In this new post to his blog, Johannes Schlüter looks at a high-performance solution to the usual storing PHP session information via a memcache frontend with a MySQL Cluster backend.
Unfortunately even such a system [using MySQL and InnoDB tables] has limits and unfortunately replication is no good solution here to scale further as we will always need a master for writing the updated session data. By using replication we can take some load from it and we can configure a slave which can be promoted to master to keep session alive if the primary master machine fails but at some point in time we need another solution ... but, happy news, again: One doesn't have to look far as MySQL cluster will be happy to help. MySQL Cluster "is a high-availability, high-redundancy version of MySQL adapted for the distributed computing environment," as the MySQL documentation states.
He describes the setup (after pointing to this post about installing MySQL Cluster for memcache) and includes some sample code/SQL/ini settings you'll need to use to get PHP's memcached functionality to cooperate with it.