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Slides from PLMCE 2014 breakout session

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As many of you already know, PLMCE is an annual MySQL
community conference and Expo organized by Percona in the month of April
(usually). It is a great conference, not only to meet new and eminent people in
MySQL and related database fields, but also to attend interesting talks, and
also to give some.

This year I spoke about synchronous replication at a higher level. The talk was
titled “ACIDic Clusters: Review of current relational databases with synchronous replication”. Having previously given talks with boring titles (but interesting content), this time I decided to go with an interesting title, and it seemed to fit well with topic being discussed.
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My twitter setup

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I have been using Twitter for a while under the handle randomsurfer.
I tend to use web interface sometimes, but regularly I use the command-line/ncurses interface.

There are two main clients that I use regularly. One is ttytter
which is a nice command-line client which offers advanced functionality such as
creating/editing lists and scripting. Refer to the linked page for more scoop. I
would say ttytter is one of the best implementations of Twitter API. It allows
for advanced scripting like:

The above invocation can be used to get the list of everyone you follow and
populate the configuration file to allow auto-completion at ttytter prompt. You
can also run ttytter as a daemon.
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Annoying access keys on web pages

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HTML access keys were introduced to improve web accessibility and
they still seem to serve that purpose. Wikipedia defines them
as “In a web browser, an access key or accesskey allows a computer user to
immediately jump to a specific part of a web page via the keyboard.” If you
view hover over the link, it should show up in tooltip; for a wikipedia page,
for instance, the ‘view history’ is mapped to ‘alt-shift-h’ by default. In HTML
source, you can see them as accesskey attribute.

Having said that, they can be really annoying many a time. This is true
primarily when you have bound that access key to a mapping in the browser or in
a plugin or even to a global binding (say, with xbindkeys). What makes it even
more worse is that on almost every site it is impossible to disable them.
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MySQL file limit, table cache and max_connections

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MySQL variables open_files_limit, table_open_cache and max_connections are
inter-related, and this is for obvious reasons: all deal with file descriptors
one way or another.

If one of the value is provided but others are left out, mysqld calculates
others using a formula and in some cases, emits a warning if not possible.

The whole calculation behind obtaining the final file descriptor limit is a bit
byzantine and is as follows (for Linux):

EDIT: This applies to MySQL 5.5, in 5.6, as Daniël in comments pointed out,
few things have changed, check comment for details. I will probably make a
followup post on the differences.

  1. First it tries to calculate a minimum requirement based on max_connections
    and table_open_cache. As for other variables, if it is not provided in cnf or
    on command line it uses defaults.

    <br />    /* MyISAM requires two file handles per table. */
    wanted_files= 10+max_connections+table_cache_size*2;

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