Date: Tue, 04 Jan 2000 11:10:39 -0500 From: Mitch Collinsworth <mkc@Graphics.Cornell.EDU> To: hardware@FreeBSD.ORG Subject: differences between SCSI and EIDE [was: wanna buy an EIDE harddisk ... 5400 or 7200 for home use (noise)] Message-ID: <200001041610.LAA15549@benge.graphics.cornell.edu> In-Reply-To: Message from Jonathan Michaels <jon@welearn.com.au> of "Tue, 04 Jan 2000 18:42:59 %2B1100." <20000104184258.A84552@phoenix.welearn.com.au>
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>anyway, thank you all for responding and sheading light on my
>confusion. i'd always thought that scsi was the better way to
>go, either fro the 'comercial' environment or the ever more
>demanding 'home' environment.
Well this is actually an interesting question. My salesman says the
HDAs are the same in SCSI and EIDE drives, so reliability-wise there
should be no difference.
So far I've stuck with SCSI but due to the significantly lower cost,
management is pushing me to use EIDE where possible in the future. I'd
like to know of any significant down-sides to using EIDE other than the
number of devices per controller.
I read on a seller's web page yesterday that:
"EIDE is not well designed for preemptive multitasking. EIDE
drives can not do more than one task at a time. In contrast,
SCSI devices are handle more jobs and SCSI controllers can
tag queue several commands."
This page appears to have been written a few years ago, so I'm concerned
the information may be dated. But assuming it is still accurate, I'd
like to better understand what is being said here. Do SCSI device
drivers typically initiate multiple commands from separate processes to
the drive without waiting for the previous command to complete? In other
words the drive logic has it's own queue management? And EIDE drives
require their device drivers to perform all queue management and only
initiate a command after the previous one has completed?
Is the bottom line result of this that the SCSI drive has a much greater
chance of servicing multiple processes during a single media revolution
while the EIDE will frequently take multiple revolutions to service the
same queue?
-Mitch
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