[systemd-devel] journald on embedded systems

Zbigniew Jędrzejewski-Szmek zbyszek at in.waw.pl
Fri Mar 13 20:03:13 PDT 2015

On Fri, Mar 13, 2015 at 06:35:38PM -0400, Chris Morgan wrote:
> Hello.
> I posted this, http://lists.freedesktop.org/archives/systemd-devel/2013-July/011926.html,
> some time ago about tiered logging for embedded systems.
> The goal is to guarantee that the flash memory will last the duration
> of the product by carefully controlling who writes to it.
> I'm back looking at the issue and wanted to re-open the discussion.
> One approach that came up would be to set "Storage=volatile", a limit
> of say "10MB" for the journal size, and then write a separate program
> that would filter out the journal entries and write them to a file on
> a physical disk.
You should be able to do something like this right now with journal-remote:
journalctl --directory /run/log/<bootid> -o export | \
   systemd-journal-remote -o /var/log/<bootid>/ -

This is a bit too hacky to put into production, and it would be better
to have a single binary which does this. But all the parts are there:

- following the journal and filtering,
- opening of specific directory as input,
- saving of state (i.e. the last cursor written),
- writing to a directory and rotating files the same as journald does.

So adding a new binary (or extending one of the existing ones)
supporting your use case would be a matter of hooking stuff together.

> The filtering portion is required as we are using the
> journal to persist some important information that we'd like to log.
> We'd also like to preserve high priority messages but don't mind if we
> lose low priority ones across reboots.
> An upside of the external program is that we can filter on both high
> priority messages as well as those with specific "FIELD=value"
> entries. Downside is a custom format file and can't use journalctl to
> search through it, no log rotation, no size limits etc.
> At the time there was some thought of putting this into journald
> itself. I'm wondering how that would fit given the desire to use more
> complicated matching to decide which entries were put into the
> persisted journal.

Adding filtering and splitting functionality to journald is another
story of course. It probably would work better: more efficent, and
journal entries would not be duplicated at all.

> If it would fit inside of journald I'd be willing to implement it but
> we would need to come up with a way to configure the filtering, where
> the files are persisted etc. The filtering is a new requirement since
> the last time this was discussed.
I think that supporting a set of similar filters to journalctl would
be a good start. Options which limit the number of messages or
filter based on time would not make sense for journald, but most
other probably would.


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