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Date:      Wed, 22 Mar 1995 11:15:44 -0800 (PST)
From:      "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com>
To:        sos@login.dknet.dk (S|ren Schmidt)
Cc:        hackers@FreeBSD.org
Subject:   Re: Why IDE is bad
Message-ID:  <199503221915.LAA09736@gndrsh.aac.dev.com>
In-Reply-To: <9503221217.AA08636@login.dknet.dk> from "S|ren Schmidt" at Mar 22, 95 01:17:17 pm

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[cc: trimmed, moved to hackers list, reply-to set to hackers]
> 
> > 
> > I don't think you will find many ide's faster than the WDC, I know
> > it can run faster on a better IDE-controller, but this is a fairly
> > standard IDE controller so it's a good indication.
> > And yes, but FreeBSD doesn't support DMA in IDE (yet ?)...
> 
> Was that my que word ??
> 
> DMA does not pay on E-IDE devices, it's not busmaster DMA, it
> is the motherboards DMA we are talking about here.
Yep.
> This is very slow on standard MB's, but faster on PCI 
> (if they implemented it right). 
Yep.
> However a "generic" driver would have to fall back to
> the std. PC DMA and that is slower than using polled mode.

Perhaps, but motherboard DMA does occur at upto 4MB/sec, and concurrent
with the CPU doing other things.  Since most drives can't sustain
this transfer rate for long periods of time it would be more efficient.

> And besides only few drives support it anyway, and the high
> rates of the PIO modes looks better in advertizing.

E-IDE DMA mode is not a function of the drive, it is a function of
the controller chip.
 
> And that is the main thing on E-IDE, the drives are designed
> with enough onboard cache, that coretest etc. reports transfer
> rates close to the interface speed (13MB sec or so), but the
> drive cannot hold this speed when it has to read from the media.

All too true, PC hardware is optimized to run the Benchmarks that
you see in all the trade rags, not for real world applications.  Being
fast on the benchmarks does often help some, but it is no where near
what you can get out of good hardware by spending the extra money to
get what is not only fast on the dumb benchmarks, but is fast no
matter what you thow at it.

> And here is the catch, in that most el cheapo IDE drives has
> inferior drive mechanics (hey they are cheap), and then some
> fancy cache/interface electronics to make up for outdated
> hardware....

I don't think this is totally true, many very good drives are avaliable
in both SCSI and IDE, they use the exact same technology just different
drive electronics.  Same samples are the Micropollis 4410, the Quantum
ProDrive, Lightning and Maverik series.   I won't call Samsung a good
drive, but they also have a line of drives that are avaliable in either
SCSI or EIDE.

> > 
> I personally think that E_IDE and friends are the cleverest
> sales trick in todays PC hardware.

BINGO!!!  And the other problems with E-IDE or even IDE is that it
is hard to get the real numbers you need to know if the drive is
really fast or not.  Drive adds always list the host bus burst
interface rate, what you REALLY want to know is the ``Data Transfer
Rate - To/From Media''.  This number is almost always avaliable in
the specs for SCSI drives, and only sometimes avaliable in the
specs for ATA drives.  I happen to have the numbers for the Samsung

				Transfer Rate
drive		ATA/SCSI	Host(SCSI/ATA)/sec	Media TO/FROM /sec

SHD-30280A	ATA only	X/11MB			38.10Mbit
SHD-30420A	ATA only	X/11MB			38.10Mbit
SHD-30560A	ATA only	X/11MB			38.10MBit
PLS-30540A	ATA only	X/11MB			46.22MBit
PLS-30730A	ATA only	X/11MB			46.64MBit
PLS-30850A/S	ATA/SCSI	11MB/10MB		48.11MBIT
PLS-31100A/S	ATA/SCSI	11MB/10MB		47.27MBIT
TRS-31080A/S	ATA/SCSI	11MB/10MB		38.5MBIT/72.4MBIT

Here is a line of drives that should do sustained tranfers of
4MB to 9MB/sec avaliable in both SCSI and ATA.  Me personally,
only use ATA when I have to and/or when the customer is cutting
cost.

I wonder what Poul's little test would like like if I compared the
two versions of that TRS-31080, how much CPU would I need to keep
up with 9MB/sec PIO :-)


-- 
Rod Grimes                                      rgrimes@gndrsh.aac.dev.com
Accurate Automation Company                   Custom computers for FreeBSD



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