pt-stalk - Collect forensic data about MySQL when problems occur.



pt-stalk [OPTIONS]

pt-stalk waits for a trigger condition to occur, then collects data to help diagnose problems. The tool is designed to run as a daemon with root privileges, so that you can diagnose intermittent problems that you cannot observe directly. You can also use it to execute a custom command, or to collect data on demand without waiting for the trigger to occur.


Percona Toolkit is mature, proven in the real world, and well tested, but all database tools can pose a risk to the system and the database server. Before using this tool, please:

  • Read the tool’s documentation
  • Review the tool’s known “BUGS”
  • Test the tool on a non-production server
  • Backup your production server and verify the backups


Sometimes a problem happens infrequently and for a short time, giving you no chance to see the system when it happens. How do you solve intermittent MySQL problems when you can’t observe them? That’s why pt-stalk exists. In addition to using it when there’s a known problem on your servers, it is a good idea to run pt-stalk all the time, even when you think nothing is wrong. You will appreciate the data it collects when a problem occurs, because problems such as MySQL lockups or spikes in activity typically leave no evidence to use in root cause analysis.

pt-stalk does two things: it watches a MySQL server and waits for a trigger condition to occur, and it collects diagnostic data when that trigger occurs. To avoid false-positives caused by short-lived problems, the trigger condition must be true at least --cycles times before a --collect is triggered.

To use pt-stalk effectively, you need to define a good trigger. A good trigger is sensitive enough to fire reliably when a problem occurs, so that you don’t miss a chance to solve problems. On the other hand, a good trigger isn’t prone to false positives, so you don’t gather information when the server is functioning normally.

The most reliable triggers for MySQL tend to be the number of connections to the server, and the number of queries running concurrently. These are available in the SHOW GLOBAL STATUS command as Threads_connected and Threads_running. Sometimes Threads_connected is not a reliable indicator of trouble, but Threads_running usually is. Your job, as the tool’s user, is to define an appropriate trigger condition for the tool. Choose carefully, because the quality of your results will depend on the trigger you choose.

You define the trigger with the --function, --variable, --threshold, and --cycles options. The default values for these options define a reasonable trigger, but you should adjust or change them to suite your particular system and needs.

By default, pt-stalk tool watches MySQL forever until the trigger occurs, then it collects diagnostic data for a while, and sleeps afterwards to avoid repeatedly collecting data if the trigger remains true. The general order of operations is:

while true; do
   if --variable from --function > --threshold; then
      if cycles_true >= --cycles; then
         if --collect; then
            if --disk-bytes-free and --disk-pct-free ok; then
               (--collect for --run-time seconds) &
            rm files in --dest older than --retention-time
      if iter < --iterations; then
         sleep --sleep seconds
      if iter < --iterations; then
         sleep --interval seconds
rm old --dest files older than --retention-time
if --collect process are still running; then
   wait up to --run-time * 3 seconds
   kill any remaining --collect processes

The diagnostic data is written to files whose names begin with a timestamp, so you can distinguish samples from each other in case the tool collects data multiple times. The pt-sift tool is designed to help you browse and analyze the resulting data samples.

Although this sounds simple enough, in practice there are a number of subtleties, such as detecting when the disk is beginning to fill up so that the tool doesn’t cause the server to run out of disk space. This tool handles these types of potential problems, so it’s a good idea to use this tool instead of writing something from scratch and possibly experiencing some of the hazards this tool is designed to avoid.


You can use standard Percona Toolkit configuration files to set command line options.

You will probably want to run the tool as a daemon and customize at least the --threshold. Here’s a sample configuration file for triggering when there are more than 20 queries running at once:


If you don’t run the tool as root, then you will need specify several options, such as --pid, --log, and --dest, else the tool will probably fail to start.



default: yes; negatable: yes

Collect diagnostic data when the trigger occurs. Specify --no-collect to make the tool watch the system but not collect data.

See also --stalk.


Collect GDB stacktraces. This is achieved by attaching to MySQL and printing stack traces from all threads. This will freeze the server for some period of time, ranging from a second or so to much longer on very busy systems with a lot of memory and many threads in the server. For this reason, it is disabled by default. However, if you are trying to diagnose a server stall or lockup, freezing the server causes no additional harm, and the stack traces can be vital for diagnosis.

In addition to freezing the server, there is also some risk of the server crashing or performing badly after GDB detaches from it.


Collect oprofile data. This is achieved by starting an oprofile session, letting it run for the collection time, and then stopping and saving the resulting profile data in the system’s default location. Please read your system’s oprofile documentation to learn more about this.


Collect strace data. This is achieved by attaching strace to the server, which will make it run very slowly until strace detaches. The same cautions apply as those listed in –collect-gdb. You should not enable this option together with –collect-gdb, because GDB and strace can’t attach to the server process simultaneously.


Collect tcpdump data. This option causes tcpdump to capture all traffic on all interfaces for the port on which MySQL is listening. You can later use pt-query-digest to decode the MySQL protocol and extract a log of query traffic from it.


type: string

Read this comma-separated list of config files. If specified, this must be the first option on the command line.


type: int; default: 5

How many times --variable must be greater than --threshold before triggering --collect. This helps prevent false positives, and makes the trigger condition less likely to fire when the problem recovers quickly.


Daemonize the tool. This causes the tool to fork into the background and log its output as specified in –log.


short form: -F; type: string

Only read mysql options from the given file. You must give an absolute pathname.


type: string; default: /var/lib/pt-stalk

Where to save diagnostic data from --collect. Each time the tool collects data, it writes to a new set of files, which are named with the current system timestamp.


type: size; default: 100M

Do not --collect if the disk has less than this much free space. This prevents the tool from filling up the disk with diagnostic data.

If the --dest directory contains a previously captured sample of data, the tool will measure its size and use that as an estimate of how much data is likely to be gathered this time, too. It will then be even more pessimistic, and will refuse to collect data unless the disk has enough free space to hold the sample and still have the desired amount of free space. For example, if you’d like 100MB of free space and the previous diagnostic sample consumed 100MB, the tool won’t collect any data unless the disk has 200MB free.

Valid size value suffixes are k, M, G, and T.


type: int; default: 5

Do not --collect if the disk has less than this percent free space. This prevents the tool from filling up the disk with diagnostic data.

This option works similarly to --disk-bytes-free but specifies a percentage margin of safety instead of a bytes margin of safety. The tool honors both options, and will not collect any data unless both margins are satisfied.


type: string; default: status

What to watch for the trigger. The default value watches SHOW GLOBAL STATUS, but you can also watch SHOW PROCESSLIST and specify a file with your own custom code. This function supplies the value of --variable, which is then compared against --threshold to see if the the trigger condition is met. Additional options may be required as well; see below. Possible values are:

  • status
Watch SHOW GLOBAL STATUS for the trigger. The value of --variable then defines which status counter is the trigger.
  • processlist

Watch SHOW FULL PROCESSLIST for the trigger. The trigger value is the count of processes whose --variable column matches the --match option. For example, to trigger --collect when more than 10 processes are in the “statistics” state, specify:

--function processlist \
--variable State       \
--match statistics     \
--threshold 10

In addition, you can specify a file that contains your custom trigger function, written in Unix shell script. This can be a wrapper that executes anything you wish. If the argument to --function is a file, then it takes precedence over built-in functions, so if there is a file in the working directory named “status” or “processlist” then the tool will use that file even though are valid built-in values.

The file works by providing a function called trg_plugin, and the tool simply sources the file and executes the function. For example, the file might contain:

trg_plugin() {
     | grep -c "has waited at"

This snippet will count the number of mutex waits inside InnoDB. It illustrates the general principle: the function must output a number, which is then compared to --threshold as usual. The $EXT_ARGV variable contains the MySQL options mentioned in the “SYNOPSIS” above.

The file should not alter the tool’s existing global variables. Prefix any file-specific global variables with “PLUGIN_” or make them local.


Print help and exit.


short form: -h; type: string

Host to connect to.


type: int; default: 1

How often to check the if trigger is true, in seconds.


type: int

How many times to --collect diagnostic data. By default, the tool runs forever and collects data every time the trigger occurs. Specify --iterations to collect data a limited number of times. This option is also useful with --no-stalk to collect data once and exit, for example.


type: string; default: /var/log/pt-stalk.log

Print all output to this file when daemonized.


type: string

The pattern to use when watching SHOW PROCESSLIST. See --function for details.


type: string

Send an email to these addresses for every --collect.


short form: -p; type: string

Password to use when connecting.


type: string; default: /var/run/

Create the given PID file. The tool won’t start if the PID file already exists and the PID it contains is different than the current PID. However, if the PID file exists and the PID it contains is no longer running, the tool will overwrite the PID file with the current PID. The PID file is removed automatically when the tool exits.


type: string

Load a plugin to hook into the tool and extend is functionality. The specified file does not need to be executable, nor does its first line need to be shebang line. It only needs to define one or more of these Bash functions:


Called before stalking.


Called when the trigger occurs, before running a --collect subprocesses in the background.


Called after running a collector process. The PID of the collector process is passed as the first argument. This hook is called before after_collect_sleep.


Called after sleeping --sleep seconds for the collector process to finish. This hook is called after after_collect.


Called after sleeping --interval seconds after each trigger check.


Called after stalking. Since pt-stalk stalks forever by default, this hook is only called if --iterations is specified.

For example, a very simple plugin that touches a file when --collect is triggered:

before_collect() {
   touch /tmp/foo

Since the plugin is completely sourced (imported) into the tool’s namespace, be careful not to define other functions or global variables that already exist in the tool. You should prefix all plugin-specific functions and global variables with plugin_ or PLUGIN_.

Plugins have access to all command line options but they should not modify them. Each option is a global variable like $OPT_DEST which corresponds to --dest. Therefore, the global variable for each command line option is OPT_ plus the option name in all caps with hyphens replaced by underscores.

Plugins can stop the tool by setting the global variable OKTORUN to 1. In this case, the global variable EXIT_REASON should also be set to indicate why the tool was stopped.

Plugin writers should keep in mind that the file destination prefix currently in use should be accessed through the $prefix variable, rather than $OPT_PREFIX.


short form: -P; type: int

Port number to use for connection.


type: string

The filename prefix for diagnostic samples. By default, all files created by the same --collect instance have a timestamp prefix based on the current local time, like 2011_12_06_14_02_02, which is December 6, 2011 at 14:02:02.


type: int; default: 30

Number of days to retain collected samples. Any samples that are older will be purged.


type: int; default: 30

How long to --collect diagnostic data when the trigger occurs. The value is in seconds and should not be longer than --sleep. It is usually not necessary to change this; if the default 30 seconds doesn’t collect enough data, running longer is not likely to help because the system or MySQL server is probably too busy to respond. In fact, in many cases a shorter collection period is appropriate.

This value is used two other times. After collecting, the collect subprocess will wait another --run-time seconds for its commands to finish. Some commands can take awhile if the system is running very slowly (which can likely be the case given that a collection was triggered). Since empty files are deleted, the extra wait gives commands time to finish and write their data. The value is potentially used again just before the tool exits to wait again for any collect subprocesses to finish. In most cases this won’t happen because of the aforementioned extra wait. If it happens, the tool will log “Waiting up to N seconds for subprocesses to finish...” where N is three times --run-time. In both cases, after waiting, the tool kills all of its subprocesses.


type: int; default: 300

How long to sleep after --collect. This prevents the tool from triggering continuously, which might be a problem if the collection process is intrusive. It also prevents filling up the disk or gathering too much data to analyze reasonably.


type: int; default: 1

How long to sleep between collection loop cycles. This is useful with --no-stalk to do long collections. For example, to collect data every minute for an hour, specify: --no-stalk --run-time 3600 --sleep-collect 60.


short form: -S; type: string

Socket file to use for connection.


default: yes; negatable: yes

Watch the server and wait for the trigger to occur. Specify --no-stalk to collect diagnostic data immediately, that is, without waiting for the trigger to occur. You probably also want to specify values for --interval, --iterations, and --sleep. For example, to immediately collect data for 1 minute then exit, specify:

--no-stalk --run-time 60 --iterations 1

--cycles, --daemonize, --log and --pid have no effect with --no-stalk. Safeguard options, like --disk-bytes-free and --disk-pct-free, are still respected.

See also --collect.


type: int; default: 25

The maximum acceptable value for --variable. --collect is triggered when the value of --variable is greater than --threshold for --cycles many times. Currently, there is no way to define a lower threshold to check for a --variable value that is too low.

See also --function.


short form: -u; type: string

User for login if not current user.


type: string; default: Threads_running

The variable to compare against --threshold. See also --function.


type: int; default: 2

Print more or less information while running. Since the tool is designed to be a long-running daemon, the default verbosity level only prints the most important information. If you run the tool interactively, you may want to use a higher verbosity level.

===== =====================================
0     Errors
1     Warnings
2     Matching triggers and collection info
3     Non-matching triggers

Print tool’s version and exit.


This tool does not require any environment variables for configuration, although it can be influenced to work differently by through several variables. Keep in mind that these are expert settings, and should not be used in most cases.

Specifically, the variables that can be set are:













For example, during collection iostat is called with a -dx argument, but because you have an NFS partition, you also need the -n flag there. Instead of editing the source, you can call pt-stalk as

CMD_IOSTAT="iostat -n" pt-stalk ...

which will do exactly what you need. Combined with the plugin hooks, this gives you a fine-grained control of what the tool does.


This tool requires Bash v3 or newer. Certain options require other programs:

--collect-gdb requires gdb

--collect-oprofile requires opcontrol and opreport

--collect-strace requires strace

--collect-tcpdump requires tcpdump


For a list of known bugs, see

Please report bugs at Include the following information in your bug report:

  • Complete command-line used to run the tool
  • Tool --version
  • MySQL version of all servers involved
  • Output from the tool including STDERR
  • Input files (log/dump/config files, etc.)

If possible, include debugging output by running the tool with PTDEBUG; see “ENVIRONMENT”.


Visit to download the latest release of Percona Toolkit. Or, get the latest release from the command line:




You can also get individual tools from the latest release:


Replace TOOL with the name of any tool.


Baron Schwartz, Justin Swanhart, Fernando Ipar, Daniel Nichter, and Brian Fraser


This tool is part of Percona Toolkit, a collection of advanced command-line tools for MySQL developed by Percona. Percona Toolkit was forked from two projects in June, 2011: Maatkit and Aspersa. Those projects were created by Baron Schwartz and primarily developed by him and Daniel Nichter. Visit to learn about other free, open-source software from Percona.


pt-stalk 2.2.6

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