From: David Goulet Date: Wed, 11 Sep 2013 15:08:51 +0000 (-0400) Subject: Add original snapshot proposal to documentation X-Git-Tag: v2.4.0-rc1~155 X-Git-Url: https://git.lttng.org/?p=lttng-tools.git;a=commitdiff_plain;h=91eb2ca3237630b5feca7d5fddbe7c58d3325f9c Add original snapshot proposal to documentation Signed-off-by: David Goulet --- diff --git a/doc/proposals/0006-lttng-snapshot.txt b/doc/proposals/0006-lttng-snapshot.txt new file mode 100644 index 000000000..399fcca2e --- /dev/null +++ b/doc/proposals/0006-lttng-snapshot.txt @@ -0,0 +1,164 @@ +RFC - LTTng snapshot + +Author: David Goulet + +Version: + - v0.1: 11/04/2013 + * Initial proposal + +Motivation +---------- + +This proposal is for the snapshot feature in lttng-tools. The idea is to be +able to snapshot a portion of the trace and write it to a specified output +(disk or network). This could be particularly useful in flight recorder mode +where you detect a problem on your system for instance and then use this +snapshot feature to save the latest buffers which should contain information to +help understand the issue. + +Requirements +----------------- + +In order to snapshot a session, it must be set in flight recorder mode meaning +that there is *no* consumer extracting the trace and writing it to a +destination. To do that, the --no-output option is added to "lttng create" +command. + + $ lttng create --no-output + Create a session with active tracing but no data being collected. + + For the API call lttng_create_session(), simply set the URL to NULL. + +Furthermore, by default, this command will set all subsequent channel in +overwrite mode. You can force the discard value (overwrite=0) but it is a bit +pointless since the snapshot does NOT remove the data from the buffers. + +Proposed Solution +----------------- + +First, the new lttng command line UI is presented followed by the new API +calls. + +This new command uses a git-alike UI, but in the form of the object first +followed by the desired action, whereas other commands only use actions such as +"enable-event". + + $ lttng snapshot [ACTION] [OPTIONS] + + ACTION: (detailed below) + +The user can specify an output destination either for the current session +(being the default) or on the spot when the command is executed. To specify an +output for the session, use the "add-output" action. + + $ lttng snapshot add-output [URL] [OPTIONS] + + OPTIONS: + -s, --session NAME + -C, --ctrl-url URL + -D, --data-url URL + -a, --alias ALIAS Name of the output in the session. + -m, --max-size SIZE Maximum bytes size of the snapshot. + +Without the -s, the current session is used. The above command adds an output +location to the session so when a snapshot is recorded later on, it's sent +there. This action command takes either a valid lttng URL (see proposal 0004) +or the -C/-D options from "lttng create" can be used to define relayd on +different ports. The alias option can be used to give a name to the output so +it's recognizable in the list command and can also be used with the del +command. + +Following that, two new actions are available to control outputs. You can list +and delete outputs. + + $ lttng snapshot list-output + [1]: file://[...] + [2]: net://1.1.1.1:8762:9123 + [3] - ALIAS: file://[...] + + $ lttng snapshot del-output ID|ALIAS + + The output identified by the ID or alias is removed from the session. In + this case the ID is the number returned by list-output (e.g.: 2). + +To specify an output destination on the spot when the snapshot is taken, use +the record action. + + $ lttng snapshot record [URL] [OPTIONS] + + OPTIONS: + -s, --session NAME Session name + -n, --name NAME Name of the snapshot to recognize it afterwards. + -m, --max-size SIZE Maximum bytes size of the snapshot. + -C, --ctrl-url URL + -D, --data-url URL + +No URL means that the default output of the session is used. The max-size is +the maximum size of the trace you want to snapshot. The name is used so you can +recognize the snapshot once taken and written on disk. Finally, the -s let the +user specify a session name or else the current session is used (in .lttngrc). + +Finally, we allow the snapshot command to be used without an action which +basically do "lttng snapshot record". + + $ lttng snapshot [OPTIONS] + + OPTIONS: + -s, --session NAME + -m, --max-size SIZE Maximum bytes size of the snapshot. + + $ lttng snapshot -s mysession -m 8192 + + Snapshot the session and puts it in the session define output directory. + +By default, the snapshot(s) are saved in the session directory in the snapshot/ directory. + + SESSION_DIR/snapshot/--