doc/snapshot-howto.txt

   1 LTTng Flight Recorder Snapshot HOWTO
   2
   3 Mathieu Desnoyers
   4 July 21st, 2013
   5
   6 This document presents how to use the snapshot feature of LTTng.
   7
   8 Snapshots allow to grab the content of flight recorder tracing buffers
   9 at the time the snapshot record command is invoked. Flight recorder
  10 tracing gather trace data in memory, overwriting the oldest information,
  11 without requiring any disk I/O. The snapshot record command exports the
  12 snapshot to the destination specified by the user.
  13
  14 Basic usage:
  15
  16 Session daemon started as root for kernel tracing:
  17
  18 # lttng-sessiond -d
  19
  20 From a user part of the tracing group (for kernel tracing):
  21
  22 $ lttng create --snapshot
  23 $ lttng enable-event -k -a      # enable kernel tracing
  24 $ lttng enable-event -u -a      # enable user-space tracing
  25 $ lttng start
  26
  27     [ do something, generate activity on the system ]
  28
  29 $ lttng snapshot record
  30
  31     [ do more stuff... ]
  32
  33 $ lttng snapshot record
  34
  35 $ lttng stop
  36 $ lttng destroy
  37
  38 Each "lttng snapshot" command records a snapshot of the current buffer
  39 state. "lttng enable --snapshot" automatically setups the buffers in
  40 overwrite mode for flight recording, and does not attach any output file
  41 to the trace. The "lttng snapshot record" command can be performed
  42 either while tracing is started or stopped.
  43
  44 As an example, this generates the following hierarchy under the
  45 directory reported by the "create" command above:
  46
  47 .
  48 ├── snapshot-1-20130721-141838-0
  49 │   ├── kernel
  50 │   │   ├── channel0_0
  51 │   │   ├── channel0_1
  52 │   │   ├── channel0_2
  53 │   │   ├── channel0_3
  54 │   │   └── metadata
  55 │   └── ust
  56 │       └── uid
  57 │           └── 1000
  58 │               └── 64-bit
  59 │                   ├── channel0_0
  60 │                   ├── channel0_1
  61 │                   ├── channel0_2
  62 │                   ├── channel0_3
  63 │                   └── metadata
  64 └── snapshot-1-20130721-141842-1
  65     ├── kernel
  66     │   ├── channel0_0
  67     │   ├── channel0_1
  68     │   ├── channel0_2
  69     │   ├── channel0_3
  70     │   └── metadata
  71     └── ust
  72         └── uid
  73             └── 1000
  74                 └── 64-bit
  75                     ├── channel0_0
  76                     ├── channel0_1
  77                     ├── channel0_2
  78                     ├── channel0_3
  79                     └── metadata
  80
  81
  82 Then, running babeltrace on, e.g.
  83
  84 babeltrace snapshot-1-20130721-141838-0
  85
  86 shows the content of the first snapshot.
  87
  88 Please refer to the lttng(1) manpage for details on the snapshot mode.