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

[David Goulet]

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 :)

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
> 
>