A workstation runs a systemd timer with `OnCalendar=*-*-* 02:00:00`, but staff shut the machine down each evening, so the 02:00 activation is missed every night. Which directive in the [Timer] section makes systemd run the missed activation as soon as the machine boots again?
LPIC-1 Exam 102-500, objective 107. Administrative tasks medium
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How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct Persistent=true
Correct. Persistent records the last time the timer fired on disk; if the calculated activation was missed while the system was down, the unit is triggered immediately after the timer starts.
Not correct WakeSystem=true
Wrong for this case. WakeSystem uses the RTC alarm to resume a suspended system so a timer can fire. It brings a machine out of suspend, not out of a full power-off.
Not correct OnBootSec=1min
Wrong. OnBootSec fires relative to boot, so the job would run one minute after every boot whether or not anything was missed, and the 02:00 schedule would no longer be honoured on its own.
Not correct AccuracySec=1us
Wrong. AccuracySec only narrows the window within which systemd may delay the activation in order to batch timers together. It has no bearing on missed runs.
Why
Persistent= is the systemd timer answer to the problem anacron solves for cron: it stores a timestamp under /var/lib/systemd/timers and compares it with the next calendar elapse at start-up. It is only meaningful together with a calendar-based trigger such as OnCalendar=; monotonic triggers like OnBootSec= and OnUnitActiveSec= are always measured from an event that just happened.
Where this comes from
- Cited
- manual page systemd.timer(5)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 107 and space the ones you get wrong.
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