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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: 4FF32066.5070000@2ndQuadrant.com (view raw or flat)
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 
instead.

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 www.2ndQuadrant.com

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