Re: Allow wal_keep_segments to keep all segments

Simon Riggs wrote:
> On Wed, 2010-06-02 at 20:28 -0400, Bruce Momjian wrote:
> > Simon Riggs wrote:
> > > On Wed, 2010-06-02 at 15:20 -0400, Bruce Momjian wrote:
> > > 
> > > > The attached patch allows wal_keep_segments = -1 to keep all segements; 
> > > > this is particularly useful for taking a base backup, where you need all
> > > > the WAL files during startup of the standby.  I have documented this
> > > > usage in the patch as well.
> > > > 
> > > > I am thinking of applying this after 9.0 beta2 if there is no objection.
> > > 
> > > It's not clear to me why "keep all files until server breaks" is a good
> > > setting. Surely you would set this parameter to the size of your disk.
> > > Why allow it to go higher?
> > 
> > Well, the -1 allows them to set it temporarily without having to compute
> > their free disk space.  Frankly, because the disk space varies, it is
> > impossible to know exactly how large the disk is at the time it would
> > fill up.
> > 
> > I think the normal computation would be:
> > 
> > 	1) How long is my file system backup and restore to standby
> > 	   going to take
> > 	2) How often do I generate a 16MB WAL file
> > 
> > You would do some computation to figure that out, then maybe multiply it
> > by 10x and set that for wal_keep_segments.  I figured allowing a simple
> > -1 would be easier.
> 
> I think its much easier to find out your free disk space than it is to
> calculate how much WAL might be generated during backup. Disk space
> doesn't vary significantly on a production database.
> 
> If we encourage that laziness then we will get reports that replication
> doesn't work and Postgres crashes.

Well, we don't clean out the archive directory so I don't see this as
anything new.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + None of us is going to be here forever. +