MK-SLAVE-PREFETCH(1)

NAME
mk-slave-prefetch - Pipeline relay logs on a MySQL slave to pre-warm caches.

SYNOPSIS
 mk-slave-prefetch
 mk-slave-prefetch --statistics > /path/to/saved/statistics
 mk-slave-prefetch /path/to/saved/statistics

DESCRIPTION
mk-slave-prefetch reads the slave's relay log slightly ahead of where the slave's SQL thread is reading, converts statements into "SELECT", and executes them. In theory, this should help alleviate the effects of the slave's single-threaded SQL execution. It will help take advantage of multiple CPUs and disks by pre-reading the data from disk, so the data is already in the cache when the slave SQL thread executes the un-modified version of the statement.

Statements that can't be converted into "SELECT" are ignored. However, there is always a chance of bugs. It would be a very good idea to connect as a read-only user. Here is an example of how to grant the necessary privileges:

   GRANT SELECT, REPLICATION CLIENT, REPLICATION SLAVE ON *.*
   TO 'prefetch'@'%' IDENTIFIED BY 'sp33dmeup!';

"mk-slave-prefetch" learns how long it takes statements to execute, and doesn't try to execute those that take a very long time. You can ask it to print what it has learned after it executes. You can also specify a filename on the command line. The file should contain the statistics printed by a previous run. These will be used to pre-populate the statistics so it doesn't have to re-learn.

This program is based on concepts I heard Paul Tuckfield explain at the November 2006 MySQL Camp un-conference. However, the code is my own work. I have not seen any other implementation of Paul's idea.

DOES IT WORK?
Does it work? Does it actually speed up the slave?

That depends on your workload, hardware, and other factors. It might work when the following are true:

•   The slave's data is much larger than memory, and the workload is mostly randomly scattered small (single-row is ideal) changes.

•   There are lots of high-concurrency "UPDATE" and "DELETE" statements on the master.

•   The slave SQL thread is I/O-bound, but the slave overall has plenty of spare I/O capacity (definitely more than one disk spindle).

•   The slave uses InnoDB or another storage engine with row-level locking.

It does not speed up replication on my slaves, which mostly have large queries like "INSERT .. SELECT .. GROUP BY". In my benchmarks it seemed to make no difference at all, positive or negative.

On the wrong workload or slave configuration, this technique might actually make the slaves slower. Your mileage will vary.

User-contributed benchmarks are welcome.

DOWNLOADING
You can download Maatkit from Google Code at <http://code.google.com/p/maatkit/>, or you can get any of the tools easily with a command like the following:

   wget http://www.maatkit.org/get/toolname
   or
   wget http://www.maatkit.org/trunk/toolname

Where "toolname" can be replaced with the name (or fragment of a name) of any of the Maatkit tools. Once downloaded, they're ready to run; no installation is needed. The first URL gets the latest released version of the tool, and the second gets the latest trunk code from Subversion.

OPTIONS
Specify at least one of "--print", "--execute" or "--stop".

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

--charset
    short form: -A; type: string

    Default character set. If the value is utf8, sets Perl's binmode on STDOUT to utf8, passes the mysql_enable_utf8 option to DBD::mysql, and runs SET NAMES UTF8 after connecting to MySQL. Any other value sets binmode on STDOUT without the utf8 layer, and runs SET NAMES after connecting to MySQL.

--check-interval
    type: Array; default: 16,1,1024

    How often to check the slave: init,min,max. This many relay log events should pass before checking the output of "SHOW SLAVE STATUS". The syntax is a three-number range: initial, minimum, and maximum. You should be able to leave this at the defaults.

    "mk-slave-prefetch" varies the check interval in powers of two, depending on whether it decides the check was necessary.

--config
    type: Array

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

--daemonize
    Fork to the background and detach from the shell. POSIX operating systems only.

--database
    short form: -D; type: string

    The database to use for the connection. The initial connection will be to this database, but mk-slave-prefetch will issue "USE" statements as required by the binary log events.

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

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

--errors
    cumulative: yes

    Print queries that caused errors. If specified once, at exit; if twice, in realtime.

    If you specify this option once, you will see a report at the end of the script execution, showing the normalized queries and the number of times they were seen. If you specify this option twice, you will see the errors printed out as they occur, but no normalized report at the end of execution.

--[no]execute
    default: yes

    Execute the transformed queries to warm the caches.

--help
    Show help and exit.

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

    Connect to host.

--io-lag
    type: size; default: 1k

    How many bytes to lag the slave I/O thread. This helps avoid "mysqlbinlog" reading right off the end of the relay log file.

--log
    type: string

    Print all output to this file when daemonized.

--max-query-time
    type: float; default: 1

    Do not run queries longer than this many seconds; fractions allowed. If "mk-slave-prefetch" predicts the query will take longer to execute, it will skip the query. This is based on the theory that pre-warming the cache is most beneficial for short queries.

    "mk-slave-prefetch" learns how long queries require to execute. It keeps an average over the last "--query-sample-size" samples of each query. The averages are based on an abstracted version of the query, with specific parameters replaced by placeholders. The result is a sort of "fingerprint" for the query, not executable SQL. You can see the learned statistics with the "--statistics" option.

    You can pre-load query fingerprints, and average execution times, from a file. This way you don't have to wait for "mk-slave-prefetch" to learn all over every time you start it. Just specify the file on the command line. The format should be the same as the output from "--statistics".

    You might also want to filter out some statements completely, or let only some statements through. See the "--reject-regexp" and "--permit-regexp" options.

    If "mk-slave-prefetch" hasn't seen a query's fingerprint before, and thus doesn't know how long it will take to execute, it wraps it in a subuery, like this:

       SELECT 1 FROM ( <query> ) AS X LIMIT 1;

    This helps avoid fetching a lot of data back to the client when a query is very large. It requires a version of MySQL that supports subqueries (version 4.1 and newer). If yours doesn't, the subquery trick can't be used, so the query might fetch a lot of data back to the client.

    Once a query's fingerprint has been seen, so it's known that the query isn't enormously slow, "mk-slave-prefetch" just rewrites the "SELECT" list for efficiency. (Avoiding the subquery reduces the query's overhead for short queries). The rewritten query will then look like the following;

       SELECT ISNULL(COALESCE(<columns>)) FROM ...

--num-prefix
    Abstract away numeric table name prefixes. This causes the following two queries to "fingeprint" to the same thing:

      select from 1_2_users;
      select from 2_3_users;

--offset
    type: size; default: 128

    How many bytes "mk-slave-prefetch" will try to stay in front of the slave SQL thread. It will not execute log events it doesn't think are at least this far ahead of the SQL thread. See also "--window".

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

    Password to use when connecting.

--permit-regexp
    type: string

    Permit queries matching this Perl regexp. This is a filter for log events. The regular expression is matched against the raw log event, before any transformations are applied. If specified, this option will permit only log events matching the regular expression.

--pid
    type: string

    Create the given PID file when daemonized. The file contains the process ID of the daemonized instance. The PID file is removed when the daemonized instance exits. The program checks for the existence of the PID file when starting; if it exists and the process with the matching PID exists, the program exits.

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

    Port number to use for connection.

--print
    Print the transformed relay log events to standard output.

--print-nonrewritten
    Print queries that could not be transformed into "SELECT".

--progress
    type: int

    Print progress information every X events. The information is the current log file and position, plus a summary of the statistics gathered.

--query-sample-size
    type: int; default: 4

    Average query exec time over this many queries. The last "N" queries with a given fingerprint are averaged together to get the average query execution time (see "--max-query-time").

--reject-regexp
    type: string

    Reject queries matching this Perl regexp. Similar to "--permit-regexp", but has the opposite effect: log events must not match the regular expression.

--run-time
    type: time

    How long "mk-slave-prefetch" should run before exiting. The default is to run forever.

--sentinel
    type: string; default: /tmp/mk-slave-prefetch-sentinel

    Exit if this file exists.

--set-vars
    type: string; default: wait_timeout=10000

    Set these MySQL variables. Immediately after connecting to MySQL, this string will be appended to SET and executed.

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

    Socket file to use for connection.

--statistics
    Print execution statistics after exiting. The statistics are in two sections: counters, and queries. The counters simply count the number of times events occur. You may see the following counters:

       NAME                    MEANING
       ======================  =======================================
       mysqlbinlog             Executed mysqlbinlog to read log events.
       events                  The total number of relay log events.
       not_far_enough_ahead    An event was not at least --offset
                               bytes ahead of the SQL thread.
       too_far_ahead           An event was more than --offset
                               + --window bytes ahead of the SQL thread.
       too_close_to_io_thread  An event was less than --iolag bytes
                               away from the I/O thread's position.
       event_not_allowed       An event wasn't a SET, USE, INSERT,
                               UPDATE, DELETE or REPLACE query.
       event_filtered_out      An event was filtered out because of
                               --permitregexp or --rejectregexp.
       same_timestamp          A SET TIMESTAMP event was ignored because
                               it had the same timestamp as the last one.
       do_query                A transformed event was executed
                               or printed.
       query_error             An executed query had an error.
       query_too_long          An event was not executed because its
                               average query length exceeded
                               --maxquerytime.
       query_not_rewritten     A query could not be rewritten to a
                               SELECT.
       master_pos_wait         The tool waited for the SQL thread to
                               catch up.
       show_slave_status       The tool queried SHOW SLAVE STATUS.
       load_data_infile        The tool found a LOAD DATA INFILE query
                               and unlinked (deleted) the temp file.
       could_not_unlink        The tool failed to unlink a temp file.
       sleep                   The tool slept for a second because the
                               slave's SQL thread was not running, or
                               because it read past the end of the log.

    After the counters, "mk-slave-prefetch" prints information about each query fingerprint it has seen, two lines per fingerprint. The first line contains the query's fingerprint. The second line contains the number of times the fingerprint was seen, number of times executed, the sum of the execution times, and the average execution time over the last "--query-sample-size" samples.

--stop
    Stop running instances by creating the "--sentinel" file.

--tmpdir
    type: string; default: /dev/null

    Where to create temp files for "LOAD DATA INFILE" queries. The default will cause "mysqlbinlog" to skip the file and the associated "LOAD DATA INFILE" command entirely.

    If "mk-slave-prefetch" sees a "LOAD DATA INFILE" command (which it won't, if this is left at the default), it will try to remove the temporary file, then skip the event.

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

    User for login if not current user.

--version
    Show version and exit.

--window
    type: size; default: 4k

    The max bytes ahead of the slave "mk-slave-prefetch" should get. Defines the window within which "mk-slave-prefetch" considers a query OK to execute. The window begins at the slave SQL thread's last known position plus "--offset" bytes, and extends for the specified number of bytes.

    If "mk-slave-prefetch" sees a log event that is too far in the future, it will increment the "too_far_ahead" counter and wait for the slave SQL thread to catch up (which increments the "master_pos_wait" counter). If an event isn't far enough ahead of the SQL thread, it will be discarded and the "not_far_enough_ahead" counter increments.

    Watching the mentioned statistics can help you understand how to tune the window. You want "mk-slave-prefetch" to run just ahead of the SQL thread, not throwing out a lot of events for being too far ahead or not far enough ahead.

SYSTEM REQUIREMENTS
You need Perl, DBI, DBD::mysql, and some core packages that ought to be installed in any reasonably new version of Perl.

ENVIRONMENT
The environment variable "MKDEBUG" enables verbose debugging output in all of the Maatkit tools:

   MKDEBUG=1 mk-....

BUGS
Please use Google Code Issues and Groups to report bugs or request support: <http://code.google.com/p/maatkit/>. You can also join #maatkit on Freenode to discuss Maatkit.

Please include the complete command-line used to reproduce the problem you are seeing, the version of all MySQL servers involved, the complete output of the tool when run with "--version", and if possible, debugging output produced by running with the "MKDEBUG=1" environment variable.

COPYRIGHT, LICENSE AND WARRANTY
This program is copyright 2007-2009 Baron Schwartz. Feedback and improvements are welcome.

THIS PROGRAM IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2; OR the Perl Artistic License. On UNIX and similar systems, you can issue `man perlgpl' or `man perlartistic' to read these licenses.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.

AUTHOR
Baron Schwartz.

VERSION
This manual page documents Ver 1.0.9 Distrib 4047 $Revision: 4045 $.