Re: Point in Time Recovery

Simon Riggs wrote:
> On Wed, 2004-07-14 at 10:57, Zeugswetter Andreas SB SD wrote:
> > > The recovery mechanism doesn't rely upon you knowing 1 or 3. The
> > > recovery reads pg_control (from the backup) and then attempts to
> > > de-archive the appropriate xlog segment file and then starts
> > > rollforward
> >
> > Unfortunately this only works if pg_control was the first file to be
> > backed up (or by chance no checkpoint happened after backup start and
> > pg_control backup)
> >
> > Other db's have commands for:
> > start/end external backup
> >
>
> OK...this idea has come up a few times. Here's my take:
>
> - OS and hardware facilities exist now to make instant copies of sets of
> files. Some of these are open source, others not. If you use these, you
> have no requirement for this functionality....but these alone are no
> replacement for archive recovery.... I accept that some people may not
> wish to go to the expense or effort to use those options, but in my mind
> these are the people that will not be using archive_mode anyway.
>
> - all we would really need to do is to stop the bgwriter from doing
> anything during backup. pgcontrol is only updated at checkpoint. The
> current xlog is updated constantly, but this need not be copied because
> we are already archiving it as soon as its full. That leaves the
> bgwriter, which is now responsible for both lazy writing and
> checkpoints.
> So, put a switch into bgwriter to halt for a period, then turn it back
> on at the end. Could be a SIGHUP GUC...or...

I don't think we can turn off all file system writes during a backup.
Imagine writing to a tape. Preventing file system writes would make the
system useless.

> - please could somebody else code that?... my time is limited

Yes, I think someone else could code this.

--
Bruce Momjian | http://candle.pha.pa.us
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