Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!convex!mic!letni!rwsys!merch!cpe!adaptex!adaptx1!neese
From: neese@adaptx1.UUCP
Newsgroups: comp.periphs.scsi
Subject: Re: Controller Cache vs. Software C
Message-ID: <283400143@adaptx1>
Date: 16 Jun 91 13:07:17 GMT
References: <31288@hydra.gatech.EDU>
Lines: 21
Nf-ID: #R:hydra.gatech.EDU:31288:adaptx1:283400143:000:1187
Nf-From: adaptx1.UUCP!neese    Jun 15 11:08:00 1991


>You mention several intersting points with regards to drive arrays. In order to
>take full advantage of a dirve array you need to have the array working on
>multiple requests simultaneously. Scsi 2 allows for command queing and
>overlapped commands, just the kind of thing necessary to maximize performance 
>from a disk array. The problem as I see it is that most OS device drivers do
>not yet support SCSI 2 and hence would limit the potential effectiveness of
>an array. Rather than change all the device drivers for all OS's might it
>be easier to implement these features with a smart caching host adaptor?

SCSI-2 does make provisions for tag queuing, but the feature is an option.
Out of all the SCSI-2 drives I have seen (4 manufacturers), only one of them
*correctly* supported tag queuing.  One other reports it supports tag
queuing, but in fact does not have it correctly implemented, the other two
do not support tag queuing and properly claim so.  So watch it when buying
a SCSI-2 device.  If you need tag queuing, be sure to specify so.

			Roy Neese
			Adaptec Senior SCSI Applications Engineer
			UUCP @  neese@adaptex
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