How to track down a Drupal max_allowed_packet error?

One of my staging sites has recently started spewing huge errors on every admin page along the lines of:

User warning: Got a packet bigger than 'max_allowed_packet' bytes query: UPDATE cache_update SET data = ' ... ', created = 1298434692, expire = 1298438292, serialized = 1 WHERE cid = 'update_project_data' in _db_query() (line 141 of /var/www/vhosts/mysite/mypath/includes/database.mysqli.inc). (where "..." is about 1.5 million characters worth of serialized data)

How should I go about tracking down where the error originates? Would adding开发者_如何转开发 debugging code to _db_query do any good, since it gets called so much?

No need to track this down because you can't fix it I think.

This is the cache from update.module, containing information about which modules have updated versions and so on. So this is coming from one of the "_update_cache_set()" calls in that module.

Based on a wild guess, I'd say it is the one in this function: http://api.drupal.org/api/drupal/modules--update--update.fetch.inc/function/_update_refresh/6

It is basically building up an huge array with information about all projects on your site and tries to store it as a single, serialized value.

How many modules do you have installed on this site?

I can think of three ways to "fix" this error:

• Increase the max_allowed_packet size. (max_allowed_packet setting in my.conf)
• Disable update.module (It's not that useful on a staging/production site anyway, when you need to update on a dev site first anyway)
• Disable some modules ;)

I had a similar error and went round and round for about an hour.

Increased memory limit to 512m and still had the issue. And figured that was enough. So went looking elsewhere.

I cleared the caches with drush, still the error, and then looked at the database tables.

I noticed that all the cache tables were cleared except cache_update. I truncated this table and bam, everything was working normally.

Before I got the memory limit error, I got a max_input_vars error since I am on PHP5.4. But this question and answer led me to this fix. Not quite sure how or why it worked, but it did.