What is Guix garbage collector root aka gc root

What is Guix garbage collector root aka gc root

[Oleg Pykhalov]

Hello Guix,

I'm trying to understand what is “garbage collector root” aka “gc root”.

1. Is “$HOME/.guix-profile” a “gc root”?

2. I need to build a package, but don't install it.  Just preserve it
for offloading.  Is “guix build --root=$HOME/.guix-profile PACKAGE” (run
as non root user) what I want?  Or if not, what FILE in “--root=FILE” do
I need to pass?

Thanks.

-- 
Best regards,

Oleg.

Re: What is Guix garbage collector root aka gc root

[Leo Famulari]

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On Sun, Aug 13, 2017 at 04:19:54PM +0300, Oleg Pykhalov wrote:
> I'm trying to understand what is “garbage collector root” aka “gc root”.

Garbage-collector roots (GC roots) are items in /gnu/store that are
registered to protect them from the garbage collector:

https://www.gnu.org/software/guix/manual/html_node/Invoking-guix-gc.html

You can see the GC root symlinks in /var/guix/gcroots. You can make a
new GC root manually by adding a symlink to the store item you want to
preserve. For example:

$ ln --symbolic $(guix build hello) /var/guix/gcroots/my-hello

> 1. Is “$HOME/.guix-profile” a “gc root”?

Yes, this is mentioned in the manual, section Invoking guix package:

https://www.gnu.org/software/guix/manual/html_node/Invoking-guix-package.html

> 2. I need to build a package, but don't install it.  Just preserve it
> for offloading.  Is “guix build --root=$HOME/.guix-profile PACKAGE” (run
> as non root user) what I want?  Or if not, what FILE in “--root=FILE” do
> I need to pass?

I've never used this feature, so I'm not sure if the value of FILE can
be anywhere on the filesystem, or if it has to be in /var/guix/gcroots.
Hopefully someone else can clarify this. Or you can read the code or
experiment :)

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Re: What is Guix garbage collector root aka gc root

[Ricardo Wurmus]

Hi Oleg,

> 2. I need to build a package, but don't install it.  Just preserve it
> for offloading.  Is “guix build --root=$HOME/.guix-profile PACKAGE” (run
> as non root user) what I want?  Or if not, what FILE in “--root=FILE” do
> I need to pass?

You can pass any file to “--root”.  It will *create* a GC root for you.

The manual says this:

--8<---------------cut here---------------start------------->8---
‘--root=FILE’
‘-r FILE’
     Make FILE a symlink to the profile for this environment, and
     register it as a garbage collector root.

     This is useful if you want to protect your environment from garbage
     collection, to make it “persistent”.

     When this option is omitted, the environment is protected from
     garbage collection only for the duration of the ‘guix environment’
     session.  This means that next time you recreate the same
     environment, you could have to rebuild or re-download packages.
--8<---------------cut here---------------end--------------->8---

Since it will create a new symlink, do *not* give it the path to an
existing profile.

--
Ricardo

GPG: BCA6 89B6 3655 3801 C3C6  2150 197A 5888 235F ACAC
https://elephly.net

Re: What is Guix garbage collector root aka gc root

[Oleg Pykhalov]

Hello Leo and Ricardo,

Leo Famulari <leo@famulari.name> writes:

…

> You can see the GC root symlinks in /var/guix/gcroots. You can make a
> new GC root manually by adding a symlink to the store item you want to
> preserve. For example:
>
> $ ln --symbolic $(guix build hello) /var/guix/gcroots/my-hello

Just for the record and other readers.

If I create “GC root” with “guix build --root=$HOME/PACKAGE PACKAGE”, I
will have two symbolic links

--8<---------------cut here---------------start------------->8---
$HOME/PACKAGE -> /gnu/store/…-PACKAGE
/var/guix/gcroots/auto/HASH -> $HOME/PACKAGE
--8<---------------cut here---------------end--------------->8---

To remove this package in next garbage collector invocation just remove
“$HOME/PACKAGE” symbolic link.  First invocation of GC like “guix gc
--list-live” will remove “/var/guix/gcroots/auto/HASH” symbolic link.

Thank you Leo and Ricardo!

-- 
Best regards,

Oleg.