If you see some of the following sample error messages when backing up PostgreSQL, then it is very likely that you need to tweak your database settings to allow replica/slave backups to complete.
pg_dump: error: Dumping the contents of table "some_table" failed: PQgetResult() failed.
pg_dump: error: Error message from server: ERROR: canceling statement due to conflict with recovery DETAIL: User was holding a relation lock for too long.
pg_dump: error: The command was: COPY public.some_table (id, created_at, ...) TO stdout;
Explanation of the issue
If you have a replica/slave configuration and you use that replica/slave for your backups, then if a query takes too long to be read, there is a possibility that the returned result no longer exists or have been changed on the master node.
Proposed solution
Edit postgresql.conf on the replica node and set the following two values:
max_standby_archive_delay = 900s
max_standby_streaming_delay = 900s
This way queries on the slave/replica with a duration less than 900 seconds won't be cancelled quickly and give it more time to complete.
max_standby_streaming_delay: sets the maximum delay before canceling queries when a hot standby server is processing streamed WAL data
max_standby_archive_delay: sets the maximum delay before canceling queries when a hot standby server is processing archived WAL data