This comment seems a bit strange here: "… units when If the user …"

Thanks for the hint!

Does this work as expected, I though you needed to have mkMerge or something? In particular, I would assume the // will remove the (any (fs: fs == "btrfs") config.boot.supportedFilesystems) condition of the above code.

This indeed broke my initrd, as I discovered on booting today, argh. 🤦‍ You're totally right.

I updated it to use mkMerge in a similar way to what the zfs module does, now I see why they're doing it that way.

I'm curious, what issues did you encounter using the template units?

(Not a blocker, just to be clear)

@joachifm the full story:

What I initially tried to achieve was this, which seems to be the usual solution on other distros (copied from Arch Linux):

btrfs-scrub@.timer:

[Unit]
Description=Monthly Btrfs scrub on %f

[Timer]
OnCalendar=monthly
AccuracySec=1d
Persistent=true

[Install]
WantedBy=multi-user.target

btrfs-scrub@.service:

[Unit]
Description=Btrfs scrub on %f

[Service]
Nice=19
IOSchedulingClass=idle
ExecStart=/usr/bin/btrfs scrub start -B %f

enabling specific units for scrubbing:

> systemctl enable btrfs-scrub@$(systemd-escape /).timer
Created symlink /etc/systemd/system/multi-user.target.wants/btrfs-scrub@-.timer → /usr/lib/systemd/system/btrfs-scrub@.timer.
> systemctl enable btrfs-scrub@$(systemd-escape /home).timer
Created symlink /etc/systemd/system/multi-user.target.wants/btrfs-scrub@-home.timer → /usr/lib/systemd/system/btrfs-scrub@.timer.

each parameterized timer unit will execute a corresponding parameterized service unit.

My issues:

Packing the units (without [Install], of course) works fine using systemd.services/timers."btrfs-scrub@". But enabling the parameterized timers was problematic: I came up with a few variants (should have kept the code to show it to you…), trying to use wants / wantedBy and aliases to create the required symlinks. But as soon as something like that is added to the timer, the timer file suddenly is empty like this:

[Unit]

[Timer]

without any error message. The only thing I noted in the logs was that sometimes it tried to start btrfs-scrub@timers.timer, but I have no idea where the timers parameterization came from - from timers.target? But how?

(I found no exapmle in NixOS where something similar is done.)

So I tried to have only a single timer, activating a list of services, but a timer can only start a single services. Which lead me to the non-templated version you see here.

Hm, alright, well I guess this can be rectified later.

Okay, I was not sure whether that is actually wanted or preffered vs letting nix do all the work.
I extended the comment a little describing the issue, and marked it as a TODO.