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In-reply-to: Your message of "Thu, 13 Nov 2008 15:46:32 +1000."
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Date: Wed, 12 Nov 2008 22:57:58 +0000
From: Dieter <freebsd@sopwith.solgatos.com>
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Subject: Re: Areca vs. ZFS performance testing. 
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>> For the array(s)
>> 9 x ST31000340AS  1tb disks
>> 1 x ST31000333AS  1tb disk (trying to swap this for a ST31000340AS)

> There seems to be little difference between enabling and disabling the
> disk cache on the Areca.  This leads me to two conclusions:
> 	1. Disabling the write cache does nothing on Seagate drives.
> 	2. IO to the drives is so slow that a write cache is irrelevant.

I have a couple of the ST31000340AS 1TB disks as well as older lower capacity
Seagates, and turning the write cache on/off makes a MASSIVE (roughly 10:1)
difference in write speed.

Jeremy reports "about 13%" with Seagate ST3120026AS:
http://lists.freebsd.org/pipermail/freebsd-hardware/2008-October/005450.html

Perhaps there is something about the Areca or the testing?  Is the write cache
really getting turned on/off?

You're getting about 2-3x the speed I'd expect if the write cache were off,
so maybe it is still on but there is a bottleneck elsewhere?

Have you tried a simple test with /dev/zero and dd to a raw drive to
eliminate the effects of the filesystem?