Sometimes, in order to provide acceptable data durability, a high availability configuration must utilize synchronous commits. Beginning with PostgreSQL 9.1, database servers can now refuse to commit a transaction until the data is located on at least one alternate server. Unlike asynchronous replication where this is optional, synchronous replicas enforce this requirement to a fault.
Discussions in the PostgreSQL mailing list suggest that there is a long-standing misconception that synchronous replication is similar to RAID-1 operation. In RAID-1, the same exact data exists on two disks (or two disk sets), and if one of the pair fails, it continues to operate in degraded mode until the problem ...
