Hello,
I'm successully using systemd with some non trivial (for me!) unit
dependencies including some performing:
custom local disk formatting and mounting at boot
additionnal nics configuration by running postscripts fetched from
the network
Infiniband initialisation
NFS remote mounts
Infiniband remote mounts
HPC scheduler and its side services activation
and I've read
https://www.freedesktop.org/software/systemd/man/systemd.special.html
Still I do not fully (or at all ?) understand the concept of passive vs
active targets and some related points:
The link above states :
"Note specifically that these passive target units are generally not
pulled in by the consumer of a service, but by the provider of the
service. This means: a consuming service should order itself after these
targets (as appropriate), but not pull it in. A providing service should
order itself before these targets (as appropriate) and pull it in (via a
Wants= type dependency)."
nd also :
"Note that these passive units cannot be started manually, i.e.
"systemctl start time-sync.target" will fail with an error. They can
only be pulled in by dependency."
Since my first look at a passive dependency was network.target which I
indeed saw was pulled in by NetworkManager.service which ordered itself
Before it and which I compared with the active network-online.target
which pulls in the NetworkManager-wait-online.service I first deduced
the following:
a) a passive target "does" nothing and serves only as an ordering checkpoint
b) an active target "does" actually something
I thought that a passive target could be seen as "published" by the
corresponding provider
But this does not seems as simple as that:
For one I see on my system that rsyslog.service also pulls in
network.target (but orders itself After it and thus does not seeems to
be the actual "publisher" of it as opposed the NetworkManager.service)
Then rpcbind.target seems to auto pull itself so without the Before
ordering we see in the NetworkManager.service pulling network.target example
Also, it seems that there are more than one way to pull in a passive
dependency (or maybe several providers which can "publish" it). Like for
instance network-pre.target wich is pulled in by both nftables.service
and/or rdma-ndd.service.
Finally, my understanding is some passive targets are not to be taken
for granted, i.e. they may not be pulled in at all and it is to the user
to check it if actually is the case if he want to order a unit againt
it. I'm not talking here about obvious targets we don't have because out
of our scope (like not having remote mounts related targets if system is
purely local) but some we could think we have but maybe not. For
instance on my system I see remote-fs-pre.target pulled in by
nfs-client.target but would be remote-fs-pre-target be pulled in (by
who?) if I had only Infiniband remote mounts ?
So my question would revolve around the above points
Can you help me figuring out the correct way to see those concepts ?
Thanks for your help
--
Thomas HUMMEL
