[systemd-devel] Why do passive target units have to exist?

[林自均]

Hi Lennart,

Yes, it makes sense to me. Thank you for your explanation.

However, I still have a question about it. If the general goal is to
minimize synchronization points, why don't we convert more active targets
into passive targets? For example: not all machines have swap, so can the
swap.target be a passive target to avoid unnecessary sync point? I guess
there must be a reason not to do so, but I don't know what it is.

John

Lennart Poettering <lennart at poettering.net> 於 2017年10月24日 週二 下午4:45寫道：

> On Mo, 16.10.17 15:15, 林自均 (johnlinp at gmail.com) wrote:
>
> > Hi folks,
> >
> > I am reading systemd documents, and I find passive target units a little
> > bit confusing.
> >
> > Take "network.target" for example:
> >
> > "systemd-networkd.service" specifies "Wants=network.target" and
> > "Before=network.target". That effectively makes starting
> > "systemd-networkd.service" brings up both "systemd-networkd.service" and
> > "network.target", and make sure that "network.target" is active after
> > "systemd-networkd.service" being active. It also implies that the
> shutdown
> > order is correct: "network.target" will be stopped before
> > "systemd-networkd.service".  Everything is fine.
> >
> > What if we use an active target unit to achieve all this? Can we specify
> a
> > "WantedBy=network.target" in "systemd-network.target"? So that we can
> > enable "systemd-network.service" (which makes a symbolic link in the
> > "network.target.wants" directory) and start "network.target" to pull in
> > "systemd-networkd.service". That also makes sure "network.target" is
> active
> > after "systemd-networkd.service" because of the target unit default
> > dependencies. And shutdown order will be correct too.
> >
> > The only difference I can tell is the units to start. With a passive
> > "network.target", we start "systemd-networkd.service". With an active
> > "network.target", we start "network.target".
> >
> > Is there any benefits of passive target units over active target units?
>
> We generally try to minimize synchronization points. "network.target"
> is one, and we don't want it around if there's no reason to have
> it. The more synchronization points you have the more serial your boot
> becomes.
>
> The logic hence is: if there's not networking stack then there's no
> reason for services to synchronize before or after it, and thus it's
> the networking stack that pulls in network.target and not its
> consumer, if you so will.
>
> Does that make sense?
>
> Lennart
>
> --
> Lennart Poettering, Red Hat
>
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