pg_start_backup and small max_wal_size

Question

What will happen, if max_wal_size gets exceeded during backup?

Specifically, let's consider the following scenario on master:

1. Set max_wal_size to a very low number.
2. Run SELECT pg_start_backup('label', true); to create a checkpoint.
3. Perform lots of transactions, so that max_wal_size is exceeded.

If I am right, PostgreSQL cannot process WAL files, because doing so would pollute the checkpoint. On the other hand, no new WAL files can be created, because of the limit.

Answer 1

max_wal_size is a soft limit - under many scenarios it gets crossed. Specifically as for pg_start_backup it just commits and outputs the wal location - if you have archiving in place than you have nothing to worry about.

Starting the backup will create more wal files but doesn't prevent rolling them - that's why you need the archiving as all wal files generated in the duration of the backup are needed for a successful restore.

See: max_wal_size documentation Also: backup documentation