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 <[email protected]> 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 ([email protected]) 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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