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Re: replication recovery/startup question

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: Rob Cowell <Rob(dot)Cowell(at)transversal(dot)com>
Cc: "'pgsql-admin(at)postgresql(dot)org'" <pgsql-admin(at)postgresql(dot)org>
Subject: Re: replication recovery/startup question
Date: 2012-07-03 16:40:06
Message-ID: (view raw or whole thread)
Lists: pgsql-admin
On 06/25/2012 11:40 AM, Rob Cowell wrote:
> Why would the output from ‘ls’ show older filenames (0....1....3D...xx)
> as newer in date than the “0....1....3F...xx” filenames?
> Does Postgres re-cycle old log filenames ?

It recycles old log files.  If you turn on log_checkpoints, you can see 
how many and how often.  It will list a count of recycled WAL files at 
each checkpoint, along with how many of the old ones were just deleted 

The weird pattern in the timestamps you're seeing is a state in the 
middle of doing that, and yes they look quite weird sometimes.  The 
files are noteed as reusable, get re-initialized to hold new data 
(they're not overwritten completely with zeros like new WAL files are), 
and renamed to a new segment number.  And each of those steps has a 
corresponding flush to disk step which makes sure the filesystem 
metadata is updated.  Some of the middle states there are unusual.

> Does the output from ‘ps’ mean the master/slave are in sync, or is the
> slave really still playing catchup (based on the names of the logfiles
> in pg_xlog) ?

Your example was in sync, with the file names just being odd due to the 
implementation of WAL file recycling.  You might also check 
pg_stat_replication to get an easier view of things, rather than relying 
on ps.  ps is correct, it's just harder to check.

Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support

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