Re: [PERFORM] Backup taking long time !!!

On Sun, Jan 22, 2017 at 6:57 AM, Stephen Frost <sfrost@snowman.net> wrote:

Greetings,

* julyanto SUTANDANG (julyanto@equnix.co.id) wrote:
> CORRECTION:
>
> "you might you pg_start_backup to tell the server not to write into the
> DATADIR"
>
> become
>
> "you might *use* pg_start_backup to tell the server not to write into the
> *BASEDIR*, actually server still writes but only to XLOGDIR "

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.

 

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.

 

Cheers,

Jeff