Re: A new paradigm for modifying operating system declarations

[raid5atemyhomework]

Hi Carlo,

> 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.

Thank you for this, this is certainly something I would prefer to simplify 
installing ZFS onto the operating system!  I wasn't aware of the 
`profile-service-type`.

I already have a `zfs-loader-service-type` here: 
https://issues.guix.gnu.org/45643#1

These are the things that the service has to do:

* Get a kernel module compiled and added to the set of kernel modules that can 
be loaded.
  * From what I understand of what you describe, this is non-existent for now.  
How difficult would this be?  Where do I start in getting this implemented?
* Load the kernel module.
* Execute `zpool import -a -l` to import all ZFS pools, mount those that are 
marked automount, and request for passwords if encrypted.
  * This has to occur after kernel module loading `(requirements 
'(kernel-module-loader))`.
  * This has to occur before `file-systems`.  In 
https://issues.guix.gnu.org/45643#2 I made `file-systems` into a target similar 
to `user-processes` (i.e. it has a `file-systems-service-type` that can be 
extended with a list of `requirements`, just like `user-procesess`) so that the 
`zpool import` service is a dependency of `file-systems`.
    * This is needed in order to get `/home` on ZFS, since `user-homes` can 
race with this and populate `/home` before the ZFS module is loaded and the 
pools are imported, and ZFS will refuse to mount on top of a non-empty 
directory.  We need to make sure that ZFS gets to mount before `file-systems` 
is started, since `file-systems` will also trigger `user-homes`.
* Add the compiled ZFS on the profile.
* Add ZFS module configuration into an `/etc/modprobe.d/zfs.conf`.

The above would not get us `/` on ZFS, since we would need to have the ZFS 
kernel module inside the `initrd`, have the kernel load the module before 
launch, probably import pools early, and then look for root pool and so on.  
The problem is getting the ZFS configuration inside the `initrd` as well, in 
addition to the userspace tools.

So, here's a sketch:

```scheme
(operating-system
  (kernel linux-libre-5.4)
  ;...

  (services
    (cons* (service zfs-service-type
             (zfs-configuration
               ; we need to compile for a specific kernel, the alternative here
               ; would be (operating-system this-operating-system) but the below
               ; is a good bit shorter, despite the DRY violation...
               (kernel linux-libre-5.4)
               (options
                 '(("zfs_arc_max" 5000000000)))))
           ;...
           %desktop-services))

  #;...)
```

Thanks
raid5atemyhomework