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From: Greg Smith <gsmith@gregsmith.com>
To: pgsql-performance@postgresql.org
Subject: Re: Lying drives [Was: Re: Which OS provides the _fastest_
Date: Thu, 23 Nov 2006 02:31:22 -0500 (EST)
Message-ID: <Pine.GSO.4.64.0611230209060.8621@westnet.com> (raw)
In-Reply-To: <20061113083205.GJ29915@esphion.com>
References: <kO24h.1260$2X4.632@trnddc04>
	<1162920135.543539.118390@h54g2000cwb.googlegroups.com>
	<2cl4h.1236$Es5.713@trnddc07>
	<1162995009.842121.125840@k70g2000cwa.googlegroups.com>
	<1163016362.882752.14140@m7g2000cwm.googlegroups.com>
	<06E4h.5421$Lt4.2413@trnddc08>
	<1163084338.373159.99280@h54g2000cwb.googlegroups.com>
	<U2_4h.3726$244.2632@trnddc01>
	<1163171668.837184.161080@e3g2000cwe.googlegroups.com>
	<4554CAC9.3000104@cheapcomplexdevices.com>
	<20061113083205.GJ29915@esphion.com>

On Mon, 13 Nov 2006, Guy Thornley wrote:

> I've yet to find a drive that lies about write completion. The problem 
> is that the drives boot-up default is write-caching enabled (or perhaps 
> the system BIOS sets it that way). If you turn an IDE disks write cache 
> off explicity, using hdparm or similar, they behave.

I found a rather ominous warning from SGI on this subject at 
http://oss.sgi.com/projects/xfs/faq.html#wcache_query

"[Disabling the write cache] is kept persistent for a SCSI disk. However, 
for a SATA/PATA disk this needs to be done after every reset as it will 
reset back to the default of the write cache enabled. And a reset can 
happen after reboot or on error recovery of the drive. This makes it 
rather difficult to guarantee that the write cache is maintained as 
disabled."

As I've been learning more about this subject recently, I've become 
increasingly queasy about using IDE drives for databases unless they're 
hooked up to a high-end (S|P)ATA controller.  As far as I know the BIOS 
doesn't mess with the write caches, it's strictly that the drives default 
to having them on.  Some manufacturers lets you adjust the default, which 
should prevent the behavior SGI warns about from happening; Hitachi's 
"Feature Tool" at http://www.hitachigst.com/hdd/support/download.htm is 
one example I've used successfully before.

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



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