Re: [PATCH 0/7] tools/kvm_stat: add logfile support

[Stefan Raspl <raspl@linux.ibm.com>]

On 2020-03-23 11:12, Paolo Bonzini wrote:
> On 23/03/20 10:58, Stefan Raspl wrote:
>> Thx!
>> As for SIGHUP: The problem that I see with logrotate and likewise approaches is
>> how the heading is being handled: If it is reprinted every x lines (like the
>> original logging format in kvmstat does), then it messes up any chance of
>> loading the output in external tools for further processing.
>> If the heading is printed once only, then it will get pushed out of the log
>> files at some time - which is fatal, since '-f <fields>' allows to specify
>> custom fields, so one cannot reconstruct what the fields were.
> 
> For CSV output, can't you print the heading immediately after SIGHUP
> reopens the files?  (Maybe I am missing something and this is a stupid
> suggestion, I don't know).

(Same for me LOL)
To be able to make use of the logfiles, we'd need to have the heading appear at
the top of each of the files.
Couldn't find much info on how logrotate works internally, but from what I
gathered, it seems it moves out the current logfile e.g. /var/log/kvm.log to
become /var/log/kvm.log.1, and sends a SIGHUP to kvm_stat so that it re-opens
/var/log/kvm.log - which would then start out with a header again.
That should work, but can you confirm that this is what you're suggesting?
If so: Keep the current semantics for the original logging mode, where we have
the heading printed every 20 lines? I would assume so, as that format is better
suited for console logs, but just in case you wanted that changed...

Ciao,
Stefan