[lttng-dev] clarification on limiting the size of the trace file

Thibault, Daniel Daniel.Thibault at drdc-rddc.gc.ca
Thu May 8 09:28:11 EDT 2014

Date: Mon, 5 May 2014 16:10:06 +0000

>   Your command sets up a single user-space channel named perf which uses a 32-part buffer of total size 32*262144=8388608 (8 MiB).
>   The trace is itself limited to 32 chunks of size 262144 each.  By default, user-space channels run in discard mode.  So in this case LTTng
> should capture about 8 MiB of events to disc, and a further 8 MiB in memory before "giving up" altogether.
>   When the trace is started the 8 MiB buffer will start filling up, and the consumer daemon will start dumping the events to the trace file.
> Once the trace file saturates at 8 MiB total size, the consumer daemon will stop emptying the buffer.  The buffer will then capture
> another 8 MiB of events.  Once the buffer is full, LTTng will systematically discard new events as they occur.
>   It would be an interesting feature if the snapshot command could be used at this point to dump the 8 MiB of otherwise-lost events to another trace file.

   My apologies, I misspoke.  The discard/overwrite setting of the channel is independent of the trace file size/count settings, in the sense that it only matters when comparing the relative data rates of the trace providers and consumers.  In the example above, the on-disc trace file does *not* saturate: rather, once all 32 file chunks have been written, the consumer daemon re-uses chunk 0, then chunk 1, etc.  The on-disc trace file in effect becomes a circular buffer itself.

   It could be an interesting feature to have a separate option, say 'tracefile-discard', which would have the trace file, when tracefile-size is specified, prevent re-use of its files (or chunks if also specified).  This could be used to capture a trace of the boot process events, for instance (which you can already do using snapshot+discard mode).  If anyone thinks this may be of use, please submit a feature request on bugs.lttng.org.

Daniel U. Thibault
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