In part one, Introduction to Clustering in PHP, we explored the concepts of load balancing, PHP sessions, and how to set up a rudimentary PHP cluster that allows for redundancy as well as load balancing. The final configuration was one load balancer exposing an NFS share for all of the client PHP servers to use for session storage. While effective, this still gives us a single point of failure (the load balancer). More load balancers can be added, but sits us squarely back on our original problem: All of the sessions are on the first load balancer, not the second.
They talk about the GlusterFS networkable file system and talk about its concepts of "drives" and "bricks". The rest of the post is centered around helping you get GlusterFS servers set up and a brief mention of pointing your PHP session storage to the resulting setup.