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Re: WAL partition filling up after high WAL activity

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: Rafael Martinez <r(dot)m(dot)guerrero(at)usit(dot)uio(dot)no>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: WAL partition filling up after high WAL activity
Date: 2011-11-12 06:31:16
Message-ID: 4EBE12B4.5020705@2ndQuadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 11/11/2011 04:54 AM, Rafael Martinez wrote:
> Your explanation in 2) sounds like a good candidate for the problem we
> had. As I said in june, I think we need to improve the documentation in
> this area. A note in the documentation about what you have explained in
> 2) with maybe some hints about how to find out if this is happening will
> be a great improvement.
>    

A new counter was added to pg_stat_bgwriter in PostgreSQL 9.1 that 
tracks when the problem I described happens.  It's hard to identify it 
specifically without a source code change of some sort.  Initially I 
added new logging to the server code to identify the issue before the 
new counter was there.  The only thing you can easily look at that tends 
to correlate well with the worst problems here is the output from 
turning log_checkpoint on.  Specifically, the "sync" times going way up 
is a sign there's a problem with write speed.

As for the documentation, not much has really changed from when you 
brought this up on the docs list.  The amount of WAL files that can be 
created by a "short-term peak" is unlimited, which is why there's no 
better limit listed than that.  Some of the underlying things that make 
the problem worse are operating system level issues, not ones in the 
database itself; the PostgreSQL documentation doesn't try to wander too 
far into that level.  There are also a large number of things you can do 
at the application level that will generate a lot of WAL activity.  It 
would be impractical to list all of them in the checkpoint documentation 
though.

On reviewing this section of the docs again, one thing that we could do 
is make the "WAL Configuration" section talk more about log_checkpoints 
and interpreting its output.  Right now there's no mention of that 
parameter in the section that talks about parameters to configure; there 
really should be.

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


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