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MySQL Forks and Patches
Features
this evaluation) shows that read access
to the images table is particularly
frequent. The next step would be for
the database user to experiment with
code optimization or other changes to
the table format to reduce access incidence
and save time. The relatively
write-intensive movies table has a
suspiciously high write-access count
and, at the same time, a large number
of index updates.
Because the statistics can be reset
easily with FLUSH TABLE_STATISTICS,
interval-based evaluation by means of
a Munin plugin that you write yourself,
or some similar method, would
be your best bet. Retrospectively, you
could investigate load peaks in relation
to table access and modification.
New Functions
New functions in the MySQL kernel
give administrators additional options
so that maintenance of the MySQL
server is more secure and convenient.
A typical task is to stop MySQL
processes with the KILL command.
Under load, you might see a process
listed as Idle by SHOW PROCESSLIST
start to handle a new query just at the
moment you kill it. The “Kill if Idle”
patch adds an option to kill a process
only if it is doing nothing: KILL IF_
IDLE Process_Id. This saves you the
embarrassment of accidentally killing
a process while it is handling a query.
LVM and ZFS snapshots are commonly
regarded as the simplest
methods for backing up an InnoDB
database on the fly without interrupting
operations. If this method is
not an option for you and you are
forced to rely on a legacy dump file,
you need to make sure that the data
on your MySQL server do not change
while a file is being dumped. The
legacy approach to doing this is FLUSH
TABLES WITH READ LOCK. However, this
might not be sufficient in the InnoDB
case because background processes
also write to the database. The InnoDB
Freeze patch executes SET
GLOBAL innodb_disallow_writes = 1
and then freezes all processes that
write InnoDB data so you can create
a backup. Afterward, SET GLOBAL in‐
nodb_disallow_writes = 0 disables
the freeze.
Performance Enhancements
Thanks to its support for transactions
and line-based locking, InnoDB has
developed into a modern alternative
for the now fairly ancient MyISAM
engine. Despite performance gains
in write access thanks to line-based
locking, the overhead for supporting
transactions, foreign keys, and
other functions (even if you don’t
use them) costs valuable CPU cycles
and hardware I/​O resources. MySQL
systems under heavy load thus need
a perfectly configured and powerful
InnoDB engine.
A variety of performance-boosting
patches are available for the version
of InnoDB that ships with MySQLM;
some of them are included in the
OurDelta version. One that is worthy
of mention is a reworked RW lock
that improves locking behavior on
multi-processor systems in particular.
A description of all the improvements
is beyond the scope of this article, but
one thing is clear; patches for InnoDB
exist that retain compatibility and
offer transparent optimization of the
engine. Typically, it is difficult to measure
the performance gain in an objective
way because the performance
of the MySQL server will depend to
a great extent on the hardware, configuration,
and data it uses. The most
exhaustive and reliable source is the
MySQL Performance Blog [4], which
regularly publishes test results.
Improvements by selectively installing
patches for the legacy InnoDB engine
are regarded as a fairly conservative
approach. The use of an alternative
Listing 1: Userstats Patch in Action
database engine holds more promise.
The InnoDB plug in and Percona
XtraDB engines are becoming increasingly
widespread.
The InnoDB Plugin
The InnoBase InnoDB plugin is an
ongoing development of the InnoDB
engine that ships with MySQL [6].
Improvements include general optimization
of CPU load and I/​O access, a
faster locking mechanism, extended
configuration and reporting options,
and optional table compression.
MySQL 5.1 introduced the option of
unloading the standard engine and
replacing it with a different version.
Starting with MySQL 5.1.38, MySQL
additionally supplies the InnoDB
plugin. As MySQL describes this as
a release candidate, administrators
do need to enable it manually. The
official MySQL documentation describes
the steps required to do so
[7]. To benefit from the combination
of distribution updates for the MySQL
kernel and the latest functions and
optimizations of the InnoDB plugin, it
is a good idea to install the latest InnoDB
plugin version from the InnoDB
website.
Ubuntu 10.04 comes with MySQL
server version 5.1.41. The /usr/lib/
mysql/plugin/ houses an InnoDB plugin
version 1.0.4 that is disabled by
default. The InnoDB website has the
current version, 1.0.6, which you can
download and unpack. Then copy
ha_innodb.so to the /usr/lib/mysql/
plugin/ directory. Because Ubuntu
uses AppArmor to protect services by
default, you need to disable or modify
AppArmor to let you load content
from the plugin directory by adding
mysql> select * from information_schema.TABLE_STATISTICS ORDER BY ROWS_READ DESC LIMIT 0,5;
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| TABLE_SCHEMA | TABLE_NAME | ROWS_READ | ROWS_CHANGED | ROWS_CHANGED_X_INDEXES |
+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+
| moviepilot | images | 13138219791 | 14778 | 118224 |
| moviepilot | events | 3957858216 | 59964 | 359784 |
| moviepilot | comments | 2650553183 | 3408 | 20448 |
| moviepilot | movies | 2013076357 | 598505 | 7780565 |
| omdb | log_entries | 1106683022 | 2737 | 5474 |
+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐+‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐+
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