[libvirt] [PATCH v2] libxl: define a per-domain logger.

[Daniel P. Berrange]

On Fri, Jan 06, 2017 at 08:15:58AM -0700, Jim Fehlig wrote:
> On 01/06/2017 01:04 AM, Cedric Bosdonnat wrote:
> > On Thu, 2017-01-05 at 15:45 -0700, Jim Fehlig wrote:
> > > Cédric Bosdonnat wrote:
> > > > libxl doesn't provide a way to write one log for each domain. Thus
> > > > we need to demux the messages. If our logger doesn't know to which
> > > > domain to attribute a message, then it will write it to the default
> > > > log file.
> > > > 
> > > > Starting with Xen 4.9 (commit f9858025 and following), libxl will
> > > > write the domain ID in an easy to grab manner. The logger introduced
> > > > by this commit will use it to demux the libxl log messages.
> > > 
> > > One thing I noticed when testing this on a Xen installation prior to that commit
> > > is log files are created that will never be written to. E.g. after starting
> > > libvirtd and one domain
> > > 
> > > # ll /var/log/libvirt/libxl/
> > > -rw-r--r-- 1 root root     0 Jan  5 15:48 Domain-0.log
> > > -rw-r--r-- 1 root root 17512 Jan  5 15:49 libxl-driver.log
> > > -rw-r--r-- 1 root root     0 Jan  5 15:49 sles12sp2-hvm.log
> > > 
> > > IMO these empty, never-to-be-written-to log files might be a bit confusing to
> > > users. AFAIK there is no way to detect up front that the underlying Xen supports
> > > this capability right?
> > 
> > No there is no way to know it since there is no public API change
> > in libxl. The only thing we can do is delaying the creation of the file
> > to the first message write.
> 
> I think that is a wise thing to do, otherwise I envision bug reports such as
> "my domain log files are 0 bytes" :-).

Or put some content in them :-)

With the QEMU driver, every single /var/log/libvirt/qemu/$GUEST.log will
at least have the QEMU command line arguments, followed by a timestamp
showing when it started.

The idea of the CLI args is that it gives an accurate record of the config
of the guest from QEMU's POV, which is helpful to support people as you
can't be sure the XML you see is the same as the XML when the guest first
started. You don't have CLI args for libxl, but perhaps there's some  confg
you can easily serialize into there. The timestamp would at least be useful
too, if nothing else.


Regards,
Daniel
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