wishlist: “repack” generations history of profile

[zimoun <zimon.toutoune@gmail.com>]

Hi,

For instance, I have these,

--8<---------------cut here---------------start------------->8---
$ guix package --list-generations -p ~/.config/guix/profiles/emacs/emacs
Generation 14	Dec 30 2021 21:49:01
Generation 15	Dec 30 2021 22:11:51
Generation 16	Dec 30 2021 22:26:48
Generation 17	Dec 30 2021 23:34:14
Generation 18	Dec 31 2021 19:10:15
Generation 19	Apr 26 2022 14:50:34
Generation 20	Apr 26 2022 14:50:45
Generation 21	May 03 2022 14:14:20	(current)
--8<---------------cut here---------------end--------------->8---

or

--8<---------------cut here---------------start------------->8---
$ guix pull --list-generations -p ~/.config/guix/current
Generation 72	Dec 29 2021 22:52:13
Generation 73	Dec 31 2021 19:02:14
Generation 74	Jan 04 2022 10:04:35
Generation 75	Jan 06 2022 10:30:48
Generation 76	Feb 04 2022 11:15:54
Generation 77	Apr 05 2022 10:27:45
Generation 78	Apr 20 2022 01:26:24
Generation 79	Apr 26 2022 13:43:13	(current)
--8<---------------cut here---------------end--------------->8---

Now, assume I am running out of space.  If I run,

    guix gc --delete-generations=2m

then I am removing the items in the store (data) and also the meta data
(manifest, date, channels, etc.).

I am fine to delete the old items in the store.  I do not want to keep
things I am not using.  However, for tracking and monitoring, I would
like to still keep these meta and potentially be able to rebuild such
generation.

The question is what to do when we delete?

I am proposing to delete the content, i.e., all but keep the meta, i.e.,
the file manifest.  We could have an option soft (keep meta) and hard
(remove all, meta included, as today) for guix gc.

Or an option “guix gc --repack=2m” which would remove all except the
manifest file.  The capacity of switching to old generation is kept,
space is saved, and it is almost transparent for the user.

Well, if I would like to switch to a previous generation, Guix would
recompute all the derivations, i.e., just fill the store (cache).


Working on projects long of several years, the old generations are
deleted and a part of the history is lost.  Obviously, I track in the
Git repo of the project some manifest.scm and channels.scm files.  Well,
it could be better if to be able to keep generations from years in a
very light and transparent way.

On machine where many users are running a lot of profiles, the sysadmin
periodically run “guix gc” to save some space and therefore Guix delete
materials for users missing to track the correct information.


WDYT?


Cheers,
simon