[lttng-dev] lttng snapshots and running traces

Thibault, Daniel Daniel.Thibault at drdc-rddc.gc.ca
Wed Sep 25 17:18:06 EDT 2013

   How does the 'lttng snapshot record' command affect a running (active) trace?  I presume very little, only to the extent that the consumer daemon servicing the request "steals" CPU cycles from the session daemon busily shoving records into the buffer.

   Say we have a trace running in flight recorder mode and a heavy flow of events into its buffers.  When the 'lttng snapshot record' command is issued, a consumer starts at the earliest (oldest) sub-buffer and starts dumping the records to a trace directory.  As its "cursor" advances around the ring of buffers, tracing continues.  By the time it wraps around the ring, the tracer may very well have re-used a number of sub-buffers, so the consumer keeps going, trying to catch up.  Am I right in supposing that if the reading and writing speeds are well matched, this could theoretically go on nearly forever, generating a huge trace?  (That's what the snapshot --max-size option is for, along with the enable-channel --tracefile-size and --tracefile-count options)

Daniel U. Thibault
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