Re: Backup taking long time !!!

* Jeff Janes (jeff(dot)janes(at)gmail(dot)com) wrote:
> On Sun, Jan 22, 2017 at 6:57 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> > Just to make sure anyone reading the mailing list archives isn't
> > confused, running pg_start_backup does *not* make PG stop writing to
> > BASEDIR (or DATADIR, or anything, really). PG *will* continue to write
> > data into BASEDIR after pg_start_backup has been called.
>
> Correct. Unfortunately it is a very common myth that it does cause
> PostgreSQL to stop writing to the base dir.

I would love a way to dispel that myth. :/

If you have any suggestions of how we could improve the docs, I'd
certainly be happy to take a look and try to help.

> > The only thing that pg_start_backup does is identify an entry in the WAL
> > stream, from which point all WAL must be replayed when restoring the
> > backup. All WAL generated from that point (pg_start_backup point) until
> > the pg_stop_backup point *must* be replayed when restoring the backup or
> > the database will not be consistent.
>
> pg_start_backup also forces full_page_writes to be effectively 'on' for the
> duration of the backup, if it is not already explicitly on (which it
> usually will already be). This affects pg_xlog, of course, not base. But
> it is an essential step for people who run with full_page_writes=off, as it
> ensures that anything in base which got changed mid-copy will be fixed up
> during replay of the WAL.

Agreed.

Thanks!

Stephen