Configuration of User/Node affinity in a Sendmail+Dovecot+Squirrelmail e-mail cluster

The storage of user e-mail boxes on a shared filesystem and their read/write access on multiple cluster nodes requires special care to benefit from file persistence in the filesystem cache on each node and to prevent system response to be significantly degraded by frequent file synchronisations onto physical storage. The simplest measure towards this goal is to divide access to nodes on a per-user basis, while keeping the cluster failsafe. In this paper we share a few non-obvious details for the configuration of such user/node affinity in sendmail, dovecot, squirrelmail.