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Re: [libvirt] [PATCH 0/6] Allow debug dump in case of crashes



On Thu, Mar 03, 2011 at 06:26:19PM +0800, Daniel Veillard wrote:
> This patch set main goal is to allow dumping
> of full debug informations for operations occuring
> in libvirt daemon before a crash.
> The principle is to save all logs to a round-robbin memory
> buffer (which we already do, except we never use that
> buffer), save the general daemon logs to a libvirtd.log
> file for which we keep an open descriptor, and upon
> reception of a fatal signal, save the memory buffer
> directly to the file descriptor and fsync it.
> 
> There is quite a few remarks about this, we already have
> that buffer but we don't log everything in, only what's
> output, that's what patch 1 changes, then we need to
> provide a function to dump this, so there is a new internal
> API for it. We also need to be cautious about what
> system call we operate from the signal handler, but
> basically we should limit ourselves here to write/fsync and
> sigaction which are safe there dixit POSIX.
> 
> Something like killall -USR2 libvirtd allows to see the
> kind of output one get, an idle libvirtd is quiet, but
> handle/timer/fdpolls tend to be very verbose, maybe we need
> an intermediate debug level, but that would also impact the
> API. Maybe now that this part is well debugged some of those
> could be suppressed or commented off.
> 
> Also the buffer is a statically allocated 64KB, maybe this
> could be made more flexible but IMHO that's not fundamental.

64 KB is unlikely to be sufficiently large if libvirt is running
under RHEV with moderate load. With a fairly strict log filter
of '1:libvirt 1:util 1:qemu' RHEV generates > 300 MB of logs
in ~10 minutes. This works out at approx 512 KB per second so
a 64 KB buffer will only capture 125 milliseconds at that log
filter level, and far far less if collecting full debug logs

Daniel
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