[lttng-dev] LTTng snapshot howto

[Mathieu Desnoyers]

* yin sun (sunyin51 at gmail.com) wrote:
> Hi Mathieu,
> 
> Can you explain how the snapshot record works. Is it going to dump the
> whole buffer even if the same buffer is dumped before.

Yes, it is going to dump everything that is still available from the
buffer (not overwritten). If you want to grab less data, you can also
use lttng snapshot add-output to set the maximum size of each snapshot
(see lttng(1) for details).

> How to do something like "tail -f" to the buffer.

So if I understand correctly, you would like the second snapshot not to
gather data from the previous snapshot (no repeated events). Am I
correct ?

Unfortunately, this is not yet implemented in lttng-tools, but I can
definitely see how this could be useful! You might want to file a
feature request on bugs.lttng.org for it.

Thanks,

Mathieu

> 
> Thanks,
> /Yin
> 
> 
> On Sun, Jul 21, 2013 at 11:27 AM, Mathieu Desnoyers <
> mathieu.desnoyers at efficios.com> wrote:
> 
> > Hi,
> >
> > I just pushed a LTTng snapshot howto (new feature of lttng 2.3-rc). I'm
> > posting it to lttng-dev, since it provides a quick insight on how to use
> > the snapshot feature. Comments are welcome!
> >
> > Thanks,
> >
> > Mathieu
> >
> >
> > LTTng Flight Recorder Snapshot HOWTO
> >
> > Mathieu Desnoyers
> > July 21st, 2013
> >
> > This document presents how to use the snapshot feature of LTTng.
> >
> > Snapshots allow to grab the content of flight recorder tracing buffers
> > at the time the snapshot record command is invoked. Flight recorder
> > tracing gather trace data in memory, overwriting the oldest information,
> > without requiring any disk I/O. The snapshot record command exports the
> > snapshot to the destination specified by the user.
> >
> > Basic usage:
> >
> > Session daemon started as root for kernel tracing:
> >
> > # lttng-sessiond -d
> >
> > From a user part of the tracing group (for kernel tracing):
> >
> > $ lttng create --snapshot
> > $ lttng enable-event -k -a      # enable kernel tracing
> > $ lttng enable-event -u -a      # enable user-space tracing
> > $ lttng start
> >
> >     [ do something, generate activity on the system ]
> >
> > $ lttng snapshot record
> >
> >     [ do more stuff... ]
> >
> > $ lttng snapshot record
> >
> > $ lttng stop
> > $ lttng destroy
> >
> > Each "lttng snapshot" command records a snapshot of the current buffer
> > state. "lttng enable --snapshot" automatically setups the buffers in
> > overwrite mode for flight recording, and does not attach any output file
> > to the trace. The "lttng snapshot record" command can be performed
> > either while tracing is started or stopped.
> >
> > As an example, this generates the following hierarchy under the
> > directory reported by the "create" command above:
> >
> > .
> > ├── snapshot-1-20130721-141838-0
> > │   ├── kernel
> > │   │   ├── channel0_0
> > │   │   ├── channel0_1
> > │   │   ├── channel0_2
> > │   │   ├── channel0_3
> > │   │   └── metadata
> > │   └── ust
> > │       └── uid
> > │           └── 1000
> > │               └── 64-bit
> > │                   ├── channel0_0
> > │                   ├── channel0_1
> > │                   ├── channel0_2
> > │                   ├── channel0_3
> > │                   └── metadata
> > └── snapshot-1-20130721-141842-1
> >     ├── kernel
> >     │   ├── channel0_0
> >     │   ├── channel0_1
> >     │   ├── channel0_2
> >     │   ├── channel0_3
> >     │   └── metadata
> >     └── ust
> >         └── uid
> >             └── 1000
> >                 └── 64-bit
> >                     ├── channel0_0
> >                     ├── channel0_1
> >                     ├── channel0_2
> >                     ├── channel0_3
> >                     └── metadata
> >
> >
> > Then, running babeltrace on, e.g.
> >
> > babeltrace snapshot-1-20130721-141838-0
> >
> > shows the content of the first snapshot.
> >
> > Please refer to the lttng(1) manpage for details on the snapshot mode.
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
> >
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> >

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com