PostgreSQL — pg_rewind

pg_rewind is a tool for synchronizing a PostgreSQL data directory with another
PostgreSQL data directory that was forked from the first one. The result is
equivalent to rsyncing the first data directory (referred to as the old cluster
from now on) with the second one (the new cluster). The advantage of pg_rewind
over rsync is that pg_rewind uses the WAL to determine changed data blocks,
and does not require reading through all files in the cluster. That makes it
a lot faster when the database is large and only a small portion of it differs
between the clusters.

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