[lttng-dev] "No space left on device" results in corrupt trace data set

[Mathieu Desnoyers]

* David Goulet (dgoulet at efficios.com) wrote:
> Hi Paul,
> 
> The problem here would be that metadata can grow arbitrarily over time
> when let say new events are enabled during tracing or a new lib is
> dynamically loaded in the application you are tracing. Same goes for the
> kernel for loaded module or CPU hotplug...
> 
> To achieve a static size of metadata during the whole life of a tracing
> session, I guess we need somekind of flag or option set to tell the
> tracers to just deny new events once started... which from there, we
> need Mathieu's opinion :)

live support will eventually allow to side-track this whole issue by
flushing metadata to disk periodically during tracing,

Thanks,

Mathieu

> 
> Cheers!
> David
> 
> Woegerer, Paul:
> > Hi,
> > 
> > embedded users sometimes have to trace to tmpfs (e.g. streaming in not
> > an option due to missing network connectivity).
> > 
> > Ramdisks of 16M to 128M size are created and as long as the ram disk
> > does not get full, tracing works fine. Unfortunately if the disk gets
> > full things fall apart:
> > 
> > PERROR: Error in file write: No space left on device [in
> > lttng_consumer_on_read_subbuffer_mmap() at consumer.c:1367]
> > 
> > The consumer daemon will not be able to write the metadata channel and
> > therefore the trace data set will be unusable (cannot be read with
> > babeltrace).
> > 
> > -rwxrwxrwx 1 testuser users        0 Nov 16 11:22 metadata
> > -rwxrwxrwx 1 testuser users 15728640 Nov 16 11:22 myevents_0
> > -rwxrwxrwx 1 testuser users     4096 Nov 16 11:22 myevents_1
> > 
> > Is it possible to somehow reserve the file size (fallocate) for metadata
> > so that in case the disk gets full at least the metadata gets written
> > and the resulting trace set is readable with babeltrace ?
> > 
> > I wonder if that is possible at all. The consumer daemon doesn't know
> > upfront how much would need to be reserved for metadata, right ?
> > 
> > Any ideas ?
> > 
> > Thanks,
> > Paul
> > 
> > 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com