Link phosphor-hostlogger and bmcweb

[Artem Senichev]

On Wed, May 26, 2021 at 09:20:38AM -0700, Nan Zhou wrote:
> >
> > > We plan to implement something similar to rotate count
> > > <https://linux.die.net/man/8/logrotate> in linux logrotate. It is
> > basically
> > > like a ring buffer in the file system. We keep N log files. The latest
> > log
> > > file is in plain text and the writer keeps appending data to it. The rest
> > > N-1 files are compressed.
> > In this case, you will keep full logs without gaps:
> > ```
> > Host start <- log is empty, start logging
> > |
> > [...] <- write file, compress and rotate file
> > |
> > Host reboot or shut down
> > ```
> > If there are too many logs, logrotate removes the oldest one and we lose
> > the
> > boot log (form host start).
> > This is the default Hostlogger mode:
> > ```
> > Host start <- log is empty, start logging
> > |
> > [line 3000] <- flush 3000 lines to the persistent file
> > |
> > [...] <- these logs are skipped (the last 3000 lines are in memory)
> > |
> > Host reboot or shut down <- flush last 3000 lines to the file
> 
> 
> Thanks for your explanation, but I didn't get it. Are you arguing that one
> can keep more logs in memory rather than on disk?

Of course not! =)

> If there are too many logs in a boot cycle, won't the current hostlogger
> lose some earlier logs (boot logs) as well?

That's the point!
Hostlogger does not lose these logs. It writes the boot messages, then skips
the middle and writes the last 3000 lines when the host shuts down.
We have two log files per host session: start and end.
It is too expensive to store all host output, so mid-session messages are
skipped.
It can be easily implemented with a buffer, but I am not sure we can achieve
this with logrotate.

> Or did me missing something?
> 
> Also, we already talked about it: there's a problem that if BMC loses the
> power before it sends out a signal to hostlogger, data in memory won't be
> persisted.

Yes, I agree that this is a problem. But there are ways to fix it without
breaking the current functionality of Hostlogger.
We can use rsyslog with external log server, or increase the buffer size
in obmc-console-server, or use systemd-cat with logrotate.
We can even add a new mode to Hostlogger that will not use the buffer, but
as I said earlier, there are not many common parts. 

-- 
Regards,
Artem Senichev
Software Engineer, YADRO.