[systemd-devel] Periodically switch volatile to persistent journald and vice versa

[Renjaya Raga Zenta]

Hi, was my email not coming through?

Regards.

On Tue, Sep 12, 2023 at 12:06 PM Renjaya Raga Zenta <
renjaya.zenta at formulatrix.com> wrote:

> Hi,
>
> We have custom embedded device using SD Card as the main storage.
> I think there are some concerns regarding how journald write the
> logs, that may cause massive write amplification and that's bad for
> a standard SD Card.
>
> Journald has Storage= option, I could use Storage=volatile and
> ForwardToSyslog=yes then the disk writes seems acceptable. I could still do
> systemctl status and journalctl, but the logs won't be available the
> next reboot.
>
> Journalctl has --flush command, to flush any log data stored in
> /run/log/journal/ into /var/log/journal/. It also has --relinquish-var
> command, to do the reverse operation of --flush.
>
> Correct me if I'm wrong, if I do journalctl --flush, journald will write
> in bulk, isn't it? So the write amplification has less effect. If it's
> true, then I think periodically switching journald storage may help.
>
> 1. First, I set Storage=volatile
> 2. After a while, I change it to Storage=persistent or Storage=auto,
> then journalctl --flush (is restarting systemd-journald necessary?)
> 3. Then, I change it again to Storage=volatile and journalctl
> --relinquish-var (again, is restarting systemd-journald required?).
>
> So, is this a good approach? Or is my assumption is wrong. Please kindly
> advise.
>
> If it isn't wrong, the next question is when should I switch from /run to
> /var? The /run is using tmpfs, so it's in memory. I need to do the
> switch before the journal log size in /run up to 50 MB for example. How
> to do that?
>
>
> Thank you.
>
>
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