[systemd-devel] Ordering a service before remote-fs-pre.target makes it quite longer

[Thomas HUMMEL]

Hello,

I'm using the xCAT (xcat.org) software to provision stateless HPC Centos 
8.1 nodes. Via a systemd service called xcatpostinit1.service, it 
enables, at boot time, to run so called postscripts for, for instance

- configure eth nic with a manual (vs dhcp) NetworkManager profile
- configure Infiniband nic
- sync files from the xcat server.

I'm using it exactly for the 3 above examples. File syncing is quite 
light as it consists in syncing pre-created ssh hostkeys.

By default this service has got the following ordering dependency:

After=network.target rsyslog.service

and doesn't pull any dependency

For my own need, I added the following:

Before=beegfs-client.service
Before=beegfs-helperd.service
After=sshd-keygen.target

This works fine.

As one of the postscript this service runs adds a NetworkManager profile 
(to autoconnect from a dhcp-originating boot profile to a manual (same 
ip address) one, and since I nfs mount some filesystems I tought I 
should order units so as to setup network first and only then mount 
remote filesystems

I thus did add:

Before=remote-fs-pre.target

This has a funny result : it ultimately works but this 
xcatpostinit1.service then takes 1min+ to end vs 20sec when the latter 
dependency is not stated.

Please find here to systemd-analyze plot svg's reflecting the with 
Before=remote-fs-pre.target (boot-dep.svg) and without it (boot-nodep.svg)

http://dl.pasteur.fr/fop/GCPbmpii/boot-dep.svg

http://dl.pasteur.fr/fop/AcfI7CSh/boot-nodep.svg

I did spend a lot of time trying to figure out why such a difference, 
all things being equal otherwise.

The part of the service which takes time is the syncing of files which

- occurs before the network reconfiguring
- consists in rsync'ing files from server to node
- triggered by a REST API call (http/80) for what I saw in sources

I did not see any cycles nor anything that caught my eye turning systemd 
in debug mode neither.

As remote-fs-pre.target is a special target I thought I may did misuse 
it for that matter.

Can you help me figure out why the difference ?

Thanks for your help

--
Thomas HUMMEL