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Message-ID: <4BCC8E76.2040105@gmail.com>
Date: Mon, 19 Apr 2010 10:10:14 -0700
From: Frederiko Costa <frederiko@gmail.com>
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To: pgsql-admin@postgresql.org
Subject: Re: archived WALL files question
References: <7965A9DCF12CC14984420BCC37B1608F25AC0B99AD@Elzar.grant.co.uk>
	<4BC6F21D0200002500030956@gw.wicourts.gov>
	<7965A9DCF12CC14984420BCC37B1608F25AC0B99E4@Elzar.grant.co.uk>
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	<4BCC415D0200002500030A6B@gw.wicourts.gov>
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I have seen new 16 MB segments files created in pg_xlog directory as 
time goes on. Honestly, I did not understand why it got created, because 
I was running it on a VM and there was no activity. I got several new 
segments.

If I run the command you asked, this is the output. Neither segment is 
copied, nor a new segment file gets created:

  select pg_switch_xlog();
  pg_switch_xlog
----------------
  0/81000088
(1 row)

Wasn't this command supposed to create a new segment file and copy the 
last non-copied segments - using command specified at archive_command - 
to the destination you defined?

I wanted to understand why the newly created wal segment files (with 16 
MB each) did not copied automatically to the new place. What's the 
criteria? They only get copied when I carry out a base backup.

Thanks for the help!

On 04/19/2010 09:41 AM, Kevin Grittner wrote:
> Frederiko Costa<frederiko@gmail.com>  wrote:
>
>    
>> I noticed the new logs files don't get copied to the directory
>> specified on the archive_command. It's only copied when I do the
>> pg_start_backup()/pg_stop_backup() base backup. Is this behaviour
>> only achieved if I set archive_timeout? I did not want to do that,
>> because I thought as soon as the 16MB WAL segment file got created
>> it would be copied to the exported directory.
>>      
>
> Have you actually been filling up any 16MB WAL file segments?  How
> have you determined that?  What happens when you run (as a database
> superuser) this command?:
>
> SELECT pg_switch_xlog();
>
> If the reason you don't want to set archive_timeout is that you
> don't want 16MB files piling up when there is little or no activity,
> you might want to consider using pglesslog or pg_clearxlogtail.
> These can eliminate nearly all the cost of the more frequent logs.
>
> -Kevin
>    

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<font face="Liberation Sans">I have seen new 16 MB segments files
created in pg_xlog directory as time goes on. Honestly, I did not
understand why it got created, because I was running it on a VM and
there was no activity. I got several new segments.<br>
<br>
</font>If I run the command you asked, this is the output. Neither
segment is copied, nor a new segment file gets created:<br>
<br>
&nbsp;select pg_switch_xlog();<br>
&nbsp;pg_switch_xlog <br>
----------------<br>
&nbsp;0/81000088<br>
(1 row)<br>
<br>
Wasn't this command supposed to create a new segment file and copy the
last non-copied segments - using command specified at archive_command -
to the destination you defined?<br>
<br>
I wanted to understand why the newly created wal segment files (with 16
MB each) did not copied automatically to the new place. What's the
criteria? They only get copied when I carry out a base backup.<br>
<br>
Thanks for the help!<br>
<br>
On 04/19/2010 09:41 AM, Kevin Grittner wrote:
<blockquote cite="mid:4BCC415D0200002500030A6B@gw.wicourts.gov"
 type="cite">
  <pre wrap="">Frederiko Costa <a class="moz-txt-link-rfc2396E" href="mailto:frederiko@gmail.com">&lt;frederiko@gmail.com&gt;</a> wrote:
 
  </pre>
  <blockquote type="cite">
    <pre wrap="">I noticed the new logs files don't get copied to the directory
specified on the archive_command. It's only copied when I do the
pg_start_backup()/pg_stop_backup() base backup. Is this behaviour
only achieved if I set archive_timeout? I did not want to do that,
because I thought as soon as the 16MB WAL segment file got created
it would be copied to the exported directory.
    </pre>
  </blockquote>
  <pre wrap=""> 
Have you actually been filling up any 16MB WAL file segments?  How
have you determined that?  What happens when you run (as a database
superuser) this command?:
 
SELECT pg_switch_xlog();
 
If the reason you don't want to set archive_timeout is that you
don't want 16MB files piling up when there is little or no activity,
you might want to consider using pglesslog or pg_clearxlogtail. 
These can eliminate nearly all the cost of the more frequent logs.
 
-Kevin
  </pre>
</blockquote>
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