Re: How does pg_basebackup manage to create a snapshot of the filesystem?

> On Mar 19, 2020, at 15:19, Dennis Jacobfeuerborn <dennisml(at)conversis(dot)de> wrote:
> I'm currently trying to understand how backups work. In the
> documentation in section "25.2. File System Level Backup" it says that
> filesystem level backups can only be made when the database if offline
> yet pg_basebackup seems to do just that but works while the database is
> online. Am I misunderstanding something here or does pg_basebackup use
> some particular features of Postgres to accomplish this?

pg_basebackup does, indeed, take an inconsistent copy of the file system while it is running; what allows it to bring the database back up to consistency is the write-ahead log segments that are created while pg_basebackup is running. That's why it is important to have all of the WAL segments created during the run (which is what --wal-method=stream provides you).
--
-- Christophe Pettus
xof(at)thebuild(dot)com