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 |
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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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