I have a .service on a separate partition which is linked with "systemctl enable" but does not automatically start on boot. Running "systemctl status" claims "failed to load" and it works if I run systemctl daemon-reload.
My guess is that this happens because it's on a separate partition and I was able to workaround this by doing "cp -t /etc/systemd/system" explicitly.
This seems like something that should "just work", what am I missing?
Yes, this is expected and it's documented in the systemctl(1) man page, under the enable command:
The file system where the linked unit files are located must be accessible when systemd is started (e.g. anything underneath
/homeor/varis not allowed, unless those directories are located on the root file system).
The reason for the limitation is that systemd scans all units and assembles a transaction at startup. Mounting other filesystems is done through units, so units in other filesystems will not be available until the transaction is already in progress. systemd will not rescan the units and recalculate the transaction after those filesystems are mounted.
Copying the units into the root filesystem is an appropriate workaround, as you already found out.
Workaround is possible. Create unit that will call:
systemctl daemon-reload; systemctl start <your service>
Make this unit to start after mounts.
/usr/local/ partition, should I use Wants=usr-local.mount or After=usr-local.mount or Requires=usr-local.mount for my timer to start upon reboots ? And in which section(s) of the service ?