zpool-trim.service fails by default in NixOS 20.03 in many scenarios

[yesbox]

Describe the bug
zpool-trim.service attempts to trim pools regardless of them being trimmable or not and whether a pool is currently being trimmed or not. This has several problems. I will herby refer to the two types as HDD (non-trimmable) and SSD (trimmable) for short.

• If the system contains both HDD pools and SSD pools and a HDD pool is listed before all SSD pools in "zpool list", the service will abort after the first HDD pool is hit and the SSD pools will never be trimmed.

• If the system contains only SSD pools the trim of all SSD pools may still fail if one SSD pool is currently being trimmed. Issuing the trim command to a pool already being trimmed will fail and the service will abort before issuing the trim command to all SSD pools.

• If the system contains only HDD pools the service which is now enabled by default in NixOS 20.03 will now periodically fail, putting the entire system in a degraded systemd state. This is a NixOS 20.03 regression.

To Reproduce
Steps to reproduce the behavior:
Recreate any of the scenarios described above. All errors return 255 and can be simulated by replacing a pool name with a non-existent pool name, which also returns 255.

Expected behavior
Either:

• zpool-trim.service should trim all SSD pools and should not degrade the system by default. This could be implemented by having zpool-trim.service ignore the exit code of every "zpool trim " command it issues.
• zpool-trim.service should be disabled by default and it be documented that it only support systems containing exclusively SSD pools.

Metadata

• system: "x86_64-linux"
• host os: Linux 5.4.23, NixOS, 20.03.git.893d745d5a0 (Markhor)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.2

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:
  - nixos/modules/tasks/filesystems/zfs.nix

[yesbox]

Ping @Mic92