[systemd-devel] --Reboot-- lines in journal

[Mantas Mikulėnas]

On Thu, May 14, 2020 at 3:55 PM Dave Howorth <systemd at howorth.org.uk> wrote:

> What do --Reboot-- lines in the journal mean and how do they get there?
>
> I can't find any explanation on
> https://www.freedesktop.org/software/systemd/man/journalctl.html or
> related pages I've tried.
>
> I should explain why I'm interested. On my openSUSE box, I can see for
> example:
>
> # journalctl --list-boots
> -1 3c9ab70ade084dfab277efe733e18949 Mon 2020-03-02 23:44:11 GMT—Sun
> 2020-03-29 08:54:38 BST
>  0 c56183ea7877444a8252dd89a32b31f3 Sun 2020-03-29 09:15:30 BST—Thu
> 2020-05-14 13:16:49 BST
> # journalctl | grep Reboot
> -- Reboot --
> #
>
> Which looks fairly sane with what I think I should expect. But on two
> Raspberry pis that I have with persistent logging enabled they both
> have a huge excess of --Reboot-- lines. For example:
>
> $ sudo journalctl --list-boots
> -3 a9346655ca5d4700ab470bfd1b94d5da Thu 2019-02-14 10:11:59 GMT—Wed
> 2020-05-13 18:31:22 BST
> -2 c4f8ab5ec73b40818b1607b3436b90b5 Wed 2020-05-13 18:32:51 BST—Wed
> 2020-05-13 18:46:29 BST
> -1 0af9c854355f4a12a64dd00e6d3d98c1 Wed 2020-05-13 19:32:57 BST—Wed
> 2020-05-13 22:33:24 BST
>  0 fc5b35dbb3604dfbb4e2cdc99e117a75 Wed 2020-05-13 22:33:24 BST—Thu
> 2020-05-14 12:46:07 BST
> $ sudo journalctl | grep Reboot | wc
>    1667    5047   22095
> $
>
> What do the apparently excess 1664 --Reboot-- messages mean?
>

The "--Reboot--" line is simply shown every time the _BOOT_ID field changes
between two entries -- even if it changes to a previously seen boot ID
(which shouldn't happen normally, but *might* be caused by lack of a RTC?).

Meanwhile --list-boots has a bit more complex logic for discovering the
boots, and it also stops the search completely if it finds a boot ID that
it has already seen.

(What do you get from, let's say, `journalctl -o json | jq -r "._BOOT_ID" |
uniq -c`? Does it show several distinct ranges for each boot ID?)

-- 
Mantas Mikulėnas
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