[opensuse] Systemd parallel startup/shutdown and dependencies (This is not a rant).

19 Jun 2014

      Hi all. Slowly getting my head around Systemd and how to "control" it. I 
understand the idea of speeding up boot/shutdown by taking advantage of modern 
processor/system capabilities by parallelising startup and shutdown wherever 
possible, but this does lead to some potential race conditions and I'm not 
sure there's a generic "patch" that can fix all of them since some are very 
system-specific.

With SysV Init, one could fairly easily control the order in which services 
were started (at boot) and killed (at shutdown) simpy by changing the Sxx or 
Kxx prefix of the scripts in /etc/rc.d/init.d (or /etc/init.d, depending on 
your distro), however this assumes that the entire startup process is 
serialised.

The same approach doesn't work with systemd, leading to one unintended side 
effect that I (and others, judging by open bug reports for other distros) have 
been dealing with.

One example - mounting remote file systems at boot time, especially Samba 
shares defined in /etc/samba/cifstab. With the default systemd configuration, 
the remote-fs and remote-fs-pre services (required by the remote-fs target) 
don't have the network.service listed in their "Requires=" parameter in the 
unit file; thus, in the default state the system can (and does) try to mount 
remote file systems before the network is started, but apparently only if 
you're bringing up the network using ifup instead of NetworkManager.

[To my mind, NetworkManager is great on a laptop or other mobile system but 
makes no sense on a desktop or server system where the network configuration 
is static.]

Furthermore, the system also takes down the network before trying to unmount 
the remote file systems, resulting in delayed shutdown until the unmount job 
times out.

The fix for the boot problem was easy; add "Requires=network.service" to the 
remote-fs.target unit file (in /usr/lib/systemd/system). This did not fix the 
shutdown problem though - the network interface was shut down before the 
remote file-systems were umounted, even though the network.service apparently 
wasn't.

I also had to add "Requires=network@eth0.service" to the remote-fs.service 
(and, for good measure, to the remote-fs-pre.target file, because it seems 
cifs.service may be started from remote-fs-pre.target rather than remote-
fs.target). Adding a dependency on the specific (and in my case, only) network 
interface ensures that the remote filesystems are unmounted before the 
interface is taken down and fixes the shutdown issue.

Herein lies the problem - creating a dependency on a specific network 
interface requires that the interface name be known when the unit file is 
created; there is no "network@eth0.service" file - on disk it exists as 
"network@.service" - the interface name is appended at runtime.

Now, consistent naming of interfaces is obviously important here but this is 
what makes it system-specific, because under the new scheme the interface 
names are system-specific (unless you override them to use the traditional 
names as my system does). That's why I'm not sure that a generic fix is 
possible, because it is so specific to each individual system. 

That means that if you are going to be mounting remote file systems at boot 
time, you'd better get familiar with systemd unit files because it's going to 
require fixing on a system-by-system basis.

That, I think, fits the definition of a broken design. It may have been a 
"corner case" that wasn't considered by the designers of systemd, I don't 
know, but there's got to be a better way. Changing the way the system handles 
the network startup/shutdown (to avoid the need to have dependencies on 
specific interfaces) is probably an area that needs addressing. 

Do I feel a bug report coming on?

-- 
==============================================================
Rodney Baker VK5ZTV
rodney.baker@iinet.net.au
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Rodney Baker

Andrey Borzenkov

Rodney Baker

jdd

Ruediger Meier

Carlos E. R.

David C. Rankin

David C. Rankin

Rodney Baker

Andrey Borzenkov

Ruediger Meier

jdd

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