Re: [systemd-devel] When is a unit "loaded"?
On Wed, Jul 11, 2018 at 06:52:12PM -0700, Filipe Brandenburger wrote: > Hey Daniel! > > On Wed, Jul 11, 2018 at 5:16 PM Daniel Wang wrote: > > I have a unit, say foo.service, on my system that's in > > /usr/lib/systemd/system, but disabled by preset. > > Not that it matters, but presets don't really matter here. The unit is > disabled, period. > > > On system boot, it doesn't show as "loaded" per `systemctl --all | grep > > foo`. > > Because there's no reference to it in units systemd sees, so systemd > doesn't need to load it. > > At bootup, systemd will simply start with default.target and then > recursively load the units it needs. > > See this link for more details: > https://www.freedesktop.org/software/systemd/man/bootup.html#System%20Manager%20Bootup > > `systemctl --all` will only show the units in memory, so foo.service > won't be listed since it's not loaded. > > > So if I override it with a file with the same name but under > > /etc/systemd/system, `systemctl cat foo.service` will show the one under > > /etc without the need for a `systemctl daemon-reload`. > > Yes, because it's not loaded. > > > If I create another service unit, bar.service, which has a After= > > dependency on foo.service, and start bar, foo.service will show as loaded. > > And then if I try to override it, `systemctl cat foo.service` will print a > > warning saying a daemon-reload is needed. > > Yes. If systemd sees a reference for that unit (even if it's an > After=), it will need to load it, since it needs to record the > dependency between the units in the internal memory structures, so it > needs a reference to the unit, and it loads it to have a complete > reference to it... > > > Nothings seems incorrect, but I have a few questions: > > - Which units are loaded on-boot and which are not? > > Only default.target and recursively any unit referred to by the loaded units. What you are saying is all correct, with two very minor nipticks: - the "default" unit can be overridden with systemd.unit= and 1/2/3, so it's not always default.target, - also, there are other units that'll be loaded, for example anything pulled in by SYSTEMD_WANTS= in udev rules (if the hardware is present), and .device/.mount/.swap units can also pull other stuff in, and such units may be created based on the state of the system. So it's not just the tree starting at the "default" unit, but also a bunch of other subtree. > > - Is the After= dependency alone enough to have systemd load a unit? Are > > there any other dependency directives that will result in the same effect? > > Yes, I believe any reference to units will trigger the unit to be > loaded. As I mentioned, systemd wants to keep the state in memory, so > loading the unit will allow it to keep complete state. > > An exception (haven't checked it, but I expect it) is references in > the [Install] section (such as Also=) since those are only used by > `systemctl enable` and are not really loaded into memory as far as I > can tell (but I might be wrong here, and these might trigger the unit > to load as well.) Yes, anything in [Install] doesn't matter (except that after the installation is done, the symlinks that are created do matter). Zbyszek ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
Re: [systemd-devel] When is a unit "loaded"?
Hey Daniel! On Wed, Jul 11, 2018 at 5:16 PM Daniel Wang wrote: > I have a unit, say foo.service, on my system that's in > /usr/lib/systemd/system, but disabled by preset. Not that it matters, but presets don't really matter here. The unit is disabled, period. > On system boot, it doesn't show as "loaded" per `systemctl --all | grep foo`. Because there's no reference to it in units systemd sees, so systemd doesn't need to load it. At bootup, systemd will simply start with default.target and then recursively load the units it needs. See this link for more details: https://www.freedesktop.org/software/systemd/man/bootup.html#System%20Manager%20Bootup `systemctl --all` will only show the units in memory, so foo.service won't be listed since it's not loaded. > So if I override it with a file with the same name but under > /etc/systemd/system, `systemctl cat foo.service` will show the one under /etc > without the need for a `systemctl daemon-reload`. Yes, because it's not loaded. > If I create another service unit, bar.service, which has a After= dependency > on foo.service, and start bar, foo.service will show as loaded. And then if I > try to override it, `systemctl cat foo.service` will print a warning saying a > daemon-reload is needed. Yes. If systemd sees a reference for that unit (even if it's an After=), it will need to load it, since it needs to record the dependency between the units in the internal memory structures, so it needs a reference to the unit, and it loads it to have a complete reference to it... > Nothings seems incorrect, but I have a few questions: > - Which units are loaded on-boot and which are not? Only default.target and recursively any unit referred to by the loaded units. > - Is the After= dependency alone enough to have systemd load a unit? Are > there any other dependency directives that will result in the same effect? Yes, I believe any reference to units will trigger the unit to be loaded. As I mentioned, systemd wants to keep the state in memory, so loading the unit will allow it to keep complete state. An exception (haven't checked it, but I expect it) is references in the [Install] section (such as Also=) since those are only used by `systemctl enable` and are not really loaded into memory as far as I can tell (but I might be wrong here, and these might trigger the unit to load as well.) I hope this helps! Cheers, Filipe smime.p7s Description: S/MIME Cryptographic Signature ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
[systemd-devel] When is a unit "loaded"?
I have a unit, say foo.service, on my system that's in /usr/lib/systemd/system, but disabled by preset. On system boot, it doesn't show as "loaded" per `systemctl --all | grep foo`. So if I override it with a file with the same name but under /etc/systemd/system, `systemctl cat foo.service` will show the one under /etc without the need for a `systemctl daemon-reload`. If I create another service unit, bar.service, which has a After= dependency on foo.service, and start bar, foo.service will show as loaded. And then if I try to override it, `systemctl cat foo.service` will print a warning saying a daemon-reload is needed. Nothings seems incorrect, but I have a few questions: - Which units are loaded on-boot and which are not? - Is the After= dependency alone enough to have systemd load a unit? Are there any other dependency directives that will result in the same effect? -- Best, Daniel smime.p7s Description: S/MIME Cryptographic Signature ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
