pt-stalkp - Online in the Cloud

PROGRAM:

NAME

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

SYNOPSIS

Usage: 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.

RISKS

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

DESCRIPTION

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
cycles_true++
if cycles_true >= --cycles; then
--notify-by-email
if --collect; then
if --disk-bytes-free and --disk-pct-free ok; then
(--collect for --run-time seconds) &
fi
rm files in --dest older than --retention-time
fi
iter++
cycles_true=0
fi
if iter < --iterations; then
sleep --sleep seconds
else
break
fi
else
if iter < --iterations; then
sleep --interval seconds
else
break
fi
fi
done
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
fi

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.

CONFIGURING

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:

daemonize
threshold=20

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.

OPTIONS

--ask-pass
Prompt for a password when connecting to MySQL.

--collect
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
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
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
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
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.

--config
type: string

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

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

--defaults-file
short form: -F; type: string

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

--dest
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.

--disk-bytes-free
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.

--disk-pct-free
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.

--function
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() {
mysql $EXT_ARGV -e "SHOW ENGINE INNODB STATUS" \
| 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.

--help
Print help and exit.

--host
short form: -h; type: string

Host to connect to.

--interval
type: int; default: 1

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

--iterations
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.

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

Print all output to this file when daemonized.

--match
type: string

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

--notify-by-email
type: string

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

--password
short form: -p; type: string

Password to use when connecting. If password contains commas they must be escaped
with a backslash: "exam\,ple"

--pid
type: string; default: /var/run/pt-stalk.pid

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.

--plugin
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:

before_stalk
Called before stalking.

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

after_collect
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".

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

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

after_stalk
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.

--port
short form: -P; type: int

Port number to use for connection.

--prefix
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.

--retention-time
type: int; default: 30

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

--run-time
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.

--sleep
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.

--sleep-collect
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".

--socket
short form: -S; type: string

Socket file to use for connection.

--stalk
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".

--threshold
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".

--user
short form: -u; type: string

User for login if not current user.

--variable
type: string; default: Threads_running

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

--verbose
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.

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

--version
Print tool's version and exit.

ENVIRONMENT

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:

CMD_GDB
CMD_IOSTAT
CMD_MPSTAT
CMD_MYSQL
CMD_MYSQLADMIN
CMD_OPCONTROL
CMD_OPREPORT
CMD_PMAP
CMD_STRACE
CMD_SYSCTL
CMD_TCPDUMP
CMD_VMSTAT

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.

SYSTEM REQUIREMENTS

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"

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