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Date: Thu, 05 Feb 2004 21:08:46 +0100
From: Ede Wolf <listac@nebelschwaden.de>
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Subject: Re: SATA RAID 5 controller for FreeBSD
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> It does not have cache as the hard disks and the OS have. There is no need.

I am by no means a RAID expert and this might be slightly off topic, but 
I've realized, that SCSI-Disks used in a RAID always have their 
write-cache disabled. Not sure why, but I guess partly for control 
meaning that if the controller flushes its cache, it can be sure that 
data is on the disk.
With write cache enabled, the controler has no real knowledge of the 
state of the drives and may be "confused"  by extremely different speeds 
(writing data to an empty cache on drive 1 while drive 2 is busy writing 
its own cache to disk).
So basically the cache on the controller is an replacement for those on 
the drives.

This may be complete bollocks, only what I've been told, but maybe 
someone would be so kind to clearify, since I am not aware one can 
disable write cache on ATA drives. Which even may not be relevant.