[systemd-devel] Confusing journal information - journal size

[Mantas Mikulėnas]

On Fri, Jul 17, 2015 at 2:13 PM, David Sommerseth <davids at redhat.com> wrote:

>
> Hi,
>
> I'm looking through some journals now, and even though I've seen it a
> few times I haven't thought about it until now.
>
>    systemd-journal[1151]: Runtime journal is using 8.0M (max allowed
>              4.0G, trying to leave 4.0G free of 63.7G available →
>              current limit 4.0G).
>
> Could this line be cleaned up so you don't have to look up a man page to
> try to figure out what this really means?  Here's my uneducated guess
> and confusion of this line:
>
> * Runtime journal is using 8.0M
>   - Okay, so currently the journal uses 8MB of disk-space.  No problem.
>
> * max allowed 4.0G
>   - Okay, so the journal should not grow beyond 4GB, makes sense.  No
>     problem.
>
> * trying to leave 4.0G free of 63.7G available
>   - Uhm, what!? So it will grow until there is 4GB left on the
>     filesystem?  Not so okay.
>

It chooses the /smallest/ limit, not largest. (Common sense...) For
example, if you had only 5 GB space available, the journal would not grow
beyond 1 GB.


> * current limit 4.0G
>   - Ehh ... okay ... so make up your mind, please!  So will the
>     journal grow until 4GB or 59.7GB.
>

This *is* it making up its mind: "min(limit 1, limit 2) → resulting limit"

But then I looked into /var/log/journal ...
>
>   # du --si -s /var/log/journal/
>   4.3G  /var/log/journal/
>
> I do see that both system,journal and user-UID.journal are both 8.4MB,
> and from that I can guess what the log entry tried to tell me with
> "Runtime journal" ... but how is /that/ information useful for me, from
> a sys-admin point of view?
>

"Runtime" here means /run, as opposed to persistent in /var. They have
separately configurable limits, since /run is in RAM and /var is usually on
disk. (Though, I'm not entirely sure what purpose the runtime journal even
serves, when /var is available.)

-- 
Mantas Mikulėnas <grawity at gmail.com>
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