I think what Takayuki is trying to say is that streaming replication works by sending the contents of the WAL archives to the standbys. If archive_mode was NOT set to always, and if you wanted to archive WAL logs in the standby you would need to rely on the process_command and make it ship the WAL logs (to the standby). This causes the same WAL log information to be shipped in two places,
1. Through Streaming Replication
2. By the process_command
This redundant shipping of the same information is expensive and consumes network bandwidth. This can be avoided with the use of archive_mode=always.
archive_mode=always makes the standby archive the WAL logs it receives, thus avoiding the requirement of having to ship it separately.
From: Adelino Silva [mailto:adelino.j.silva@googlemail.com] > What is the advantage to use archive_mode = always in a slave server compared > to archive_mode = on (shared WAL archive) ? > > I only see duplication of Wal files, what is the purpose of this feature ?
This also saves you the network bandwidth by not sending the WAL archive from the primary to the standby(s). The network bandwidth can be costly between remote regions for disaster recovery.
