🚀 go-pugleaf

In article <29@eplrx7.UUCP> lad@eplrx7.UUCP (lad) writes:
>From article <1449@ucsfcca.ucsf.edu>, by rampil@cca.ucsf.edu (Ira Rampil):
>> In article <16@eplrx7.UUCP> lad@eplrx7.UUCP (lad) writes:

>>>If you're looking to compare the performance of SCSI disks
>>>Disktimer is NOT the way to do it.  Disktimer is not a benchmark,
>>>it favors disks with a 1:1 interleave and it's results do not
>>>reflect real-world disk performance.  Supermac based their whole
>>>marketing scheme around Disktimer, saying their disks were faster
>>>because Disktimer said so.

>> Could it be that DiskTimer favors 1:1 interleave disks because
>> they ARE faster?  Is this in dispute?

>1:1 disks are faster only if the machine they're working on supports
>1:1.  For example, Apple claims that a 3:1 interleave is optimal for
>a Mac Plus, so a disk formatted to 1:1 is going perform slower.
>Disktimer's problem is that it totally bypassed the driver and made
>calls directly to the low-level SCSI routines, totally ignoring the
>OS.  Any disk with a 1:1 interleave performed better in Disktimer no
>matter what machine it was being run on.  There are several other
>reasons why Disktimer should not be seriously considered when
>evaluating SCSI disk performance, but they are very technical in
>nature.

>Disktimer II:  Just say NO.

>        Lawrence A. Deleski             |       E.I. Dupont Co.

Larry has pointed out to me in private correspondence that my initial
response was inappropriately worded.  He's entirely correct in that
respect, and I apologize.  If ever I feel the need to insult Larry in
the future, I'll take care to do it privately.

That said, I'd like to correct some factual errors in Larry's posting
and explain my view of disk benchmarking.

DiskTimer II does not bypass the device driver.  It does not use the
SCSI manager traps or touch the SCSI hardware directly.  Instead, it
calls the device driver via the device manager.  (This should be
obvious from the fact that it works on non-SCSI devices.  If you have
any doubts, you can read the publicly available source code.)  Since
this what the file manager does, this is the basis for any resemblance
between DiskTimer's results and actual system performance.

Apple did not simply claim that a 3:1 interleave is optimal for a Mac
Plus.  They claimed that a 3:1 interleave is optimal for a Seagate
ST225N on a Mac Plus when the disk driver operates through the
Apple-supplied SCSI manager.  I've tested this claim using DiskTimer
and similar programs and they agree with Apple.

Provided that your disk driver uses the SCSI manager, Apple's advice
is applicable to a particular class of drives: MFM drives without
significant cache.  If you're not using the SCSI manager (and many
people don't) or if your drive is RLL, has a substantial cache or
FIFO, then Apple's advice simply doesn't apply.

Larry's claim about DiskTimer II using "low-level SCSI routines" fails
to make sense in another way.  The device drivers, after all, use the
low-level SCSI routines or manipulate the hardware directly.  If using
the low-level routines gives the fastest data transfer rates with 1:1
interleaving, why don't the device drivers get that result?

So far as I know, DiskTimer II's greatest failing is that it's
trivially deceived by caching, either in the disk controller or in the
device driver.  SuperMac's claims relied on the former case; FWB's
grossly misleading advertising depended on the latter case.  I'm sure
other companies made equally deceptive claims, but I didn't read all
the ads.  DiskTimer II repeatedly reads and writes the same 18K block.
Any cache over 18K immediately gains a 100% hit rate and a spuriously
good score.  Obviously, this isn't realistic.

Does SCSI Evaluator avoid this trap?  I don't know.  It does give you
the opportunity to compare performance through the device driver
against "raw" SCSI manager calls.  I expect (though I don't know) that
you'll find little advantage to the raw calls.  With some disks,
you'll find that the device driver has the advantage because it
bypasses the SCSI manager.

The greatest failing with all benchmarks is that it requires a great
deal of thought and knowledge to make sense of the results and apply
them to "the real world."  DiskTimer II boils everything down to three
numbers.  I personally feel that the data transfer rates are of only
modest importance of a fairly wide range.  That's what the programmer
can tweak, though, so that's where the hot competition is.

In many Mac operations, such as launching an application, much of the
time is spent seeking.  Seek time is entirely up to the drive
manufacturer.  A remarkable number of "different" Mac drives have the
same mechanisms inside, so there's little competition here.  (BTW,
DiskTimer II's seek test is also trivially defeated by caching.)

As other posters have pointed out, SCSI Evaluator gives the user a
great number of options and delivers a graph with numerous data
points.  If you're a developer tweaking your disk driver, or a user
benchmarking a variety of drives at once, this is fantastic.  If
you're trying to compare notes with strangers via usenet, it's very
nearly hopeless.

Claimers and disclaimers: I have sold disk driver software to several
companies, including Jasmine (San Francisco), Colby Systems (Fresno),
DataSpace (Markham, Ontario), and others that never went anywhere.
I'm out of that business now.  Larry Deleski is (was? what's the
latest, Larry?) a principal of IDT, which owns Micah.

Ephraim Vishniac					  ephraim@think.com
Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142-1214

     On two occasions I have been asked, "Pray, Mr. Babbage, if you put
     into the machine wrong figures, will the right answers come out?"