repmgr 4 is a complete rewrite of the existing repmgr code base and implements repmgr as a PostgreSQL extension. It supports all PostgreSQL versions from 9.3 (although some repmgr features are not available for PostgreSQL 9.3 and 9.4).
repmgr 3.x builds on the improved replication facilities added in PostgreSQL 9.3, as well as improved automated failover support via repmgrd, and is not compatible with PostgreSQL 9.2 and earlier. We recommend upgrading to repmgr 4, as the repmgr 3.x series will no longer be actively maintained.
repmgr 2.x supports PostgreSQL 9.0 ~ 9.3. While it is compatible with PostgreSQL 9.3, we recommend using repmgr 4.x. repmgr 2.x is no longer maintained.
Replication slots, introduced in PostgreSQL 9.4, ensure that the primary server will retain WAL files until they have been consumed by all standby servers. This makes WAL file management much easier, and if used repmgr will no longer insist on a fixed minimum number (default: 5000) of WAL files being retained.
However this does mean that if a standby is no longer connected to the primary, the presence of the replication slot will cause WAL files to be retained indefinitely.
Normally at least same number as the number of standbys which will connect to the node. Note that changes to max_replication_slots require a server restart to take effect, and as there is no particular penalty for unused replication slots, setting a higher figure will make adding new nodes easier.
Before PostgreSQL 10, hash indexes were not WAL logged and are therefore not suitable for use in streaming replication in PostgreSQL 9.6 and earlier. See the PostgreSQL documentation for details.
From PostgreSQL 10, this restriction has been lifted and hash indexes can be used in a streaming replication cluster.
For minor version upgrades, e.g. from 9.6.7 to 9.6.8, a common approach is to upgrade a standby to the latest version, perform a switchover promoting it to a primary, then upgrade the former primary.
For major version upgrades (e.g. from PostgreSQL 9.6 to PostgreSQL 10), the traditional approach is to "reseed" a cluster by upgrading a single node with pg_upgrade and recloning standbys from this.
To minimize downtime during major upgrades, for more recent PostgreSQL versions (PostgreSQL 9.4 and later), pglogical can be used to set up a parallel cluster using the newer PostgreSQL version, which can be kept in sync with the existing production cluster until the new cluster is ready to be put into production.
It means the repmgr extension code is not installed in the PostgreSQL application directory. This typically happens when using PostgreSQL packages provided by a third-party vendor, which often have different filesystem layouts.
Either use PostgreSQL packages provided by the community or 2ndQuadrant; if this is not possible, contact your vendor for assistance.