Re: A new paradigm for modifying operating system declarations

[Carlo Zancanaro]

Hi raid5atemyhomework,

On Tue, Jan 05 2021, raid5atemyhomework wrote:
> What `install-zfs` does is that it installs the same 
> kernel-specific package in three different points:
>
> * `kernel-loadable-modules`, because ZFS needs to get into the 
> kernel somehow.
> * `packages`, because the kernel module is useless if you don't 
> have the userland tools to interact with the kernel module.
> * `services`, because ZFS is well-documented outside of Guix as 
> automatically mounting its filesystems at bootup, but that 
> actually requires a bit of magic in the `init` system, 
> specifically you need to actually **load** the module, then 
> execute `zpool import -a -l` to have it scan for all filesystems 
> and mount those that need automounting.
>
> Thus, an `install-zfs`, that is a *single* form that inserts the 
> correct bits in the correct ***three*** places, makes the 
> experience of adding ZFS to your `operating-system` easier 
> because there's less scope for error in actually adding the 
> package.  You just add a single `install-zfs`, not add three 
> things (plus an extra `(define my-zfs (make-zfs-package 
> linux-libre-5.4))` before your form).

In principle, I think this should all be handled by a service. 
Services have a number of extension points where they can impact 
the operating system being declared, by extending other services. 
For example, adding a package into the global profile is done by 
extending profile-service-type (which you can find in 
gnu/services.scm). Adding a shepherd service to manage a process 
is done by extending shepherd-root-service-type (in 
gnu/services/shepherd.scm).

This is how many services work. As an example, sddm-service-type 
extends services to: (a) start a process on the running system, 
(b) put files in /etc, (c) install some pam services, (d) add an 
account on the system, and (e) install packages in the global 
profile.

As far as I can tell, the only thing missing for a 
zfs-service-type to do what you want is that services can't 
currently add new kernel modules (although they can load them via 
kernel-module-loader-service-type). I may have missed a mechanism 
for this, though. If we added the ability to do this, then it 
should be possible to add zfs support by adding a single (service 
zfs-service-type) to your services list.

The approach of using services in this way has some advantages 
which are outlined in a blog post from 2015[1]. For me the most 
compelling advantage is that an zfs-service-type is more 
restricted in what it can do, and must be more explicit. An 
install-zfs procedure has free-reign over the entire 
operating-system definition that it gets passed, which makes it 
harder to reason about the composition of such procedures.

Carlo

[1]: https://guix.gnu.org/blog/2015/service-composition-in-guixsd/