4266).
>
> Implementation question: how would nix know when it can delete a file
> in /nix/store/.links?

If you can find some difference by looking at the files (to find if it 
shares some data), then it's an impurity. Maybe there are FS-specific 
tools for such queries; or we could maintain the information in some 
separate database, but that's probably too complex to be worth of 
implementing (from my point of view).

For anyone interested in space cost of /nix/store/.links on btrfs:
I just tried to remove my ~0.6 mil. links, which freed ~0.19 GB space of 
metadata (reported by btrfs fi df). That's fairly small amount compared 
to the total usage of ~80 GB; moreover it's on rotating HDD, so the 
difference should be even halved on an SSDs (no meta duplication in 
there by default).


Vladimir


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