contents/using-lttng/controlling-tracing/persistent-memory-file-systems.md

   1 ---
   2 id: persistent-memory-file-systems
   3 since: 2.7
   4 ---
   5
   6 <a href="https://en.wikipedia.org/wiki/Non-volatile_random-access_memory" class="ext">Non-volatile random-access memory</a>
   7 (NVRAM) is random-access memory that retains its information when power is turned off (non-volatile).
   8 Systems with such memory can store data structures in RAM
   9 and retrieve them after a reboot, without flushing to typical _storage_.
  10
  11 Linux supports NVRAM file systems thanks to either
  12 <a href="http://pramfs.sourceforge.net/" class="ext">PRAMFS</a> or
  13 <a href="https://www.kernel.org/doc/Documentation/filesystems/dax.txt" class="ext">DAX</a>&nbsp;+&nbsp;<a href="http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html" class="ext">pmem</a>
  14 (requires Linux 4.1+).
  15
  16 This documentation does not describe how to operate such file systems;
  17 it is assumed that you have a working persistent memory file system.
  18
  19 When creating an LTTng tracing session, you can use the `--shm-path`
  20 option to specify the path of the shared memory holding the ring
  21 buffers. Specifying a location on an NVRAM file system makes it possible
  22 to retrieve the latest recorded trace data when the system reboots
  23 after a crash.
  24
  25 Example:
  26
  27 <pre class="term">
  28 lttng create <strong>--shm-path /path/to/shm</strong>
  29 </pre>
  30
  31 The binary layout of the ring buffer files is not exactly the same as
  32 the trace files layout. To view the events of ring buffer files after
  33 a system crash, use the `lttng-crash` utility:
  34
  35 <pre class="term">
  36 lttng-crash /path/to/shm
  37 </pre>
  38
  39 This extracts the trace data behind the scenes and runs
  40 <a href="http://diamon.org/babeltrace" class="ext"><code>babeltrace</code></a>
  41 to view the events. To extract the trace data to an LTTng trace without
  42 viewing the events, use the `--extract` option:
  43
  44 <pre class="term">
  45 lttng-crash <strong>--extract /path/to/trace</strong> /path/to/shm
  46 </pre>
  47
  48 See the <a href="http://localhost:9000/man/1/lttng-crash" class="ext"><code>lttng-crash</code> man page</a>
  49 for the complete list of options.