Highly available databases often come in pairs for redundancy purposes. These servers can have any number of procedures to keep the data synchronized, and this book suggests direct connections when possible. Direct connections between servers ensure fast communication between redundant servers, and it resembles the following network design:

In some cases, it can be advantageous to connect the database servers to a general network fabric. Depending on the interaction of the upstream network devices, this can significantly increase the network packet's round-trip-time (RTT). This is usually fine for PostgreSQL replication, but OLTP systems may be more sensitive. Block-level replication systems, such as DRBD, which operate beneath the filesystem, fare even worse.

Each of our database servers should be equipped with at least two independent network interfaces. In order to prevent downtime, these interfaces must be linked with a bond. Network bonds act as an abstract...