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Too many MySQL connections and MySQL server has gone away (14 posts)

  1. Kate
    Member
    Posted 1 year ago #

    We've been having a lot of issues with errors like "WordPress database error MySQL server has gone away for..." I included some examples below. We *think* this may be related to maxed out MySQL connections. We have a server of about 100+ WordPress sites running. Any ideas on how we can configure MySQL/the server to handle this better?

    mysql> show status like '%onn%';
    +--------------------------+---------+
    | Variable_name            | Value   |
    +--------------------------+---------+
    | Aborted_connects         | 835     |
    | Connections              | 3998523 |
    | Max_used_connections     | 533     |
    | Ssl_client_connects      | 0       |
    | Ssl_connect_renegotiates | 0       |
    | Ssl_finished_connects    | 0       |
    | Threads_connected        | 3       |
    +--------------------------+---------+
    7 rows in set (0.00 sec)
    
    [Wed May 29 13:21:44 2013] [error]  WordPress database error MySQL server has gone away for query SELECT ID\n\t\t\tFROM xy27abwp_posts\n\t\t\tWHERE guid = 'http://urlhere/wp-content/uploads/logo-MO2.png'\n\t\t\tLIMIT 1 made by require('wp-blog-header.php'), wp, WP->main, WP->send_headers, apply_filters('wp_headers'), call_user_func_array, UserAccessManager->redirect, UserAccessManager->getFile, UserAccessManager->_getFileSettingsByType, UserAccessManager->getPostIdByUrl, referer: http://urlhere.com/
    
    [Wed May 29 13:21:44 2013] [error] WordPress database error MySQL server has gone away for query SELECT ID, post_name, post_parent, post_type FROM xy27abwp_posts WHERE post_name IN ('wp-content','uploads','logo-mo2-png') AND (post_type = 'page' OR post_type = 'attachment') made by require('wp-blog-header.php'), wp, WP->main, WP->query_posts, WP_Query->query, WP_Query->get_posts, WP_Query->parse_query, get_page_by_path, referer: http://urlhere.com/
    
    [Wed May 29 13:25:44 2013] [error] WordPress database error MySQL server has gone away for query SELECT option_value FROM summer_options WHERE option_name = 'feedwordpress_diagnostics_show' LIMIT 1 made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/mytheme2010/courses.php'), get_footer, locate_template, load_template, require_once('/themes/mytheme2010/footer.php'), wp_footer, do_action('wp_footer'), call_user_func_array, debug_out_feedwordpress_footer, FeedWordPress::diagnostic_on, get_option, referer: http://urlhere.com/
  2. We *think* this may be related to maxed out MySQL connections

    It is.

    We have a server of about 100+ WordPress sites running. Any ideas on how we can configure MySQL/the server to handle this better?

    You need to dig into server admin and configure MySQL (via my.cnf) for high loads and apache (httpd,conf) for keep alive and number of servers started. You may also not have enough hardware - RAM and CPU - for what you're doing with over a hundred sites, depending on the amount of traffic you're getting and the kind of database-intensive plugins you're using, like Feed WordPress.

  3. Yui
    Member
    Posted 1 year ago #

    thus, try to decrease sql activity for wp's
    if you have access to them all, a good point will be using one of

    * Object cache (memcached)
    * DB Cache reloaded

    or W3 Total Cache, it has option to cache db to disk or use memcached

  4. Kate
    Member
    Posted 1 year ago #

    Thanks so much! We have a SE that does server config. I'll talk to him about the my.cnf and httpd.conf settings. Do you have any recommendations for resources to optimize these for high load WordPress?

    The server itself is pretty beefy I think, so I'm guessing it is more a configuration issue.

    $ top
    top - 16:50:29 up 5 days, 18:36, 5 users, load average: 0.40, 0.37, 0.31
    Tasks: 655 total, 1 running, 653 sleeping, 0 stopped, 1 zombie
    Cpu(s): 1.9%us, 0.2%sy, 0.0%ni, 97.9%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    Mem: 65923008k total, 30762620k used, 35160388k free, 299820k buffers
    Swap: 10239992k total, 0k used, 10239992k free, 27128324k cached

    Lulu, we don't have access to each install of WordPress (central IT office). Departments manage their own installs for the most part. We can however recommend a caching plugin to decrease the loads.

  5. @Kate: you've got plenty of RAM, and the CPU is not paging to disk, so that's good, so use mysqltuner.pl https://github.com/major/MySQLTuner-perl to configure the MySQL tuner.

    Clean out your post/page revisions, too. I've seen databases drop to 10% of their original size with a resulting huge increase in performance. Use a plugin to delete revisions or run in phpmyadmin:

    DELETE a,b,c
    FROM wp_posts a
    LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id)
    LEFT JOIN wp_postmeta c ON (a.ID = c.post_id)
    WHERE a.post_type = 'revision'

    @lulu said:

    * Object cache (memcached)
    * DB Cache reloaded

    Object caching is php, not directly related to the database server. And a WP plugin that tries to do DB caching is not nearly as efficient as setting caching in my.cnf.

  6. Kate
    Member
    Posted 1 year ago #

    Great - I'm on it!
    Thanks much to you and Lulu. Sorry I didn't get the Object cache reference. I rely on the systems engineers for all the server specific configs.

  7. Mike Bijon
    Member
    Posted 1 year ago #

    @sondogtech, your recommendations for optimizing the database are right on, but are you advocating for @Kate to not even consider an object cache or plugin?

    The best performance results should actually come from using all 3 options.

    DB Optimization
    @sondogtech's DELETE query will work, but it can lead to mistakes if you run manually. Instead grab @dd32's Revision Control plugin & it will limit revisions permanently: http://wordpress.org/plugins/revision-control/

    I'd recommend tools.percona.com to get you started too. I start with their configs all the time: https://gist.github.com/mbijon/4701099

    Caching Plugins
    Most caching plugins now create large sets of .htaccess rules if you have an Apache-based web server. Using .htaccess rules & cached static content the plugins prevent themselves or PHP from loading very often (static file serving is actually far more efficient than anything my.cnf can do). Grab W3 Total Cache or Super Cache for good general-purpose plugins that work well on Apache servers:
    * http://wordpress.org/plugins/w3-total-cache/
    * http://wordpress.org/plugins/wp-super-cache/

    (Note: Logged-in users and Admins will always skip the caches, so a site running Buddypress, a forum, or even P2 comments may not be that much faster with these plugins)

    Object Cache
    Object Caching both reduces your DB query-count and prevents large chunks of PHP that handle query results from running. In this manner it uses "some PHP" to run less PHP ... and can deliver a big speedup.

    Installing memcache or APC is pretty simple for most sysadmins & there are a ton of great articles about how to do it for WordPress.

    Hybrid Systems (aka: All of the Above)
    I recommend using all the above options, but unless you can be patient, start with plugin & object caching. DB tuning tends to be more time consuming, especially since the WP core team has done a great job of optimizing everything really well already.

    For an example of hybrid caching boosts, the Batcache plugin is the core of what they use on WordPress.com, and combines a page cache with object cache for about a 4,000% speedup (A site I use it on was hit by frontpage Reddit traffic last month & the 2CPU web server + 4 CPU DB server spiked over 800 requests/sec with page generation under 900ms the whole time).

    If you want something even faster, switch from Apache to nginx. It requires a lot more setup time, but enables microcaching by the webserver (which takes the DB & PHP out of the loop): http://markjaquith.wordpress.com/2012/05/15/how-i-built-have-baby-need-stuff/.

    Another WP core contributor wrote a great post about using nginx & Redis caching to serve pages in under 5ms, http://eamann.com/tech/ludicrous-speed-wordpress-caching-with-redis/. We run a very similar set at another client of mine & can handle about 8,000 req/sec across 5 severs. And I'd estimate that on a single dedicated server you can probably spike over 1,000 req/sec with a similar configuration.

  8. but are you advocating for @Kate to not even consider an object cache or plugin?

    Of course not. Read the OP:

    Any ideas on how we can configure MySQL/the server to handle this better?

    And her answer:

    We have a SE that does server config. I'll talk to him about the my.cnf and httpd.conf settings.

    I'll assume for this that their SE knows a little about server admin and APC, etc.

    And, you said:

    @sondogtech's DELETE query will work, but it can lead to mistakes if you run manually.

    Exactly how can it lead to "mistakes"?

  9. Yui
    Member
    Posted 1 year ago #

    >Object caching is php, not directly related to the database server.

    you meant opcode caching

    Object caching is related to query caching by using wordpress transients api (http://codex.wordpress.org/Transients_API) and can use
    Memcache (recommended for big servers)
    Xcache/APC/APCU (for small servers)
    via separate plugins (Tribe cache plugin has all-in-one collection)

  10. Mike Bijon
    Member
    Posted 1 year ago #

    @Kate, to pull this back out of the techie-vs-techie stuff, were any of the above posts helpful?

    ---

    As a 2nd-pass at helping:

    Disk I/O tends to be the first thing that limits WordPress site speed.

    Your top results "load average: 0.40, 0.37, 0.31" and "Mem... 35160388k free" show the server CPU and RAM aren't fully-used. To me that also indicates your disk I/O is the issue.

    The simplest caching setup to reduce disk usage is probably the Memcached object cache, http://wordpress.org/plugins/memcached/.

    Two of the other plugins mentioned (Tribe & W3 Total Cache) have that as part of their functionality. Any of the 3 should buffer your speed problem quickly, then you can proceed with more detailed analysis.

  11. Kate
    Member
    Posted 1 year ago #

    Thanks for checking in Mike. I trying to plan a meeting with the engineer to discuss changes to config based on these recommendations. I'm hoping to meet with him on Monday and will post what worked (or follow-up questions!) after we test out a few things. I really appreciate the awesome response.

  12. As a "3rd-pass" at helping:

    Mike said:

    I'd recommend tools.percona.com to get you started too. I start with their configs all the time:

    You're absolutely wrong. You can't start with a boilerplate my.cnf and hard values for RAM and caching because you have to consider your total RAM first, or your performance will go down because MySQL will run out of memory. Use https://github.com/major/MySQLTuner-perl to determine what you need, walk the values up and down and test.

    Disk I/O tends to be the first thing that limits WordPress site speed.

    Absolutely wrong again. Look at the disk swap. It's zero right now. What limits WP long before disk I/O is plain old RAM and CPU and database speed.

    What is Kate asking about? Not about memcaching or APC, but MySQL maxing out connections. That's MySQL configs.

    Yes, APC, etc will help, but if it's a box that's already handling 100 sites, the admin probably already has that configured. And configuring such additions to php is best done on the box and not by using a WP plugin.

    Kate: use phpinfo to see what you're got in terms of php configs (APC, etc) right now:
    short link to KB article at Media Temple: http://bit.ly/143q7s1

    And cache plugins help, of course, but they can't make up for server inefficiencies to begin with. WP Super Cache is far more reliable and easier to configure that W3TotalCache. Search these forums for feedback on both. Super Cache has 4.6+ million downloads; it's the best out there.

  13. Mike Bijon
    Member
    Posted 1 year ago #

    @sondogtech, swap usage and disk I/O are completely independent, http://unix.stackexchange.com/a/2663. But I'm not going to break down whatever it is you're posting. A good SE should see through the smoke.

    For @Kate and anyone else in need of performance tuning, skip phpinfo and grab the free trial of New Relic, http://www.newrelic.com. Should help you get your server tuned in well under the 30-day limit: https://newrelic.com/docs/server/using-the-server-monitor-ui#server-disks

  14. Frederick Townes
    Member
    Posted 1 year ago #

    @Kate, At the start, this is not an issue for a caching plugin. You need to analyze the slowest queries in your network of sites and tune MySQL or your Themes / Plugins to make them more efficient and cacheable by your query cache in MySQL. From there you can look at W3TC to help you with caching your object caching using memcached for example, which will work very well reducing database calls on your site and not have the same saturation issues as APC does. Obviously W3TC's page caching will be a large help. Please write us if you have issues.

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