Re: [HACKERS] gitlab post-mortem: pg_basebackup waiting for checkpoint

Hi,

one take-away from the Gitlab Post-Mortem[1] appears to be that after
their secondary lost replication, they were confused about what
pg_basebackup was doing when they tried to rebuild it. It just sat there
and did nothing (even with --verbose), so they assumed something was
wrong with either the primary or the connection, and restarted it
several times.

AFAICT, it turns out the checkpoint was written on the master (they
probably did not use -c fast), but this wasn't obvious to them:

ISTM that even with WAL streaming, nothing would be written on the
client server until the checkpoint is complete, as do_pg_start_backup()
runs the checkpoint and only returns the starting WAL location
afterwards.

The attached (untested) patch is to kick of a discussion on how to
improve the situation, it is supposed to mention the checkpoint when
--verbose is used and adds a paragraph about the checkpoint being run to
the Notes section of the documentation.