Date: Wed, 11 Jul 2007 14:53:56 -0400 (EDT)
From: Greg Smith <gsmith@gregsmith.com>
To: pgsql-performance@postgresql.org
Subject: Re: WALL on controller without battery?
In-Reply-To: <200707110847.44612@hal.medialogik.com>
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References: <cone.1184165284.909164.90107.1000@zoraida.natserv.net>
	<20070711153620.GP1241@phlogiston.dyndns.org>
	<200707110847.44612@hal.medialogik.com>
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On Wed, 11 Jul 2007, Alan Hodgson wrote:

> Presumably he'll have the 2-port configured for write-through operation.

This is the real key to his question.  In order to get acceptable 
performance for the operating system, Francisco may very well need the OS 
disks to be configured in write-back mode.  If that's the case, then he 
can't put the WAL there; it has to go onto the array with the BBU.

> I would spring for a 4-port with a BBU, though, and then put the WAL on the
> drives with the OS.

This is certainly worth considering.  When putting multiple RAID 
controllers into a system, I always try to keep them of a similar grade 
because it improves the possibility of data recovery in case of a 
controller failure.  For example, if he had a 4-port with BBU and an 
8-port with BBU, the 8-port could be split into two 4-disk RAID-6 volumes, 
and then in an emergency or for troubleshooting isolation you could always 
get any data you needed off any 4-disk set with either controller.  The 
little 2-disk unit is providing no such redundancy.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD