[lttng-dev] Workflow to view old and current trace data

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Fri Nov 8 11:49:13 EST 2019

----- On Nov 8, 2019, at 3:49 AM, Benjamin Poirier benjamin.poirier at gmail.com wrote:

> Hi, long time no see!
> Consider a system that's recording trace data continuously from a
> userspace application. A problem is noticed and I'd like to investigate
> it. What workflow would allow me to view past trace data to analyze the
> cause of the problem and view current (live) trace data while working on
> fixing the problem, all the while keeping a continuous record of events
> (ie. not stopping the trace)?
> I thought of the following approaches but both seem to have
> disadvantages:
> 1)
> Have one continuous tracing session in normal mode. When a problem is
> noticed, use lttng-rotate to be able to read old trace data. Start a
> second tracing session in live mode with the same event rules to view
> live trace data. IIUC, there's no option to prevent lttng-relayd from
> writing the traces to disk, so we end up writing two sets of identical
> traces to disk during the time we want to look at live trace data.
> 2)
> Have one tracing session running in live mode all the time. When a
> problem is noticed, use the viewer on whatever lttng-relayd has flushed
> to disk to access old trace data (session rotation is not available).
> Furthermore, this workflow has the disadvantage that the trace data is
> going through lttng-relayd all the time (which I guess is less efficient
> than writing it from lttng-consumerd) merely to support the rare case
> where data needs to be seen live while analyzing a problem.
> Is there some better approach that I didn't think about? What's your
> recommendation to support this workflow?
> I'm comparing lttng to the situation where the userspace application is
> writing its logs directly to a file. In that case, it's simple to read
> the file to access old and "live" data. Of course I realize that lttng
> shines in other areas where simple file writing does not ;)

Why do you need the "live" mode at all ? What do you intend to do with the
live stream that cannot be done through notifiers invoked on rotation
completion and use size-based or timer-based rotations ?



> Thank you,
> -Benjamin
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Mathieu Desnoyers
EfficiOS Inc.

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