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diff --git a/docs/swclock.md b/docs/swclock.md new file mode 100644 index 0000000..40ce0c0 --- /dev/null +++ b/docs/swclock.md @@ -0,0 +1,25 @@ +# Software Pseudo RTC + +If the configure flag `--enable-swclock` is set, a few service and cron jobs +are enabled that try to help with systems that don't have a hardware real +time clock. + +The software pseudo RTC uses a file in `/var/lib` (exact path can be +configured) as backing store for the current date and time. + +When booting the system, a service called `swclock` restores the current time +from the file. When performing a reboot or shutdown, a service called +`swclocksave` writes the current time back to the file. + +A cron job is enabled that writes the current time to the backing file hourly, +so in case the system momentarily loses power, it loses "only" up to one hour. +The time will drift much worse anyway if the system is powered off (even +intentionally) for a while. + +All this is ensures that the system clock is monotonically increasing and only +somewhat behind the actual wall clock time. + +To catch up with real time, an additional cron job is enabled that tries to +update the time from an NTP server every four hours. This functionality is +implemented in a small shell script, that is also called from a dhcpcd hook +once a lease is obtained, assuming the DHCP client configure option was set. |