I've been looking at New Relic lately to try to identify some performance issues and I've been seeing quite a bit of issues between memcached and mdl_sessions. When the servers are running at their slowest there's always a bunch of session renewals happening.
The weird thing is the database table mdl_sessions is being hammered hardcore and I have a memcached server set-up just for sessions. I'm unsure why my DB is getting hit so hard if I am using memcached?
Most of the time the DB server is running at about 20% capacity, but when it's having this session lock renewal situation, it maxes out both Disk I/O and Network I/O.
Anyone got any ideas?