Re: Maximum number of WAL files in the pg_xlog directory

On Tue, 3 Mar 2015 11:15:13 -0500
Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Tue, Oct 14, 2014 at 01:21:53PM -0400, Bruce Momjian wrote:
> > On Tue, Oct 14, 2014 at 09:20:22AM -0700, Jeff Janes wrote:
> > > On Mon, Oct 13, 2014 at 12:11 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > >
> > >
> > > I looked into this, and came up with more questions.  Why is
> > > checkpoint_completion_target involved in the total number of WAL
> > > segments?  If checkpoint_completion_target is 0.5 (the default), the
> > > calculation is:
> > >
> > >         (2 + 0.5) * checkpoint_segments + 1
> > >
> > > while if it is 0.9, it is:
> > >
> > >         (2 + 0.9) * checkpoint_segments + 1
> > >
> > > Is this trying to estimate how many WAL files are going to be created
> > > during the checkpoint?  If so, wouldn't it be (1 +
> > > checkpoint_completion_target), not "2 +".  My logic is you have the
> > > old WAL files being checkpointed (that's the "1"), plus you have new WAL
> > > files being created during the checkpoint, which would be
> > > checkpoint_completion_target * checkpoint_segments, plus one for the
> > > current WAL file.
> > >
> > >
> > > WAL is not eligible to be recycled until there have been 2 successful
> > > checkpoints.
> > >
> > > So at the end of a checkpoint, you have 1 cycle of WAL which has just
> > > become eligible for recycling,
> > > 1 cycle of WAL which is now expendable but which is kept anyway, and
> > > checkpoint_completion_target worth of WAL which has occurred while the
> > > checkpoint was occurring and is still needed for crash recovery.
> >
> > OK, so based on this analysis, what is the right calculation? This?
> >
> > (1 + checkpoint_completion_target) * checkpoint_segments + 1 +
> > max(wal_keep_segments, checkpoint_segments)
>
> Now that we have min_wal_size and max_wal_size in 9.5, I don't see any
> value to figuring out the proper formula for backpatching.

I guess it worth backpatching the documentation as 9.4 -> 9.1 will be supported
for somes the next 4 years

--
Jehan-Guillaume de Rorthais
Dalibo
http://www.dalibo.com