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I have a WordPress site with well over 10k posts, and things are starting to get very slow whenever I am adding and editing posts. Pages load nice and fast for users, along with the admin lists of posts, but it is when writes or updates occur the server goes to 100% CPU and takes a long time (sometimes longer than PHP's timeout of 60s).

I am thinking that this is likely to do with the table level locking of MyISAM, and am thinking of switching this to InnoDB. What are the implications of doing this?

Some stats:

select  - per hour ~22k
update  - per hour ~7.6k
set option  - per hour ~7k

I know there are lots of other optimizations I can make, but my feelings are that this might have the biggest impact.


Edit: I have found one of the major problems causing the slowness, it was YARPP (Yet Another Related Posts Plugin) that was regenerating the "relatedness" each time, and this seemed to be due to the 2k+ tags we have. I turned off the "consider tags" option and it has sped up considerably.

Also, other plugins that regenerate things can cause these kind of issues, such as some XML sitemap plugins.

So, my immediate issue is resolved, although I would still love to hear a good answer to InnoDB vs MyISAM for Wordpress!

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2 Answers

InnoDB probably won't help you - page/row level locking helps mitigate contention, but it doesn't feel like that's your issue.

There's a lot of stuff out there that suggests MyISAM is slower than InnoDB in the average blog scenario (many more reads than writes).

Before making a switch, you should at least do the following

  • run mysqltuner which will give you some configuration advice (it's not infallible or all knowing though)
  • turn on slow query logging, leave it for a day or so, and then start sifting through the log, and EXPLAINing the queries to see what's going on

From personal experience, I found that adding an index to an unindexed field on wp_comments helped massively in my particular situation (periods of bursty commenting, where 10 or so people could be trying to comment at the same time), and it's possible that finding out what queries are running slowly and why may lead you to a better understanding of the problem, and a REAL solution!

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I would indeed switch to InnoDB. Table locking/row locking has long been discussed by many. I would always choose InnoDB hands down. However, there is another profound reason for choosing InnoDB...CACHING.

While most people boast that MyISAM is faster for reads, most people forget that the many cache for MyISAM, which is called the key cache (set by key_buffer_size), only caches index pages from .MYI files. It never caches data pages. It has an official maximum of 4GB in 32-bit Systems. 8GB is best maximum for 64-bit.

The InnoDB Buffer Pool caches the data and index pages. Depending on the server your have, you can cache up to the entire dataset in RAM. You can tune InnoDB for up to 80% RAM and 10% for DB Conenctions, and leave 10% for the OS. This is true even for different operating systems.

I have recommended these things for Drupal customers with marvelous success. It applies to Wordpress just as well. I have provided DB support for clients with WordPress. Same improvements.

You can always configure memory for InnoDB more effectively that you can more MyISAM. There is always a way to tweek InnoDB to suit your performance needs. As your data grows, it will eventually become a requirement.

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