timer: Import defaults from systemd-sysupdate.timer

[cgwalters]

Inspired by coreos/rpm-ostree@db108a3

[jeckersb]

👍, one minor correction if you want to fix (assuming I'm not misunderstanding things).

[jeckersb]

every 6h should be every 4h here? If it's randomly picking between 2h and 6h. Maybe I am misunderstanding something though.

[cgwalters]

The comment here is mostly copied verbatim, so any errors are in the original. But it is good to double check. In looking at this...I think offhand you're right that the comment here is wrong or misleading.

It does seem offhand to me that the timer is going to be actually on average 2h (after initial bootup and completion of the timer, ignoring the time it takes to actually execute the check).

[jeckersb]

I am going to just jot down my flimsy understanding here of how this would play out. Corrections very much welcomed 😄

• T-0: system boots
• T+1h: OnBootSec=1h is "triggered", but doesn't fire immediately due to RandomizedDelaySec=4h being applied to all "subtimers" within this timer?
• T+~3h: (assume RandomizedDelaySec=4h always just produces 2h) The timer actually fires for the first time, activating the associated service unit
• T+~5h: OnUnitActiveSec=2h is "triggered" but as above with OnBootSec doesn't fire immediately due to delay
• T+~7h: The timer fires the service for the second time
• (loop here)
• T+11h: service fires
• T+15h: service fires
• (and so on looping)
• I guess Saturday at 00:00 the OnCalendar bit fires, but is still subjected to the ~2h delay
• Saturday 2am: service fires
• (subtimer for OnUnitActive resets?)
• Saturday 4am: OnUnitActive triggered but delays
• Saturday 6am: service fires

Something like that?