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Subject: [bug#51427] [PATCH] nix: libstore: Do not remove unused links when
 deleting specific items.
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From: Ludovic =?UTF-8?Q?Court=C3=A8s?= <ludo@gnu.org>
References: <20211027034918.4591-1-maxim.cournoyer@gmail.com>
Date: Thu, 28 Oct 2021 16:16:22 +0200
In-Reply-To: <20211027034918.4591-1-maxim.cournoyer@gmail.com> (Maxim
 Cournoyer's message of "Tue, 26 Oct 2021 23:49:18 -0400")
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Hi,

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> Deleting unused links can be a very costly operation, especially on rotat=
ive
> hard drives.  As removing single store items is often used for experiment=
ation
> rather than for cleaning purposes, this change allows it to run without t=
he
> links cleanup.
>
> * nix/libstore/gc.cc (LocalStore::collectGarbage): Do not clean up links =
when
> the specified action is GCOptions::gcDeleteSpecific.
> ---
>  nix/libstore/gc.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/nix/libstore/gc.cc b/nix/libstore/gc.cc
> index e1d0765154..7d872d8cc1 100644
> --- a/nix/libstore/gc.cc
> +++ b/nix/libstore/gc.cc
> @@ -771,7 +771,7 @@ void LocalStore::collectGarbage(const GCOptions & opt=
ions, GCResults & results)
>      deleteGarbage(state, state.trashDir);
>=20=20
>      /* Clean up the links directory. */
> -    if (options.action =3D=3D GCOptions::gcDeleteDead || options.action =
=3D=3D GCOptions::gcDeleteSpecific) {
> +    if (options.action =3D=3D GCOptions::gcDeleteDead) {

I believe the effect is that =E2=80=98guix gc -D /gnu/store/=E2=80=A6-disk-=
image=E2=80=99 would
remove nothing: /gnu/store/.links would still contain a copy of that big
disk image, so as a result, you=E2=80=99ve freed zero bytes.

Am I right?

Perhaps what we could do is, upon =E2=80=98gcDeleteSpecific=E2=80=99, only =
look at the
relevant entry in .links instead of traversing all of them.

WDYT?

Thanks,
Ludo=E2=80=99.