26 lines
1.2 KiB
Markdown
26 lines
1.2 KiB
Markdown
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# Software Pseudo RTC
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If the configure flag `--enable-swclock` is set, a few service and cron jobs
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are enabled that try to help with systems that don't have a hardware real
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time clock.
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The software pseudo RTC uses a file in `/var/lib` (exact path can be
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configured) as backing store for the current date and time.
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When booting the system, a service called `swclock` restores the current time
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from the file. When performing a reboot or shutdown, a service called
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`swclocksave` writes the current time back to the file.
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A cron job is enabled that writes the current time to the backing file hourly,
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so in case the system momentarily loses power, it loses "only" up to one hour.
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The time will drift much worse anyway if the system is powered off (even
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intentionally) for a while.
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All this is ensures that the system clock is monotonically increasing and only
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somewhat behind the actual wall clock time.
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To catch up with real time, an additional cron job is enabled that tries to
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update the time from an NTP server every four hours. This functionality is
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implemented in a small shell script, that is also called from a dhcpcd hook
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once a lease is obtained, assuming the DHCP client configure option was set.
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