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Re: Why I lost the last pg_xlog file?

From: Shaun Thomas <sthomas(at)peak6(dot)com>
To: Waldomiro <waldomiro(at)shx(dot)com(dot)br>
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Why I lost the last pg_xlog file?
Date: 2011-01-27 14:17:23
Message-ID: 4D417E73.7040901@peak6.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 01/27/2011 06:10 AM, Waldomiro wrote:

> 3) I check the pg_xlog, there was created 6 logs, but in the archive
> there was only 5.

xlogs are only "archived" when they'd normally be deleted. If you have 
really high data turnover or very frequent checkpoints, that effectively 
happens constantly. I'm not sure where the cutoff is, but there's a 
certain amount of "reserve" xlog space based on your checkpoint_segments 
setting.

It is an archive, after all. pg_start_backup/pg_stop_backup ensure your 
backup is consistent, nothing else. Depending on your archives to 
capture everything isn't going to work. If you really want everything, 
you can either copy the xlogs manually (not safe) or initiate another 
backup. You can kinda fudge it doing this:

1. Call pg_current_xlog_location to get the current xlog.
2. Call pg_switch_xlog to force a new xlog.
3. Copy the file from step 1 and anything older than it to your 
archive/slave. Doing this *may* confuse the built-in archive system if 
your archive_command is too strict.
4. Profit.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
312-676-8870
sthomas(at)peak6(dot)com

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