Re: [Xen-devel] [libvirt] [PATCH] libxl: open libxl log stream with libvirtd log_level

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Wed, Nov 11, 2015 at 04:57:59PM +0100, Marek Marczykowski-GÃrecki wrote:
> On Tue, Sep 15, 2015 at 11:21:00AM -0600, Jim Fehlig wrote:
> > Daniel P. Berrange wrote:
> > > On Tue, Sep 15, 2015 at 10:57:50AM -0600, Jim Fehlig wrote:
> > >   
> > >> Daniel P. Berrange wrote:
> > >>     
> > >>> On Tue, Sep 15, 2015 at 09:26:23AM -0600, Jim Fehlig wrote:
> > >>>   
> > >>>       
> > >>>> Instead of a hardcoded DEBUG log level, use the overall
> > >>>> daemon log level specified in libvirtd.conf when opening
> > >>>> a log stream with libxl. libxl is very verbose when DEBUG
> > >>>> log level is set, resulting in huge log files that can
> > >>>> potentially fill a disk. Control of libxl verbosity should
> > >>>> be placed in the administrator's hands.
> > >>>>
> > >>>> Signed-off-by: Jim Fehlig <jfehlig@xxxxxxxx>
> > >>>> ---
> > >>>>  src/libxl/libxl_conf.c | 18 +++++++++++++++++-
> > >>>>  1 file changed, 17 insertions(+), 1 deletion(-)
> > >>>>     
> > >>>>         
> > >>> ACK, this makes sense as default behaviour. As a future enhancement
> > >>> you might also consider supporting a config setting in 
> > >>> /etc/libvirt/libxl.conf
> > >>> to explicitly control the libxl library logging behaviour independantly.
> > >>>   
> > >>>       
> > >> I had actually thought of adding it there first, but then took this
> > >> approach assuming it would be more receptive upstream :-). Personally,
> > >> I'm on the fence. I like the idea of a single knob to control log level
> > >> throughout the daemon, making it a bit easier on admins. On the other
> > >> hand, individual knobs are more friendly to those pouring through logs.
> > >> I can add a knob in libxl.conf if preferred.
> > >>     
> > >
> > > After thinking about it some more, I could actually see value in
> > > create a dedicated virLogSource() instance, solely for libxl
> > > library messages. If we then created a virLogSourceGetPriority()
> > > method, you could query that to see if to turn on logging for
> > > the libxl library. That would ultimately allow you to turn on
> > > debug for just the libxl library if desired, eg
> > >
> > >  static virLogSource virLogLibXL = {
> > >      .name = "libxl.libxl_library",
> > >      ....
> > >  }
> > >
> > > LIBVIRT_LOG_FILTERS="1:libxl_library"
> > >   
> > 
> > Ah, good idea. I'll look into it.
> 
> Is it done anywhere? If not, how can I help?
> 
> This the above change (setting libxl log level to libvirtd global one),
> makes almost impossible to get libxl debug, because the rest of libvirtd
> trashes the logs (hundreds virEvent* and virObject* messages, at
> "info"(!) level).

Presumably that's because you are using LIBVIRT_DEBUG=1 or setting
log_debug=1 in /etc/libvirt/libvirtd.log. That is generally discouraged
because it turns on all logging, which is essentially unusably verbose.

It is also better to use more specific log settings, eg when I debug
the QEMU driver I usually use

  log_filters="1:qemu 1:security 3:event 3:object 1:util"
  log_outputs="1:file:/var/log/libvirt/libvirtd.log"

This generates log messages from every source file in qemu and
security directories, and also log messages from every file
in the util/ directory *except* for event & object files.
So it is much more managable to read.

The log filter strings just do a wildcard match against the
VIR_LOG_INIT("...."); lines in each source file, so you can
make things more or less specific as you desire.

Regards,
Daniel
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