If your using memcache as your caching layer in magento, but have this snippit in your etc/local.xml
then your in for trouble. First off, this is a magento recommend setting for all 1.9 Enterprise customers. If you use this setting with even a moderate level of traffic hitting your site ~don't~ make any changes via the admin on your catalog products. It'll cause a deadlock situation and bring down your site for 5-15 minutes. We are still experimenting with a solution, but there are two possible ones:
-segregate your Admin traffic to an isolated server. On this server remove all of the slow_backend config settings in your etc/local.xml
-it seems that turning slow_backend_store_data off ~may~ solve it.
Your not getting deadlocks you say? Make sure you have your errors/local.xml setup to email you when you have a platform exception, you'll start noticing all of the lock timeouts and deadlocks.