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Date: Tue, 2 Apr 1996 21:47:18 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199604021147.VAA11954@godzilla.zeta.org.au>
To: luigi@labinfo.iet.unipi.it, msmith@atrad.adelaide.edu.au
Subject: Re: HDD cpu usage (IDE vs. SCSI).
Cc: davidg@Root.COM, dutchman@spase.nl, freebsd-hackers@FreeBSD.org
Sender: owner-hackers@FreeBSD.org
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>No, you're not understanding.  For a given CPU, IDE will _always_ use more
>CPU time than SCSI.  Period.  

Really?  Please give numbers for a PIO mode 4 IDE controller vs an ST01
SCSI controller :-).  Please give numbers for your choice of controllers
vs my choices of applications an i/o access patterns.  I'll choose a
memory intensive application that stalls the CPU waiting for the SCSI
controller.  I'll arrange the i/o so that memory caching is defeated
at strategic places.

>If you have lots of free CPU, then IDE is fine, but if you feel that your 
>CPU has better things to do with its time than copy data to and from
>your disk, then SCSI is the only solution that makes sense.

I think lots of free CPU is the usual case.  E.g., right now on freebsd.org:

    3:40AM  up 19 days, 11 mins, 16 users, load averages: 0.41, 0.32, 0.31

I would prefer lower latency to lower overhead in most cases.  IDE disks
have natural advantages in this area (no complicated SCSI protocol to
interpreted by the slow i/o processor on the controller).

Bruce