Re: journald on a system without real-time clock

[Dave Howorth <systemd@xxxxxxxxxxxxxx>]

On Mon, 21 Sep 2020 10:16:38 +0200
Magnus Berglund <wmmabeu@xxxxxxxxx> wrote:
> Hello,
> 
> I have an embedded system that does not have a real-time clock. I was
> hoping to run journald on it, but have run into some problems.
> 
> My problem is that my system currently does not guarantee a
> monotonically increasing time, and that seems to confuse journald a
> bit.
> 
> Btw: I've tested this on v243-stable.
> 
> (The system syncs over NTP, but there are some issues with this as
> well, more on that later)
> 
> I originally found the problem since there were boots missing. E.g)
> 
> # journalctl --list-boots
> -4 44cabeed86e34d09a4eca0dbff43b19f Mon 2020-08-03 14:27:59 UTC—Tue
> 2020-09-15 09:13:52 UTC
> -3 71164e7d83a9448c9e70bb59b6190a45 Tue 2020-09-15 09:13:52 UTC—Tue
> 2020-09-15 09:14:48 UTC
> -2 2e565a1c8dc84d4e95055e4cb4d0cc25 Tue 2020-09-15 09:14:48 UTC—Tue
> 2020-09-15 09:16:11 UTC
> -1 3c672d6fb8084f5fa5923a1ae5e0e53d Tue 2020-09-15 09:16:11 UTC—Tue
> 2020-09-15 09:31:52 UTC
>  0 afc0d98f275e4999aa061c7bb61b85d2 Tue 2020-09-15 09:33:35 UTC—Tue
> 2020-09-15 09:56:12 UTC
> 
> # journalctl -F _BOOT_ID
> 44cabeed86e34d09a4eca0dbff43b19f
> 71164e7d83a9448c9e70bb59b6190a45
> 1465a36c753f43df92bcc0a76d8e763e
> 2e565a1c8dc84d4e95055e4cb4d0cc25
> 3c672d6fb8084f5fa5923a1ae5e0e53d
> afc0d98f275e4999aa061c7bb61b85d2
> 302cc41b146c4b3c88f06df102913c3f
> 
> I've poked around in the source code think I found the reason for
> this: journal_file_compare_locations() (in journal-file.c) compares
> seqnum if within the the same seqnum_id, but uses current_realtime if
> not within the same seqnum_id. Since my realtime can't be trusted I
> sometimes have journalfiles within the same seqnum_id which "jumps"
> back in time. That combined with the traversal in real_journal_next()
> gives a stochastic behaviour.
> 
> I have tried fixing my clock to give a monotonically increasing
> clock. But: I want my system to boot as fast as possible, even without
> a NTP-syncronized time. And I've noticed that if I shutdown before
> getting a NTP time timesyncd will not have touched the "clock" file,
> and I end up in the situation above.
> 
> Now to my question: What is the best practice in using journald on
> systems without RTC? It it even "supported"?
> 
> /Regards,
> Magnus Berglund

There was recently a long thread about some of these issues, but
specifically as they apply to Raspberry Pis. I think it's worth reading
as a first step, though. The thread starts at

https://lists.freedesktop.org/archives/systemd-devel/2020-August/045118.html
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