October 9th, 2007

WP-Cache Bug Strikes Back: Site Down 17 Hours

In case you visited the site a few hours back, it was down with internal server errors. Unlike last time when the site was down 36 hours, this time I knew what the problem was and as soon as I disabled Wp-cache, the site was instantly back online…

QOT server errors

See in the image above how the site downtime occured. I had recently upgraded to the new wordpress 2.3 and decided to give wp-cache another try forgetting the site downtimes issues it had caused the previous time. The site worked fine for the last 2 days, and I thought that wp-cache and wordpress 2.3 finally worked well together. After all Wp-cache is a very fast cache module (is highly recommended for shared web hosting users), and can easily help your shared server survive a slashdotting or Digg effect better.

The problem was same as before. The cached and expired pages were of the same number and that somehow interfered with the functioning of the plugin, preventing further caching and deletion of expired pages also.

cache contents

I simply disabled the plugin, emptied the cache and expired pages and deactivated the wp-cache plugin and the site was instantly back online. I hope the plugin author fixes the issue. Have you encountered a similar problem with WP-cache?

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Comments

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1. Ryan Wagner | 10/10/07 #

Nope, I haven’t ever had that issue. We’re currently running WordPress 2.3 as well with the latest version of wp-cache.
2. Gary | 10/10/07 #

Your first mistake was upgrading to 2.3, which is a plugin serial killer.
3. QuickOnlineTips | 10/10/07 #

I have been encountering this issue for the last one year. Wp-Cache works well, but when cached pages = expired pages - the site stops.
4. Ashish Mohta | 10/10/07 #

HiPC,

I did face this problem but initially when started using it, Guess with 2.2 but then disabling and enabling of the plugin worked again.

My number of pages were not similar but then the caching was stopped.

Ashish
5. QuickOnlineTips | 10/10/07 #

You are right Ashish. Once you disable and enable the plugin it works well again…. but you never know when the bug will strike again. I found out after 17 hours and fixed it; if I am on vacation, the site may be down for days!
6. Shivaranjan | 10/10/07 #

PC-
My suggestion would be use it when your site gets dugg or slashdotted rather than every day.
7. QuickOnlineTips | 10/10/07 #

“Use it when your site gets dugg or slashdotted rather than every day.” - now that is a great idea Shivaranjan. It works for me that way…
8. Armen | 13/10/07 #

I think the problem would be fixed by simply clearing the cache every so often. If that could be automated, then that would probably fix the issue.
9. QuickOnlineTips | 13/10/07 #

Armen - The WP-cache is set to clear every 1 hour by default. Maybe setting it up to a lesser time of half an hour might help. But the issue of cached pages = expired pages will still occur, though at a lower page number.
10. Armen | 13/10/07 #

Oh right…
That’s an interesting bug.
11. Shivaranjan | 16/10/07 #

You can use it in this same way until the plugin author fixes the bug.
12. Robert Accettura | 28/10/07 #

I’ve hard of this bug before but never seen it myself. It sounds as if the age of the cache isn’t being correctly handled.

wp-cache seems to use filemtime(), making me wonder if this is an issue mostly for those who use webhosts that mount their filesystem using NFS, or have some sort of file caching mechanism in place that can cause funny behavior from time to time.
13. Beetlejuice | 6/11/07 #

Dreamhost have a fix for this problem in their Help pages:

1. Open /wp-content/plugins/wp-cache/wp-cache-phase2.php file* in your favourite text editor, where is the domain that you’re having troubles with EG: dreamhost.com/wp-content/plugins/wp-cache/wp-cache-phase2.php
2. Alternatively if that file is not in that location for some reason you can issue this command to find it from the directory: find . -name wp-cache-phase2.php
3. Locate the wp_cache_ob_end function
4. then inside that function find the line with: ob_end_clean(); (it should be line 219 or about)
5. and finally replace that line with: ob_end_flush();

http://wiki.dreamhost.com/WordPress#Blank_Pages