Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!killer!elg
From: elg@killer.Dallas.TX.US (Eric Green)
Newsgroups: comp.sys.amiga
Subject: Re: DiskPerf results
Message-ID: <7985@killer.Dallas.TX.US>
Date: 3 May 89 01:52:43 GMT
References: <6720@cbmvax.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 54

in article <6720@cbmvax.UUCP>, steveb@cbmvax.UUCP (Steve Beats) says:
$ In article <7965@killer.Dallas.TX.US>  writes:
$>I suspect that the limiting factors are a) the file system, and b) the
$>A2090A. I got about the same results from a 2090A and a Quantum 80S.
$>The 2090A limits speed because it is limited to the old 1mb/sec
$>asynchronous SCSI protocol (meaning that however hot your drive is,
$ While it's true that the filing system will eat some of the performance from
$ a SCSI drive, it's not reasonable to expect a synchronous mode controller to
$ run at 5.0Mb/s.  Synchronous mode is relatively new (to controller vendors,

Too true. But going synchronous can probably get you to the magic
1mb/sec Diskperf ;-).  (actually, anything over 500k/sec is probably
just window dressing as far as most Amiga users will be able to tell,
but you must admit that it DOES look neat on advertising ;-). 

$ not the SCSI spec) and as such it can become quite a pain to support.  I
$ attempted to bring up a driver with full sync xfer support and found that a
$ LOT of drives simply crashed when given a sync xfer request.  For this reason
$ I had to stop initiating the message and just handle it when the target sent
$ the request first, (an extremely rare occurence).

Was this driver, by any chance, for the 2090A? Just curious... the
2090 is a fast controller, under good conditions (i.e. DMA'ing into
FAST), but it IS a rather old design. Didn't know they had sync SCSI
back then. 

Doesn't surprise me to hear that a lot of drives crashed given the
sync xfer request. From what I've heard on the net, most drives barely
support standard SCSI, much less extensions. That's what comes of
letting the engineers write the code ;-). 

$ are NOT going to get 5.0Mb/s transfer rates.  Most drives can pull data off
$ the media at 1.25Mb/s at 1:1 interleave.  That is the maximum

Note that some drives, e.g. the Quantum (?), have a track buffer. What
this means is that if you request block 1025 and then two sectors
later find that you need block 1020, it's already there, and can be
sent "ASAP" so to speak. Of course, you had to wait for the disk to
rotate all the way to 1025 in the first place.... 

$ will still be limited to the Amigas bus speed.  With a 16 bit DMA engine on
$ a stock a2000 this will come out to around 3.58Mb/s.  That's the ABSOLUTE
$ maximum data throughput you are going to get no matter how fast the drive
$ can supply data.  The fastest drive I have found to date is the CDC Wren V
$ That baby can sustain 1400K/s direct from the drive.  

I think 3.58MB/s is plenty fast ;-). (note what I said about all
speeds > 500K/sec... massive overkill!). 

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