It was discovered after we upgraded bugzilla.mozilla.org to version 2.20 that some of the architectural changes in how Bugzilla talks to the database servers had quite by accident sideswiped the ability to run searches against a replicated database server, causing those searches to always run against the master. The net effect on Bugzilla is that on large installations (such as ours), Bugzilla would periodically lock up for a bit because of having to wait for database locks.
I applied the patch from bug 313695 to correct this problem to bugzilla.mozilla.org about 3 hours ago, and so far things are looking pretty good. I haven’t seen any freezeups yet since then, and things are staying pretty snappy. On the other hand, we’re also right at the bottom end of our off-peak time of day for traffic levels.
I also bumped up the send timeout in apache to match what we had before we moved to the new server a few months back (bug 299577). This allows some longer-running searches (and sending of bugmail on bugs with lots of CCs) to actually complete.
If you notice any new problems with Bugzilla as of this evening, come find me on #bmo on irc.mozilla.org.
For any non-Mozilla Bugzilla folks reading this, yes, this fix will be included in 2.20.1. 🙂