lttng-snapshot - Man Page

Take a snapshot of an LTTng recording session

Synopsis

Take a recording session snapshot:

lttng [GENERAL OPTIONS] snapshot record [--max-size=SIZE]
      [--name=NAME] [--session=SESSION]
      [--ctrl-url=URL --data-url=URL | URL]

Add a snapshot output to a recording session:

lttng [GENERAL OPTIONS] snapshot add-output [--max-size=SIZE]
      [--name=NAME] [--session=SESSION]
      (--ctrl-url=URL --data-url=URL | URL)

Show the snapshot output of a recording session:

lttng [GENERAL OPTIONS] snapshot list-output [--session=SESSION]

Remove the snapshot output from a recording session:

lttng [GENERAL OPTIONS] snapshot del-output [--session=SESSION] 1

Description

The lttng snapshot command can take a snapshot of, add or remove a snapshot output, and show the snapshot output of:

With the --session=SESSION option

The recording session named SESSION.

Without the --session option

The current recording session (see lttng-concepts(7) to learn more about the current recording session).

See lttng-concepts(7) to learn more about recording sessions.

A snapshot is a dump of the current sub-buffers of all the channels of the selected recording session.

When LTTng takes a snapshot, it sends the sub-buffer dump of the selected recording session to the local file system or over the network to a listening relay daemon (lttng-relayd(8)). See the “Snapshot output” section below to learn more.

When LTTng takes a snapshot, it does NOT clear the sub-buffers of the selected recording session. In other words, different snapshots of the selected recording session can contain the same event records.

You must have created the selected recording session in snapshot mode (see the --snapshot option of the lttng-create(1) command as well as lttng-concepts(7) to learn more about recording session modes) to use the snapshot command.

A snapshot-session trigger action can also take a recording session snapshot (see lttng-add-trigger(1)).

See the “Examples” section below for usage examples.

If you want, instead, to keep all the trace data, but divide it into archived chunks which are then, like snapshots, ready to be processed, see the recording session rotation feature in lttng-concepts(7). Trace chunk archives do NOT overlap like snapshots can.
Note

Before you take a snapshot on a system with a high event throughput, the LTTng project recommends that you first run the lttng-stop(1) command. Otherwise, the snapshot could contain “holes”, the result of the tracers overwriting unconsumed trace packets during the snapshot operation.

After LTTng writes the snapshot trace data, you can restart the recording session with the lttng-start(1) command.

Snapshot output

When you take a recording session snapshot with the record action, LTTng writes the snapshot trace files to:

If you specify the URL non-option argument or the --ctrl-url and --data-url options

The output defined by the URL non-option argument or by the arguments of the options.

See lttng-create(1) for the format of URL.

Otherwise

The snapshot output of the selected recording session.

Add a snapshot output to a recording session with the add-output action. As of LTTng 2.13.7, you may only add one snapshot output to a given recording session.

When you create a snapshot mode recording session with the --snapshot option of the lttng-create(1) command, and without its --no-output option, the create command automatically adds a snapshot output named snapshot-1 to the created recording session:

With its --output, --set-url, --ctrl-url, or --data-url options

Equivalent to using the add-output action with the provided or equivalent URL(s) immediately after creating the recording session.

Otherwise

A subdirectory, under the $LTTNG_HOME/lttng-traces ($LTTNG_HOME defaults to $HOME) directory, of which the name contains the recording session name and the date/time.

Show the current snapshot output of a recording session with the list-output action.

Remove the snapshot output of a recording session with the del-output action.

For both the record and add-output actions:

  • Assign a name to a snapshot output with the --name=NAME option.

    NAME becomes part of the snapshot trace file names which LTTng sends to this output.

  • By default, the snapshot files can be as big as the sum of the sizes of all the sub-buffers of all the channels of the selected recording session.

    Set the maximum total size of all the snapshot trace files LTTng writes with the --max-size option.

Options

See lttng(1) for GENERAL OPTIONS.

Recording target

-s SESSION, --session=SESSION

Take a snapshot of the sub-buffers of the recording session named SESSION instead of the current recording session.

Output

See the “Snapshot output” section above.

-C URL, --ctrl-url=URL

Set the control path URL to URL.

You must also use the --data-url option.

See lttng-create(1) for the format of URL.

-D URL, --data-url=URL

Set the trace data path URL to URL.

You must also use the --ctrl-url option.

See lttng-create(1) for the format of URL.

-m SIZE, --max-size=SIZE

Set the maximum total size of all the snapshot trace files LTTng writes when taking a snapshot to SIZE bytes.

The k (KiB), M (MiB), and G (GiB) suffixes are supported.

-n NAME, --name=NAME

Assign the name NAME to the snapshot output.

Program information

-h,  --help

Show help.

This option attempts to launch /usr/bin/man to view this manual page. Override the manual pager path with the LTTNG_MAN_BIN_PATH environment variable.

--list-options

List available command options and quit.

Exit Status

0

Success

1

Command error

2

Undefined command

3

Fatal error

4

Command warning (something went wrong during the command)

Environment

LTTNG_ABORT_ON_ERROR

Set to 1 to abort the process after the first error is encountered.

LTTNG_HOME

Path to the LTTng home directory.

Defaults to $HOME.

Useful when the Unix user running the commands has a non-writable home directory.

LTTNG_MAN_BIN_PATH

Absolute path to the manual pager to use to read the LTTng command-line help (with lttng-help(1) or with the --help option) instead of /usr/bin/man.

LTTNG_SESSION_CONFIG_XSD_PATH

Path to the directory containing the session.xsd recording session configuration XML schema.

LTTNG_SESSIOND_PATH

Absolute path to the LTTng session daemon binary (see lttng-sessiond(8)) to spawn from the lttng-create(1) command.

The --sessiond-path general option overrides this environment variable.

Files

$LTTNG_HOME/.lttngrc

Unix user’s LTTng runtime configuration.

This is where LTTng stores the name of the Unix user’s current recording session between executions of lttng(1). lttng-create(1) and lttng-set-session(1) set the current recording session.

$LTTNG_HOME/lttng-traces

Default output directory of LTTng traces in local and snapshot modes.

Override this path with the --output option of the lttng-create(1) command.

$LTTNG_HOME/.lttng

Unix user’s LTTng runtime and configuration directory.

$LTTNG_HOME/.lttng/sessions

Default directory containing the Unix user’s saved recording session configurations (see lttng-save(1) and lttng-load(1)).

/usr/local/etc/lttng/sessions

Directory containing the system-wide saved recording session configurations (see lttng-save(1) and lttng-load(1)).

Note

$LTTNG_HOME defaults to the value of the HOME environment variable.

Examples

Example 1. Take a snapshot of the current recording session, sending the trace files to its snapshot output.

$ lttng snapshot record

Example 2. Take a snapshot of a specific recording session, giving it a custom name.

See the --session and --name options.

$ lttng snapshot record --session=my-session --name=SNAP

Example 3. Take a snapshot of the current recording session, sending the trace files to a custom location.

See the “Output directory” section of lttng-relayd(8) to understand where the relay daemon to connect to (3.96.87.215) writes the received traces.

$ lttng snapshot record net://3.96.87.215

Example 4. Replace the snapshot output of a specific recording session with a local file system path.

See the --session option.

$ lttng snapshot del-output --session=monk 1
$ lttng snapshot add-output --session=monk \
                 file:///path/to/snapshots

Example 5. Take a snapshot of the current recording session, making sure its size is not over 4 MiB.

See the --max-size option.

$ lttng snapshot record --max-size=4M

Resources

Thanks

Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal for the LTTng journey.

Also thanks to the Ericsson teams working on tracing which helped us greatly with detailed bug reports and unusual test cases.

See Also

lttng(1), lttng-create(1), lttng-concepts(7)

Referenced By

lttng(1), lttng-add-trigger(1), lttng-concepts(7), lttng-create(1), lttng-rotate(1).

14 June 2021 LTTng 2.13.7 LTTng Manual