PostgreSQL Binary Replication -do i really need to copy pg_xlog separated?

Question

I am doing postgresql replication by issuing:

On the master:

>select pg_start_backup('clone', true);
>\q 
#tar -cz main -f main.tar.gz

(Explanation: main is the data directory on my system /var/lib/postgresql/9.6/main)

Then I transfer main.tar.gz to the slave....

On the slave:

--Uncompressing main.tar.gz.... 
#tar -zxf main.tar.gz 
--Copying previously created recovery.conf 
#cp recovery.conf main/
--And then just starts the slave
#service postgresql start

On the master:

--And stoping backup mode on the master:
>select pg_stop_backup(); 

What I get is that replication is working fine, and it really works nice.

But I am with doubt after reading the official documentation that after the copy I should copy pg_xlog in an additional step... https://wiki.postgresql.org/wiki/Binary_Replication_Tutorial

But I never do that, and never experienced a problem - do a lot of tests, like counting every table lines, in both master and slave and much more, but what worst can happen?

Answer 1

It works fine, until it doesn't. Then you get a backup which won't start, will start but won't connect the master to stream new data, or perhaps one which will start but is silently corrupted (unlikely, but I wouldn't rule it out). Have you tested this method for a very large server under very heavy load?

If you use pg_basebackup, it will either take care of it for you, or will notify you that the backup failed before you discover that the hard way, or notify you that you need to have a separate WAL archive. Depending on which version of PostgreSQL and which options you give to pg_basebackup.