Re: About /var/guix/profiles and guix pull generations

[Chris Marusich <cmmarusich@gmail.com>]

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Hi Pierre,

Awesome, thank you for the patch!

Pierre Neidhardt <mail@ambrevar.xyz> writes:

> -generations and collecting garbage, along these lines:
> +generations, deleting old Guix checkouts and collecting garbage, along these
> +lines:

I'm not sure what you mean by "Guix checkouts".  Do you mean a Git
checkout of Guix's Git repository?  If so, why would deleting it help?

>  @example
> -guix package -p ~s --delete-generations=1m
> -guix gc
> -@end example\n")
> -                                profile))
> +guix package --profile=~s --delete-generations=1m
> +guix package --profile=~s --delete-generations=1m
> +guix gc --free-space=5G
> +@end example

This part seems reasonable to me.  We might also mention that others
users may have default profiles of their own.  Something like:

"If there are multiple users of Guix on this system, you might want to
repeat these commands for each user."

> +You might also want to delete old non-default profiles in
> +/var/guix/gcroots/auto.")

This is a little tricky.  This might make people think they should
delete the links directly from /var/guix/gcroots/auto.  For example, if
you create a profile at /tmp/myprofile via

  guix package --profile=/tmp/myprofile -i coreutils

you will get the following symlink structure:

--8<---------------cut here---------------start------------->8---
[0] marusich@garuda.local:~/guix
$ ls -al /tmp/myprofile*
lrwxrwxrwx 1 marusich users 16 Dec  9 13:56 /tmp/myprofile -> myprofile-1-link
lrwxrwxrwx 1 marusich users 51 Dec  9 13:56 /tmp/myprofile-1-link -> /gnu/store/l4vnx2fpi0b090bg7prjbq5hzlb3skpg-profile
[0] marusich@garuda.local:~/guix
$ ls -al /var/guix/gcroots/auto | grep myprofile
lrwxrwxrwx 1 root root   21 Dec  9 13:56 f2j55vpd15brhsqdwjzshwcxid1g8q9q -> /tmp/myprofile-1-link
[0] marusich@garuda.local:~/guix
$ 
--8<---------------cut here---------------end--------------->8---

If you delete /var/guix/gcroots/auto/f2j55vpd15brhsqdwjzshwcxid1g8q9q,
liveness no longer flows through /tmp/myprofile-1-link because you've
removed the GC root, and the guix-daemon won't know about the profile in
/tmp.  However, the profile is still there!  You can still use coreutils
via /tmp/myprofile-1-link, even though coreutils might get garbage
collected.  That isn't good.

In the case of a non-default profile like /tmp/myprofile, it's safe to
remove /tmp/myprofile and /tmp/myprofile-1-link directly (e.g., via "rm
/tmp/myprofile*").  When guix-daemon next runs GC, it will notice that
/var/guix/gcroots/auto/f2j55vpd15brhsqdwjzshwcxid1g8q9q is now broken,
and it will remove the broken symlink.

Maybe we could change

"You might also want to delete old non-default profiles in
/var/guix/gcroots/auto."

to

"You might also want to delete old non-default profiles pointed to by
the symlinks in /var/guix/gcroots/auto (broken symlinks in this
directory will be automatically removed)."

What do you think?  I know I tend to write more verbose error messages,
but I just don't want to encourage people to shoot themselves in the
foot.  If you can think of a better way to convey the message, that'd be
great.  Maybe we should just say "don't delete the symlinks in the auto
directory"?

-- 
Chris

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