Date: Wed, 9 Apr 2014 12:51:27 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Reza Taheri <rtaheri@vmware.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: PGSQL, checkpoints, and file system syncs
Message-ID: <20140409165127.GD8686@momjian.us>
References: 
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 <533DA1E4.30401@vmware.com>
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On Thu, Apr 3, 2014 at 09:01:08PM +0300, Heikki Linnakangas wrote:
> >Is there something I can set in the PGSQL parameters or in the file
> >system parameters to force a steady flow of writes to disk rather
> >than waiting for a sync system call? Mounting with "commit=1" did not
> >make a difference.
>
> Try setting the vm.dirty_bytes sysctl. Something like 256MB might be a
> good starting point.
>
> This comes up fairly often, see e.g.:
> http://www.postgresql.org/message-id/flat/27C32FD4-0142-44FE-8488-9F36
> 6DC75966@mr-paradox.net

Uh, should he set vm.dirty_bytes or vm.dirty_background_bytes?  It is my
understanding that vm.dirty_background_bytes starts the I/O while still
accepting writes, while vm.dirty_bytes stops accepting writes during the
I/O, which isn't optimal.  See:

	https://www.kernel.org/doc/Documentation/sysctl/vm.txt
	
	dirty_bytes
	
	Contains the amount of dirty memory at which a process generating disk
	writes will itself start writeback.

	dirty_background_bytes
	
	Contains the amount of dirty memory at which the background kernel
	flusher threads will start writeback.
	
I think we want the flusher to be active, not necessarly the writing
process.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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