Re: Is full_page_writes=off safe in conjunction with PITR?

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> >> Think harder: we are only safe if the first write to a given page after
> >> it's mis-copied by the archiver is a full page write.  The requirement
> >> therefore continues after pg_stop_backup.  Unless you want to add
> >> infrastructure to keep track for *every page* in the DB of whether it's
> >> been fully written since the last backup?
> 
> > I am confused.  Since we checkpoint during pg_start_backup(), isn't any
> > write to a file while the tar backup is going on going to be a full page
> > write?  And once we pg_stop_backup(), do we need full page writes?
> 
> Hm.  The case I was concerned about was where a page is never written
> to while the backup occurs (thus not triggering any full-page WAL
> entry), and then the first post-backup write is partial.  However, if
> the backup is guaranteed to have captured a non-torn copy of such a page
> then there shouldn't be any problem.  So if we consider the initial
> checkpoint to be a *required part* of pg_start_backup (right now it is
> not) then maybe we can get away with this.  It needs more eyeballs on it
> though ... after having been burnt once by full_page_writes, I'm pretty
> shy ...

Right.  The comment in pg_start_backup() has to be updated:
   /*    * Force a CHECKPOINT.  This is not strictly necessary, but it seems like    * a good idea to minimize the
amountof past WAL needed to use the    * backup.  Also, this guarantees that two successive backup runs will    * have
differentcheckpoint positions and hence different history file    * names, even if nothing happened in between.    */
RequestCheckpoint(true,false);
 

This is a much simpler fix than people talking about writing their own
backup programs.

--  Bruce Momjian   http://candle.pha.pa.us EnterpriseDB    http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +