could not receive data from WAL stream: ERROR: requested WAL segment XXXXXX has already been removed I did a vacuum on the master server today and the read / slave fell behind.

When I check, that WAL file is in the pg_xlog directory on the slave and in the wal_archive directory which I created on the master.

Why is Postgres not "seeing" that WAL file.

(I saw an answer on Postgres requested WAL segment has already been removed (however it is actually in the slave&#39;s directory) however I'm not keen to copy the file into the pg_xlog of the master in case it causes an issue on the master.)
Add a. restore_command that can restore the required WAL file from the archive to recovery.conf (or postgresql.conf from v12 on) and restart the standby.